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Old 09-21-2020, 01:19 PM   #81
momaka
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Default Re: Task TK-940TX-DF Blowing fuse

Buzzing for single-transistor forward PSUs typically means over-load condition.

However, since you have a series light bulb, it could also be a false over-load condition, as some PSUs simply don't like high-impedance on their input.

So I think there are three possibilities for what you are seeing here:

1) There is a short-circuit or over-current condition on the main PS secondary side, causing the primary to over-load. (Could be a faulty rectifier on one of the rails, for example, or an open feedback path.)

2) There is a component fault on the primary side of the main PS, causing the PWM IC to over-drive main PS MOSFET until its protection kicks in (open opto-coupler, open opto-coupler feedback path, faulty snubber component, or sense resistor going high resistance.)

3) PSU's main PS design might be sensitive to high-impedance on the input, which means the series light bulb could be the reason why you're seeing this behavior. (On that note, Active PFC PSUs... or PSUs with APFC... are even more sensitive, and generally cannot be tested with a series bulb less than 200-300 Watts.) If that's the case, then using several 60-100 Watt incandescent bulbs in parallel... or one 200-300 Watt heating element... should provide sufficiently lower impedance on the output to make the PSU function.

I suggest you start by assuming case #1 above and check all secondary-side components. Start with the large power-handling components first, like large rectifiers and diodes for the 3.3V, 5V, 12V, and -12V rails. To do that, check resistance between each of these rails and ground. Note that there _may_ be minimum-load resistors on the output, which could possibly make you think that a rail is shorted to ground (common mistake when someone uses continuity test on their multimeter to check if a rail is shorted to ground, since most multimeters will beep at resistance under 100 Ohms, and load resistors on the 3.3V and 5V rails typically are 100 Ohms or less.) An alternative (and somewhat easier) method to checking the rails for short-circuit is to use a 20-40 Watt, 12V, incandescent/halogen bulb and backfeed an external voltage to each rail. If you have a spare ATX PSU, simply wire the output of that PSU to the rail you want to test. For example, if you want to test if the 12V rail is shorted, connected ground on both PSUs together, then connect the 12V rail of one PSU to the 12V rail of the other PSU _through_ the indancesence/halogen bulb (i.e. similar to how you use the series light bulb on the input.) Then, plug in only the working PSU and turn it On (the PSU under test should remain unplugged from wall.) If the bulb glows steadily, there is likely a short-circuit on that rail. If not, then rail is probably OK. Just make sure to always connect the right rails together - i.e. only use 12V rail to test 12V rail on the PSU in question, or 5V rail to test the 5V rail, and 3.3V rail to test the 3.3V rail. If you accidentally mix then up (particularly inserting a higher voltage on a lower-voltage rated rail), you could blow up some caps.

If you find that none of the rails are shorted with the above tests, then we could assume case #3 for a bit, and simply add a few more incandescent bulbs in parallel to your single bulb on the input, or simply replace that with a 200-300 Watt heating element. Then try powering up the PSU and see if it buzzes again. If it does, go back to assuming case #1, and check all secondary-side components again. After this, assume case #2 and check all primary side components related to the main PS.

Quote:
Originally Posted by socketa View Post
As far as i can hear, the light buzzing is coming from the small 472 Mohm blue cap (red line), and after i turn it off and on until the loud buzzing happens, it is coming from the adjacent side of the big white resistor (green line), possibly the other small 221 ohm small blue cap or the larger one.
The numbers on those caps indicates their capacitance and not their resistance.

caps with numbers xx5 = x.x uF
caps with numbers xx4 = 0.xx uF
caps with numbers xx3 = 0.0xx uF = xx nF
caps with numbers xx2 = 0.00xx uF = x.x nF
caps with numbers xx1 = 0.000xx uF = 0.xx nF

* where xx = 2-digit number
(Also, nF = nanoFarads, uF = microFarads)

So 472 = 4.7 nF and 221 = 0.22 nF

Last edited by momaka; 09-21-2020 at 01:27 PM..
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Old 09-22-2020, 01:36 PM   #82
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Default Re: Task TK-940TX-DF Blowing fuse

----------
Last night
----------

No shorts on the output rails. (3.3V 15ohm, 5V 43ohm, 12V 233 ohm, and no shorts on the blue, white, or grey wires)
So added a 100W bulb in parallel with the current 60W bulb.
Turned it on, the fan spins and keeps spinning, there is buzzing for a half second and then the bulbs light up and the buzzing increases in volume, but the voltages are now good.
Swapped the 60W with a 75W, same result as before except that the bulb takes a second to light up.
The the voltages are good from the moment the PSU is turned on, to the moment that i turn it off.

-----------
This morning
-----------

Added another bulb holder (so now have all three bulbs in parallel) and the buzzing was still there.
Wiggled a primary cap and noticed a spark, and noticed that i hadn't soldered it in so great.

But, foolishly, instead of just soldering it up good , i removed them and put the originals back in.
The buzzing didn't happen, the bulb briefly lights up, the fan spins for a second then stops, and i hear a brief tiny squeak sound from the PSU after turning it off for about 10 seconds.
No squeak sound when turning it on.

If i just use the 100W bulb alone, the bulb stays lit, i hear a much louder squeak sound when turning it on, the fan twitches, and i hear the same soft squeak (an un-squeak?) sound after about 10 seconds, as before, after turning it off.

I get the same result now, regardless of which of the three pairs of caps i use.

So i don't know if replacing the caps caused another fault.
I suspect that it fixed the buzzing, but the buzzing (which i'm assuming was a loose primary cap leg that was arcing with the surrounding solder) may have created a condition whereby the PSU could power up with good voltages. So maybe there was two problems and i fixed one, which subsequently brought an underlying one to the surface.
Or maybe the impedance of the parallel bulbs is still too high for the PSU to start?

Quote:
The numbers on those caps indicates their capacitance and not their resistance.
Doh! I got tricked by the "472M" that was printed on the cap, and bypassed my logic gate.
Thanks for the additional info.

P.S. When i put the components back on, i replaced two shorted zeners with ones that looked exactly the same, from the primary of another PSU

Last edited by socketa; 09-22-2020 at 02:07 PM..
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Old 09-24-2020, 11:44 AM   #83
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Last night
----------

No shorts on the output rails. (3.3V 15ohm, 5V 43ohm, 12V 233 ohm, and no shorts on the blue, white, or grey wires)
So added a 100W bulb in parallel with the current 60W bulb.
Turned it on, the fan spins and keeps spinning, there is buzzing for a half second and then the bulbs light up and the buzzing increases in volume, but the voltages are now good.
Swapped the 60W with a 75W, same result as before except that the bulb takes a second to light up.
The the voltages are good from the moment the PSU is turned on, to the moment that i turn it off.
That's an interesting result.

It suggests the PSU might just be OK, but just not liking the still relatively high impedance of the bulbs on the input. However, I haven't seen a PSU that doesn't work with 2-3 bulbs in parallel, so there may still be something iffy going on - in particular something pulling too much current on the secondary side that is making the primary overload. The fact that the output voltages are stable and have correct values means the secondary side feedback is OK. Given the results, though, I suggest to check the secondary side snubber circuits for each rail. These snubber circuits would be a low-value resistor (typically 2.2 to 10 Ohms and about 1/2 Watt rating) placed in series with either a ceramic or small metal film capacitor (generally in the range of 0.01 to 0.1 uF... or 103 to 104 printed on the cap.) You'll find these snubber circuits for each of the major rails (5V, 12V, and maybe 3.3V) close to either the heatsink-mounted rectifiers or close to the secondary side output of the main transformer. Typical failure mode for these snubber circuits is either the resistor will overheat and go open-circuit or the ceramic/film cap will go short-circuit. Since these snubber circuits are connected directly across the transformer output pins, you won't be able to check the components in circuit - you will need to remove either the cap or the resistor, or lift one leg on these two components from the PCB to take proper measurement.

Quote:
Originally Posted by socketa View Post
This morning
-----------

Added another bulb holder (so now have all three bulbs in parallel) and the buzzing was still there.
Wiggled a primary cap and noticed a spark, and noticed that i hadn't soldered it in so great.

But, foolishly, instead of just soldering it up good , i removed them and put the originals back in.
The buzzing didn't happen, the bulb briefly lights up, the fan spins for a second then stops, and i hear a brief tiny squeak sound from the PSU after turning it off for about 10 seconds.
No squeak sound when turning it on.
Is the squeak sound heard when simply disconnecting the PS_ON wire from ground (to turn Off the main PS) or when you unplug the PSU from the wall?

The latter (unplugging PSU from wall outlet) is actually normal for 2-transistor 5VSB circuits. I have a lot of PSUs with a 2-transistor 5VSB circuit, and at least half of them do a fairly loud chirp/squak (typically ending with a raise in pitch as time passes) as the PSU is unplugged from the wall.

Quote:
Originally Posted by socketa View Post
If i just use the 100W bulb alone, the bulb stays lit, i hear a much louder squeak sound when turning it on, the fan twitches, and i hear the same soft squeak (an un-squeak?) sound after about 10 seconds, as before, after turning it off.

I get the same result now, regardless of which of the three pairs of caps i use.
Well at least we have consistency now... and the bulb is no longer glowing right away when the PSU is plugged in.

Quote:
Originally Posted by socketa View Post
P.S. When i put the components back on, i replaced two shorted zeners with ones that looked exactly the same, from the primary of another PSU
That is certainly something worth investigating. Zener diodes can have different Zener voltage ratings, despite looking all the same way. Need to know which Zener diodes this is in the circuit so hopefully we can try to guess what voltage it should be rated for - that is, if you don't have the original shorted Zener diodes, which will tell you that by reading the parts numbers written on them.
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Old 09-25-2020, 12:15 AM   #84
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Is the squeak sound heard when simply disconnecting the PS_ON wire from ground (to turn Off the main PS) or when you unplug the PSU from the wall?

The latter (unplugging PSU from wall outlet) is actually normal for 2-transistor 5VSB circuits. I have a lot of PSUs with a 2-transistor 5VSB circuit, and at least half of them do a fairly loud chirp/squak (typically ending with a raise in pitch as time passes) as the PSU is unplugged from the wall.
Yes, that is indeed the the case here - switching off at the wall

I added another bulb (60W), so that's four bulbs in total - no noticeable change from when there was three.

The PSU will not power up, when turning on at the wall, until after the un-squeek squeek is heard (which happens when the primary caps have discharged to approx 30V)
Monitoring the rail voltages, i notice that 12V rail gets to 12.2V, 5V rail gets to 5.08V and the 3.3V rail gets to only 2.7V, before the PSU shut's itself down - so maybe there is a clue, or pointer, there that suggests looking at the 3.3V snubber circuit.

I'll investigate those snubber circuits to see if the resistors and ceramic/film caps are free of the faults that you mentioned, and then post results here when done.

--------

Zener Diodes

Looking back through the posts, the two zeners that i replaced (i eventually threw the originals away) are:

ZD8, next to the heatshrinked resistor,
Quote:
the shorted Zener is probably part of the problem. But since it appears connected to the Gate drive of the main PS MOSFETs
and ZD1
Quote:
I removed zener diode, ZD1, and it tests as a shorted; and the short across R23A dropped right off.

Last edited by socketa; 09-25-2020 at 12:38 AM..
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Old 09-26-2020, 03:34 AM   #85
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Default Re: Task TK-940TX-DF Blowing fuse

I found the 3 snubber circuits, all 4.7 Ohm 1/2 watt sized resistors. And there are no shorted caps or open resistors

Quote:
Since these snubber circuits are connected directly across the transformer output pins, you won't be able to check the components in circuit
I saw no need to lift any component, because, according to the following logic:
Each circuit is essentially a resistor and capacitor connected together in a closed loop.
If you place the meter leads across the resistor (which is the same as "placing them across the capacitor") and get a measurement that matches the resistor, then that means that the resistor is good, and that the capacitor is not shorted. If the capacitor was shorted then the meter would read as a short because one could rightly say that it's parallel with the resistor.

For quite some time i've being inclined question what i'm told, because from a young age i refused to eat animal meat, but, at that time, someone told me that sausages were mostly bread crumbs, so i became somewhat addicted to them, and sausage rolls, for many years; not really understanding, or considering, what i was doing - until one day not long after i started eating mince pies, i noticed a piece of artery inside one. That was the end of that 'venture', and also the beginning of being vegetarian. I still didn't know about why cows make milk (incredibly many folk don't!) ... could go on with a long story, but suffice it to say that i'm constantly amazed at how easy it can be to miss stuff, that in hindsight, is blatantly obvious after the application of minimal thought, quietude, or/and assistance

Attached is some photos of that resistors (which all tested good, at 4.7 ohms) and caps.
The first pic looks like "102" is printed on the cap, and the other two caps have "472" printed on them.
The colours on the three resistors are yellow, violet, gold; although due to lack of sunshine, the yellow looks green in the photos.
Also attached, is a photo of the back of the board, with the three snubber circuits drawn in yellow.
The snubber at the bottom has a jumper on the right hand side (vertical yellow line) which i initially missed, and so i was looking for the path back to the transformer (hence the irrelevant diode that i drew in)






Attached Images
File Type: jpg 2020-09-26_05-43-08-p.m..jpg (33.6 KB, 55 views)
File Type: jpg 2020-09-26_05-44-57-p.m..jpg (36.6 KB, 54 views)
File Type: jpg 2020-09-26_05-45-29-p.m..jpg (29.0 KB, 4 views)
File Type: jpg 2020-09-26_05-45-59-p.m..jpg (36.6 KB, 54 views)
File Type: jpg 2020-09-26_05-46-49-p.m..jpg (41.5 KB, 54 views)
File Type: jpg 20200926_165412.jpg (397.7 KB, 54 views)
File Type: jpg Snubbers.jpg (375.2 KB, 53 views)

Last edited by socketa; 09-26-2020 at 04:15 AM..
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Old 09-29-2020, 11:35 AM   #86
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Yes, that is indeed the the case here - switching off at the wall

I added another bulb (60W), so that's four bulbs in total - no noticeable change from when there was three.

The PSU will not power up, when turning on at the wall, until after the un-squeek squeek is heard (which happens when the primary caps have discharged to approx 30V)
Monitoring the rail voltages, i notice that 12V rail gets to 12.2V, 5V rail gets to 5.08V and the 3.3V rail gets to only 2.7V, before the PSU shut's itself down - so maybe there is a clue, or pointer, there that suggests looking at the 3.3V snubber circuit.
The PSU not able to start until you hear the "un-squeak squeak" is probably because a short-circuit or over-power protection is latching.

Yes, the 12V and 5V rails coming up but the 3.3V rail not regulating properly is definitely a clue here.

Since this PSU derives the 3.3V rail through a mag-amp circuit, it relies on the primary side's switching action to regulate the 3.3V rail through a transistor and a saturation coil on the secondary side. If something is not quite right with the PWM switching, the 3.3V rail can get affected by that and not regulate properly. Or it could also be that something on the 3.3V rail's mag-amp circuit is bad, causing it to not regulate properly. If you trace the entire power-side of the 3.3V rail (i.e. only big traces that carry power and not small feedback/signal traces), you should find a transistor, a diode or two, and also a 431 shunt regulator with some resistors all connected to the 3.3V rail - all of these parts are for the mag-amp circuit and should be checked out.

You should also try feeding back 3.3V from another PSU into this PSU's 3.3V rail and see if anything gives or overheats (particularly the transistor responsible for regulating the 3.3V rail.)

Quote:
Originally Posted by socketa View Post
I found the 3 snubber circuits, all 4.7 Ohm 1/2 watt sized resistors. And there are no shorted caps or open resistors
...
I saw no need to lift any component, because, according to the following logic:
Each circuit is essentially a resistor and capacitor connected together in a closed loop.
If you place the meter leads across the resistor (which is the same as "placing them across the capacitor") and get a measurement that matches the resistor, then that means that the resistor is good, and that the capacitor is not shorted. If the capacitor was shorted then the meter would read as a short because one could rightly say that it's parallel with the resistor.
That is true.

So looks like your snubber circuits are OK.

Quote:
Originally Posted by socketa View Post
Zener Diodes

Looking back through the posts, the two zeners that i replaced (i eventually threw the originals away) are:

ZD8, next to the heatshrinked resistor, and ZD1
Ok, looking back at the circuit once again...

ZD8 is related to the 5VSB circuit only: it appears to be used as a limiter for the voltage on the Gate of the 5VSB MOSFET. Since the 5VSB seems to be working normally, I suppose everything is good there.

However, that does raise the question: when you try to power on the main PS and you hear the PSU squeak, what happens to the 5VSB? In other words, try measuring the voltage on the 5VSB when you do this experiment. We want to see if the 5VSB drops out or not. If it is, that would be a problem. To get more accurate result, first put a load on the 5VSB - something in the range of 0.25 to 0.5 Amps should do (so connect something like a 10-20 Ohm resistor to the output of the 5VSB - that 1.5 KW heating element should have about 6.5 Ohms resistance, and probably even that will do the trick.) Then plug in the PSU, but keep main PS turned OFF, then see if the 5VSB still outputs 5V. If it does, try powering on the main PS and see if the 5VSB continued to supply 5V. The reason we want to do this is because the 5VSB circuit also has a primary side auxiliary winding that feeds the PWM controller on the primary side for the main PS. If 5VSB is unstable or dropping out, the main PS will not work either.

As for ZD1.....
That one is for Gate voltage protection for the main PS MOSFETs.

With that said, both ZD1 and ZD8 should have Zener voltage ratings that are less than the maximum rated Gate voltage of the 5VSB and main PS MOSFETs.
i.e.
ZD1 Zener voltage < V_GSS of main PS MOSFETs
and
ZD8 Zener voltage < V_GSS of 5VSB MOSFET

However, the Zener voltage of these Zener diodes should also not be too low, because then the MOSFET's may not get fully driven On.

This means the Zener voltage of these Zener diodes should be close to the rated V_GSS of the MOSFETs, but just slightly lower.

So if you used an improper Zener diode for ZD1, that could cause the MOSFET to not get fully driven On and thus not switch properly, leading to OPP/SC protection kicking in.

Last edited by momaka; 09-29-2020 at 11:37 AM..
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Old 10-01-2020, 02:12 AM   #87
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
when you try to power on the main PS and you hear the PSU squeak, what happens to the 5VSB?
5VSB is good after the squeak
And it's good irregardless of whether i switch the PSU off and on straight away, or off and then back on, straight after after the unsqueak.
The unsqueak that happens after switching the PSU off, coincides with the 5VSB voltage reading dropping off the multimeter screen.

I back-fed 3.3V from another PSU into the Task 3.3V output rail, initially with a 12V 23W lamp (no glow), then a 12V 8W lamp (no glow), then a 12V 3.4W lamp (glowed when i briefly connected it)
So am guessing that that shows that there is no shorts on that rail.

I removed ZD8, and it looks like it has 18C printed on it,
which looks to be a zener voltage of 18V
V_GSS of K2746 = ± 30V
So the zener that i used is quite a lot lower.
I've now replaced it with a C27 Zener instead, but no change in test result

Mind you, i wasn't super hopeful about that completely fixing it; because why would the 5V and 12V rails briefly spark up as good, but the 3.3V rail be way under spec? You'd think that they would all be affected if either of the main switching transistors were having consequential issues.

So that just leaves that mag-amp circuit that you mentioned.
I traced out what i could, removed the minimum load resistor which measured 15 ohms, so that i could check the other resistors (see photo), but when i put one lead of my meter on 3.3V rail and the other one on the earth rail to test the 3 resistors (the first one doesn't count, because the +S wire is connected to the 3.3V rail in the motherboard connector), their total resistance should be (330 + 110 + 22)ohms, but the meter reading keeps on going up to over 1K.

Those two diodes, that i drew in test good.
No shorts on the shunt regulator, or the transistor

Before i remove the transistor and the shunt regulator to test out of circuit, would you say that the zener diode that i replaced with the C27 diode was an acceptable replacement (was the closest value zener that i could scavenge of other parts PSUs), and wouldn't be the cause of PSU shutting down?
Attached Images
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File Type: jpg 20201001_183503.jpg (639.0 KB, 7 views)
File Type: jpg mag-amp circuit.jpg (202.7 KB, 8 views)
File Type: jpg 20201001_183941.jpg (555.2 KB, 6 views)

Last edited by socketa; 10-01-2020 at 02:36 AM..
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Old 10-02-2020, 04:58 PM   #88
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
5VSB is good after the squeak
And it's good irregardless of whether i switch the PSU off and on straight away, or off and then back on, straight after after the unsqueak.
The unsqueak that happens after switching the PSU off, coincides with the 5VSB voltage reading dropping off the multimeter screen.
Ah OK.
Sounds more like a more-or-less normal 2-transistor 5VSB function.

Quote:
Originally Posted by socketa View Post
I back-fed 3.3V from another PSU into the Task 3.3V output rail, initially with a 12V 23W lamp (no glow), then a 12V 8W lamp (no glow), then a 12V 3.4W lamp (glowed when i briefly connected it)
So am guessing that that shows that there is no shorts on that rail.
Yes, looks OK.
In addition to backfeeding a rail, you can also monitor the voltage on that rail to see if it's dropping voltage and how much / to what value. But since you tried 3 different bulbs, with the largest being 23W (about 6.26 Ohms when bulb is fully lit and lower when cold) I'm pretty sure you would have seen/felt something overheat and the bulb glow if there was a bad component. Sometimes the transistor that does the regulation on mag-amp 3.3V rails can partially short and run very hot, with 3.3V regulation being all whack... but again, I don't think that's the case here anymore. Still worth mentioning it, though, in case you take other PSUs for repair.

Quote:
Originally Posted by socketa View Post
I removed ZD8, and it looks like it has 18C printed on it,
which looks to be a zener voltage of 18V
V_GSS of K2746 = ± 30V
So the zener that i used is quite a lot lower.
I've now replaced it with a C27 Zener instead, but no change in test result
OK, that's good to know. At least we eliminate that as being a possibility as well.

Quote:
Originally Posted by socketa View Post
Mind you, i wasn't super hopeful about that completely fixing it; because why would the 5V and 12V rails briefly spark up as good, but the 3.3V rail be way under spec? You'd think that they would all be affected if either of the main switching transistors were having consequential issues.
Well, the only reason I thought about checking the 3.3V rail is because the mag-amp transistor basically switches On and Off to create "counter currents" through the mag-amp coil, which then is what reduces the voltage from the 5V AC tap to 3.3V.

Quote:
Originally Posted by socketa View Post
So that just leaves that mag-amp circuit that you mentioned.
I traced out what i could, removed the minimum load resistor which measured 15 ohms, so that i could check the other resistors (see photo), but when i put one lead of my meter on 3.3V rail and the other one on the earth rail to test the 3 resistors (the first one doesn't count, because the +S wire is connected to the 3.3V rail in the motherboard connector), their total resistance should be (330 + 110 + 22)ohms, but the meter reading keeps on going up to over 1K.
Ok, in that case pull out those resistors as necessary and test if they read proper resistance out of circuit too.

Also, it just crossed my mind, but....
This PSU may be one of those units that doesn't like to run with no load connected to it, and maybe that's why the 3.3V rail was low.

To test if that is the case, add your 8 Watt, 12V bulb on the 5V rail and put the 4.3 Watt, 12V bulb on the 12V rail. Leave the 3.3V rail without a load, other than its 15-Ohm minimum load resistor. Then connected the PSU with the series bulbs on the input (try 2 or 3 60-Watt bulbs first) and see if the voltages are normal.

Quote:
Originally Posted by socketa View Post
Before i remove the transistor and the shunt regulator to test out of circuit, would you say that the zener diode that i replaced with the C27 diode was an acceptable replacement (was the closest value zener that i could scavenge of other parts PSUs), and wouldn't be the cause of PSU shutting down?
From what I understood, there was no change in behavior when going from the 18C to C27 Zener, correct?

Actually, there are two factors when considering the Gate protection Zener diodes: one is, as mentioned in the previous post, the maximum voltage the Gate of the MOSFET is rated for. So the Zener needs to have a Zener voltage less than the gate. The other factor is the drive voltage. In the case of ZD1, this would be the Vcc voltage going to the PWM controller (UC3843 IC). Note that the UC384x family of PWM controllers is rated for 30V absolute max for Vcc. So with that in mind, we know the voltage that the PWM controller outputs could be up to 30V, depending on Vcc, but not more than that. So Z1 needs to be selected to have a Zener voltage that is higher than whatever Vcc actually is in the circuit (but not more than 30V, because UC384x PWM IC is not expected to output more than that anyways.)

Thus, the C27 Zener you used should be a good fit. Not only should it protect the MOSFET Gate from going over its maximum V_GSS voltage (30V), but should also protect the PWM IC in cases where the MOSFET can go bad and conduct Drain-to-Gate (opening the possibility of a high voltage leaking back from Drain into Gate.)

Last edited by momaka; 10-02-2020 at 05:01 PM..
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Old 10-03-2020, 12:15 AM   #89
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Default Re: Task TK-940TX-DF Blowing fuse

Just as well i checked before desoldering and testing the other components,
and just as well:
Quote:
Also, it just crossed my mind, but....
This PSU may be one of those units that doesn't like to run with no load connected to it, and maybe that's why the 3.3V rail was low.
Good thinking, Batman! (0% alcohol version)
Even though it has "minimum load"(?) resistors on the three rails?

Quote:
To test if that is the case, add your 8 Watt, 12V bulb on the 5V rail and put the 4.3 Watt, 12V bulb on the 12V rail. Leave the 3.3V rail without a load, other than its 15-Ohm minimum load resistor. Then connected the PSU with the series bulbs on the input (try 2 or 3 60-Watt bulbs first) and see if the voltages are normal.
OK, i put the 15-Ohm minimum load resistor back on, and carried out the above test, with the following results:

I used two 60W bulbs and a 70W bulb,
and connected the 12V bulbs as you suggested, turned on the PSU, and got good voltages on all three rails.
The PSU didn't shut down.
Only the 12V bulbs glowed.

Then i removed one 60W bulb and turned on the PSU, and one of two thing happened:

1. All bulbs glowed and the PSU made a high pitched squealing noise (what causes that?) with poor voltages (3.3V rail=2.3V, 5.5V rail = 2.4V , 12V rail = 6V) The PSU didn't shut down. The fan may or may not start, but turns at about 1/2 speed if i give it a push with my hand


2. All bulbs glowed. No high pitched squeal.The fan spins at about half-speed, (3.3V rail = 3V, 5.5V rail = 3.1V , 12V rail = 7.5V).

------------
Was the reason for the higher wattage bulb across the lower voltage rail, because this PSU is one that "5V heavy"?

Just out of interest, is the 3.3V derived via a mag-amp in this schematic,
http://danyk.cz/s_atx01u.png,
and if so, where is the mag-amp?

During the course of all this soldering/desoldering, i found one tip that someone may find useful:
Cleaning off flux with isopropyl alcohol doesn't do a very good job with "kingbo" flux; so i tried epoxy solvent clean up, which i think is xylol, and it does a great job.

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Old 10-03-2020, 12:17 PM   #90
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Good thinking, Batman! (0% alcohol version)
Even though it has "minimum load"(?) resistors on the three rails?
Never assume the designer necessarily did their job properly - at least with lesser known PSU brands.

Sirtec PSUs, for example, are known to oscillate badly with a heavy 12V load and light 5V load, despite having a 12V rail strong enough for a 12V-based system and the label claiming them being suitable for "Pentium 4" (goes to show how old some of these are )

Quote:
Originally Posted by socketa View Post
OK, i put the 15-Ohm minimum load resistor back on, and carried out the above test, with the following results:

I used two 60W bulbs and a 70W bulb,
and connected the 12V bulbs as you suggested, turned on the PSU, and got good voltages on all three rails.
The PSU didn't shut down.
Only the 12V bulbs glowed.
Check your connections on the 12V bulbs. They should all light up or at least glow a little.
Otherwise this result suggests the PSU may be OK.

Try running the PSU longer with this test - something like 10-15 minutes to see if anything overheats. Run the test in 5 minute intervals while checking heatsink temperatures - and on that note, unplug PSU while it is still running to discharge primary caps so that the primary heatsink is not live or at a dangerous voltage when you try to check its temperature.

If all is good, try putting the 23W, 12V bulb on the 12V rail and the 8W, 12V bulb on the 5V rail. Then switch the three parallel-series bulbs on the input of the PSU with that 1.5 KW heating element you had before (and make sure you have a 4-5 Amp fuse in the PSU, in case something shorts hard so the element will disconnects.) Now see if the PSU runs and if anything gets hot. Note: you may or may not have to switch the 23W and the 8W bulbs between the 12V and 5V rails, depending if this PSU prefers a more 12V-heavy or 5V-heavy load.

Quote:
Originally Posted by socketa View Post
Then i removed one 60W bulb and turned on the PSU, and one of two thing happened:

1. All bulbs glowed and the PSU made a high pitched squealing noise (what causes that?) with poor voltages (3.3V rail=2.3V, 5.5V rail = 2.4V , 12V rail = 6V) The PSU didn't shut down. The fan may or may not start, but turns at about 1/2 speed if i give it a push with my hand
Ouch!
Whatever protections there are in this PSU, they don't seem to be engaging. Makes me wonder if the supervisory IC on the secondary side is still good or not (maybe it got taken out from all of the ripple of the previously bad caps or when the 5VSB went bad.)

The squealing is the PSU trying to keep up with the "load"... whether that means it's loaded too hard (given the three series bulbs on the input possibly lowering the input AC voltage too much - more on that in a bit), or loaded too lightly (unlikely - you'd see at least one rail with a much higher voltage then), or loaded unevenly (unlikely - same as loaded too lightly)... it will just have to be determined by experiments.

On that note, while trying the above tests with the three series bulbs or heating element on the input...
1) Check your line AC voltage at a plug to get a reference of what it is. Post the value here.
2) When the PSU turns On with stable voltages, check the line AC voltage that the PSU is getting _after_ the series bulbs / heating element. Note the value here as well. Some PSUs don't work well when the AC voltage drops too much. I'm curious if that is the case here...

Hope I'm not annoying you to death with these tests.

Quote:
Originally Posted by socketa View Post
2. All bulbs glowed. No high pitched squeal.The fan spins at about half-speed, (3.3V rail = 3V, 5.5V rail = 3.1V , 12V rail = 7.5V).
Yeah, something is not right with this thing. If you can't get proper output voltages with the 1.5 KW heating element as the series current limit, then it might be time to change the supervisory IC... and possibly PWM controller too. While at it, grab a few spare 431 shunt regulators as well.

If this doesn't revive the PSU, I really don't know what else can... at least for the time being (it's going to be at least a few more bathroom sessions with me sitting on the toilet and pondering about circuits stuff )

Quote:
Originally Posted by socketa View Post
Was the reason for the higher wattage bulb across the lower voltage rail, because this PSU is one that "5V heavy"?
Yeah, it could be.
If you can determine with which bulbs on what rails the PSU can run and output normal voltages and with which it can't, then that should give you a clue whether it's a 5V-heavy or 12V-heavy design... or just a heavy oversized paperweight.

Quote:
Originally Posted by socketa View Post
Just out of interest, is the 3.3V derived via a mag-amp in this schematic,
http://danyk.cz/s_atx01u.png,
and if so, where is the mag-amp?
Yes, it is.

In that circuit...
SD1 is the 3.3V rail rectifier
L4 and L(?) are the 3.3V rail filter coils
C18 and C19 are the output filter caps
R28 is the minimum load resistor

And the mag-amp circuit components...
L2 is the saturation coil/toroid (the white one next to the main transformer in your PSU - looks like L7, I think.)
Q5 is the mag-amp switching transistor
Q6 (TL431 - it should really be labeled as "IC" and not "Q" in the schematic) is the current shunt regulator for the 3.3V rail.
D8 and D9 are the diodes for the mag-amp coil
R26 and R27 (1k VR) are the resistors forming a resistive divider to set the 3.3V rail voltage to 3.3 Volts.

But really, the easiest way to recognized when a PSU has a mag-amp circuit for the 3.3V rail is that you will see either one or two saturation coils in the proximity of the main transformer (sometimes can be behind the secondary heatsink) in addition to a main toroid filter inductor and a separate (and slightly smaller) toroid filter inductor for the 3.3V rail - both located behind the secondary heatsink.

Quote:
Originally Posted by socketa View Post
During the course of all this soldering/desoldering, i found one tip that someone may find useful:
Cleaning off flux with isopropyl alcohol doesn't do a very good job with "kingbo" flux; so i tried epoxy solvent clean up, which i think is xylol, and it does a great job.
Interesting.
I guess Kingbo quality can vary quite a bit... which shouldn't be too surprising.
We (me and the techs at a former shop I worked at long time ago) used to think that Kingbo was some high quality flux. But over the years, I've learned that it's just a common name used by many different Chinese manufacturers, and thus its quality and properties can vary somewhat. Overall, though, it does appear to work well (at least mine does), even for SMD and BGA rework. I have no problems cleaning mine with IPA and small swabs/balls of paper towel. It does take at least 99% IPA, though, to clean it. Anything less and I have to go over each joint and clean it several times.
Acetone works great too, but it can be a bit too aggressive on some PCB coatings.
Don't think I have tried Xylol.
Denatured alcohol also worked, I think.
I don't remember much about MEK (Methyl Ethyl Ketone) though.

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Old 10-04-2020, 12:57 AM   #91
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Default Re: Task TK-940TX-DF Blowing fuse

Before i get immersed in all of that,

After thinking about this, if i use only a 60W and a 75W bulb in parellel, in which 1/R = 1/(0.00113 + 0.00142) = 392 ohms > 134W, instead of the three bulbs, and the PSU malfunctions, then that means that there is still a problem with the PSU, because the amount of bulbs that are now present, which is still well able to handle the required current, should not be relevant to the functioning of the PSU.

The actual metered Watts are 24W with three bulbs (so no surprise that that three bulbs dont glow?)

After doing some ohms law calcs etc, and Watt Meter measurements, i can see that the load of the PSU is the determining factor for current - not any bulbs that are inserted in the live AC line - thus, considering only that fact, removing one bulb shouldn't make any difference to the well-functioning "three bulbed" PSU,


i.e., what does that result narrow the cause down to?
The PSU seems to be aware that a bulb on the AC live has being removed - could it be the input impedance sensitivity that you previously mentioned?
Why would a PSU care about the impedance of any bulbs on the primary filtering section?
Would it cause some imballance in the two primary input coils?
Another thought is that removing one parallel bulb, would increase the input impedance - so that correlates with the test result and the theory of failure due to increased input impedance.
According to the calcs that i did, the Wattage of the combined three parallel bulbs will equal 194W, and after removing a 60W bulb it equals 134W - so maybe that is the reason why two bulbs both light up (and the PSU goes AWOL), while three bulbs don't.
When i remove a 60W bulb and power on the PSU (fan doesn't spin with two bulbs now), the metered wattage increases to 127W (was 24W with the three bulbs)
127W is pretty close to what the two parallel bulbs would naturally draw (134W)

With just the 60W and 75W bulbs connected, and the 3.4W bulb on the 12V rail, and 8W bulb on the 12V rail, all four bulbs glow, and the rail voltages are not high enough
and if i switch the small bulbs around, the PSU squeaks for half a second then shuts down with only the two 60 and 70W bulbs glowing.

---

My calcs are as follows:
The 3 parallel bulbs that are connected in series with the AC live wire briefly glow (due to in-rush current?)

so the max combined output resistance of the PSU with the two 12V bulbs, in post #89 was:
using P=VČ/R
15 ohms resistor + (43 ohms resistor + 5V^2/8W) + (233ohms resistor + 12V^2/3.4W)
= 15 ohms on 3.3 rail + 2.91 ohms on 5V rail + 42.36 ohms on the 12V rail
Converting to watts: 3.3^2/15 + 5^2/2.91 + 12^2/42.36
= 0.72W + 14W + 3.4
So that means that the PSU should draw close to 19W with those two 12V bulbs

But the wall power meter says that it's drawing 24W - which is only 5W difference, which could be attributable to heating of the PSU resistors, coils, and other components.

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Old 10-04-2020, 02:30 AM   #92
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Check your connections on the 12V bulbs. They should all light up or at least glow a little.
Otherwise this result suggests the PSU may be OK.
The connections are good, they do glow, except, or course, for when the PSU shuts itself down after swapping the 12V bulbs around (actually, just swapping the wires that are inserted into the 5V and 12V molex connector holes), and there is no voltage on the 3.3V, 5V and 12V rails.

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Old 10-04-2020, 01:37 PM   #93
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Default Re: Task TK-940TX-DF Blowing fuse

From my understanding, the reason for the bulbs or the element on the AC Live input, is to restrict the current so that you don't keep wasting fuses

The three parallel bulbs is 194W, which is 0.84A, which is nicer working result than an issue that would necessitate usage of the 4-5A fuse that you previously mentioned.
But what is the reason for using a 1500W heating element, (for which current would be 6.5A in the event of a completely shorted PSU?

So i used 5A fuse wire, which can handle (5A X 230V) 1150W (yes, i know that it was risky since the PSU is only rated for 400W),
1.5A fuse wire would have being ideal, since that would melt at 345W

Why does this PSU have a 400W rating?, when the info in the label says
+5V = 30A
+3.3V = 28A
+12V = 24A
150W + 288W + 92W = 530W, which exceeds the 400W max for this PSU

Anyway, i was feeling confident about this being one of those PSU's that requires an external load to operate, and whatever it is about the input impedance sensitivity that you mentioned, so connected a hard drive, and used just the 5A fuse wire on the AC Live wire.

Turned it on and it went good.
Turned it off after a couple of minutes, being grateful that nothing went "pop" or "bang"
The voltages fluctuate plus or minus 0.03V

This is the first PSU that i've had that appears to require an external load to function

Can you confirm that this PSU has no balancing resistors, because, unless i'm delusional, this PSU has no resistors attached to the middle terminals trace of the primary caps.
So maybe it's the first PSU that i've had that doesn't have balancing resistors.
Would it be a good idea to add the resistors on the underside of the board?

And why would input impedance of the bulbs affect the operation of the PSU?

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Old 10-05-2020, 08:00 PM   #94
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Default Re: Task TK-940TX-DF Blowing fuse

The ratings (current draw?) of the hard drive is 5V 0.85A, 12V 0.75A (= 13.2W)
The current drawn by the Task PSU + Hard drive = 22.5W
So tried a Bestec PSU + Hard drive, and got 20.3W
And then tried an Aywun (that couldn't fully power a particular non-faulty optical mouse (was only recognized by windows when i changed the PSU) when plugged into the GA-78LMT-USB3 that you helped me get working) + Hard drive, and got 13.3W

The Task and the Bestec seem to be significantly inefficient, compared to the Aywun - but maybe that is because the Aywun is gutless?

Also, bearing in mind that the big pink 0.1 ohm resistor that i lost, has being replaced with one that is 0.2 ohms

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Old 10-05-2020, 10:07 PM   #95
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
From my understanding, the reason for the bulbs or the element on the AC Live input, is to restrict the current so that you don't keep wasting fuses
Yes, but thinking that the dim bulb device is for limiting current is not quite the correct term here. Rather, the bulb(s) limit the amount of power to the DUT (device under test)... and with that, of course, the current also becomes limited. Keep this small distinction in mind for the discussion that is to follow.

Quote:
Originally Posted by socketa View Post
The three parallel bulbs is 194W, which is 0.84A, which is nicer working result than an issue that would necessitate usage of the 4-5A fuse that you previously mentioned.
But what is the reason for using a 1500W heating element, (for which current would be 6.5A in the event of a completely shorted PSU?
So here is why I mentioned the distinction above: the series bulb(s) limit power, whereas a fuse limits (interrupts) current once it goes past a certain threshold.

Not only that, but the fuse acts pretty much like a short-circuit (extremely low impedance device) right up until the maximum current is exceeded. And on that note, most fuses (even fast blow ones) will usually take a long time (could be up to several seconds) to pop when running at or near their maximum current. It's only when you go significantly above their rating that they will act fast and blow right away. Meanwhile, the voltage going to the device is mostly unaffected (that is, there's no significant voltage drop over the fuse). And because of that, the power going to the device is mostly still unlimited and the device can draw as much as it pleases... or at least until the fuse blows. So in the relatively long time it takes for the fuse to act / blow, you can have a large spike of surge current pass through it for a few moments, and thus also have a large surge / spike in power draw. These surges in power and current can be so high that other parts downstream may get damaged.

With that said, fuses are not necessarily meant to protect parts from blowing up. Rather, they are a "last resort" device so that components don't overload uncontrollably for an extended period of time, leaving the possibility of starting a fire.

On the other hand, an incandescent bulb or heating element has a fairly defined and relatively high resistance (at least compared to a fuse.) Because of that, the moment your device tries to draw more power (and more current) through the bulb / heating element, the voltage drop across the bulb / element will increase proportionally with the increase in current draw (because of Ohm's law), and so will the bulb's / heating element's power. So in other words, the bulb / heating element imposes a power limit and this power can only be shifted between the DUT and the bulb / heating element. Moreover, everything happens instantly and there is no delay or surge in current (and power) like there can be with the fuse. As such, there is much less chance of blowing components... though if the current is too high for some components, they can overheat and even burn / start a fire if there is no fuse to interrupt the current. This is why I suggested to use the 4-5 Amp fuse if testing with the 1.5 KW heating element - that way, you have a current and power limiting.

Quote:
Originally Posted by socketa View Post
And why would input impedance of the bulbs affect the operation of the PSU?
Right.

So try to keep in mind everything mentioned in the above discussion.

Before I continued with more on that topic, though, first consider this:
most devices and electrical appliances are rated for a specific voltage input and frequency. In the case of *most* SMPS's, the input frequency isn't really that important (the exceptions, of course, being ones with APFC and large PPFC chokes.) However, the input/line voltage is very important.

While APFC and many flyback PSUs are typically specified to work over a "wide range input" - i.e. a line voltage anywhere in the range of 90 to 240V AC... older PSUs with a voltage doubler circuit (like yours in this thread) are typically specified to work either with 115V (110-120V AC) or 230V (220-240V AC), +/- 6%. Going below 110V (with the voltage selector switch set to 115V) or 220V (with voltage selector switch set to 230V) may render the PSU inoperable, just like yours did with the bulbs.

How low can you go on the line voltage before the PSU stops working?
- That tends to vary between different PSUs with different designs. In my experience, 100-105V is about as low as I could go on most PSUs (with their switch on the "115V" position, since I live in the USA and have 120V AC lines.) But it's not a set number. I've seen some very old half-bridge PSUs work on as low as 90V AC and also saw one that stopped below 108V AC.

Now, the reason I brought this discussion up is because with the incandescent bulb(s) or heating element in series with the PSU, you're going to see a voltage "drop" across the bulb(s) / element as the PSU starts to draw current. An increase in the current draw by the PSU will cause an increase in current through the bulb(s) / heating element. Because the bulb(s) / element have a significant resistance/impedance (unlike a fuse), Ohm's law dictates that the voltage across the bulb(s) / element will also increase. This means the voltage across the PSU will actually decrease. In other words, the AC voltage across the PSU (let's call it V_psu_ac) plus the voltage across the bulb / heating element (let's call that V_bulb_ac) is always equal to the line voltage (230/240V in your case), or:
V_psu_ac + V_bulb_ac = 230V

When you have less bulbs in series with the PSU, the resistance/impedance of the bulbs is higher, and so with the same amount of current drawn by the PSU, the voltage drop across the bulbs will be higher with less bulbs, and that means PSU getting lower voltage input. Thus, it's only a matter of how much current the PSU draws before the voltage across it decreases too much and falls out of the specified working range.
.
... and I'm splitting posts here, because I got hit with 10k char limit. lol.
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Old 10-05-2020, 10:14 PM   #96
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Default Re: Task TK-940TX-DF Blowing fuse

Now, you were wondering when you had the 1x 60W bulb + 1x 70W bulb, why did the PSU not work, since the total power of both bulbs combined is 130 Watts - i.e. much higher than what the PSU was drawing.

To answer that, consider this much simpler example (and you are welcome to try and recreate it if you like):
Two resistors in series, R1 and R2, connected across a power supply Vs.
. ___________
.|...I -->.......|
.|...............[R2]
(Vs)..............|
.|.................|
.|................[R1]
.|___________|

(^ please ignore the "...." dots above - they're just for spacing my beautiful text art )
Imagine R1 represents the resistance of the PSU at any given moment, and R2 representing the resistance of the incandescent bulb / heating element.

To make math simple, let's have Vs = 6V and R2 (bulb) = 1 KOhms.

Now imagine the first case is where the PSU is short-circuited (i.e. R1 = 0 Ohms.) Then the total resistance is R1 + R2 = 1 KOhms and the current, I, is Vs / (R1 + R2) = 6 / 1K = 6 mA.
Using V=IR again, you can see that the voltage across R1 is V1 = I * R1 = 6 mA * 0 = 0V
and the voltage across R2 is V2 = I * R2 = 6 mA * 1k = 6V
... which makes sense, because V1 + V2 must always equal to Vs
and power draw of the PSU is P_psu = V1 * I = 0V * 6 mA = 0 mW

Next, let's repeat that with the PSU trying to work "hard" and having R1 = 0.5 KOhms. Then the total resistance is R1 + R2 = 1.5 KOhms and the current, I, is Vs / (R1 + R2) = 6 / 1.5K = 4 mA.
Using V=IR again, you can see that the voltage across R1 is V1 = I * R1 = 4 mA * 0.5k = 2V
and the voltage across R2 is V2 = I * R2 = 4 mA * 1k = 4V
as a check..... V1 + V2 = 2V + 4V = 6V, which is equal to Vs.
and power draw of the PSU is P_psu = V1 * I = 2V * 4 mA = 8 mW

Now let's try again with the PSU resistance being the same as the bulb's - i.e. R1 = R2 = 1 KOhms. Then the total resistance is R1 + R2 = 2 KOhms and the current, I, is Vs / (R1 + R2) = 6 / 2K = 3 mA.
Using V=IR, the voltage across R1 is V1 = I * R1 = 3 mA * 1k = 3V
and the voltage across R2 is V2 = I * R2 = 3 mA * 1k = 3V
as a check..... V1 + V2 = 3V + 3V = 6V, which is equal to Vs.
and power draw of the PSU is P_psu = V1 * I = 3V * 3 mA = 9 mW

We can continue this "trend" with increasing PSU resistance R1. Let's try 2 KOhms. Then the total resistance is R1 + R2 = 3 KOhms and the current, I, is Vs / (R1 + R2) = 6 / 3K = 2 mA.
Using V=IR, the voltage across R1 is V1 = I * R1 = 2 mA * 2k = 4V
and the voltage across R2 is V2 = I * R2 = 2 mA * 1k = 2V
as a check..... V1 + V2 = 4V + 2V = 6V, which is equal to Vs.
and power draw of the PSU is P_psu = V1 * I = 4V * 2 mA = 8 mW

Once more... let's set R1 = 5 KOhms. Then the total resistance is R1 + R2 = 6 KOhms and the current, I, is Vs / (R1 + R2) = 6 / 6K = 1 mA.
Using V=IR, the voltage across R1 is V1 = I * R1 = 1 mA * 5k = 5V
and the voltage across R2 is V2 = I * R2 = 1 mA * 1k = 1V
as a check..... V1 + V2 = 5V + 1V = 6V, which is equal to Vs.
and power draw of the PSU is P_psu = V1 * I = 5V * 1 mA = 5 mW

And let's say the PSU is very "lightly" loaded... perhaps R1 = 11 KOhms. Then the total resistance is R1 + R2 = 12 KOhms and the current, I, is Vs / (R1 + R2) = 6 / 12K = 0.5 mA.
Using V=IR, the voltage across R1 is V1 = I * R1 = 0.5 mA * 11k = 5.5V
and the voltage across R2 is V2 = I * R2 = 0.5 mA * 1k = 0.5V
as a check..... V1 + V2 = 5.5V + 0.5V = 6V, which is equal to Vs.
and power draw of the PSU is P_psu = V1 * I = 5.5V * 0.5 mA = 2.75 mW

If you look at the pattern of the values in this example, you can see that as the PSU power draw decreases, the voltage across it increases, whereas the voltage across the bulb decreases. Not only that, but you can see that the maximum power draw from the PSU occurs when the PSU resistance matches that of the light bulb. However, do note that the voltage across the two is equally split in half. In the real world, this would correspond to the PSU getting only 115V when set to operate on 230V AC line and only 57.5V when set to operate on 115V AC line. Clearly, this isn't going to work!

So as you can see, the PSU must be very lightly loaded, if at all, to use the series bulb trick. Alternatively, you must use higher-power bulbs / element, if you want the PSU to draw more power without the AC across it dropping out of spec.
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Old 10-05-2020, 10:19 PM   #97
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Why does this PSU have a 400W rating?, when the info in the label says
+5V = 30A
+3.3V = 28A
+12V = 24A
150W + 288W + 92W = 530W, which exceeds the 400W max for this PSU
Ah, that's a classic "play on the numbers" by manufacturers.
This doesn't mean the above numbers are bogus or made up. But I didn't see you post a picture of the label of this PSU, so I can't say for sure if they are or aren't.

With that said, reputable PSU manufacturers will usually specify not only the maximum power the PSU can provide, but also the maximum power available from each rail and all of the rails combined.

Nowadays, most good quality modern PSUs typically have just a single 12V rail output, and they derive the 5V and 3.3V rails from it with switching / buck converters. But older PSUs, like the one in this thread (or modern cheaper ones) often use a "group regulated" design, where the main transformer outputs a 5V and a 12V rail (and sometimes, but not always, a 3.3V rail). In many cases, regardless if there is a 3.3V rail tap on the transformer or not, the 3.3V and 5V rails usually share turns on the same winding on the main transformer. For this reason, group regulated PSUs will usually specify a "3.3V + 5V combined power" rating. Here is a good example of that:
https://www.badcaps.net/forum/attach...1&d=1425959741

... which is the PSU found here:
https://www.badcaps.net/forum/showpo...postcount=2230
The Inno Power PSU in the above link has a 30 Amp rectifier for the 5V rail and a 30 Amp rectifier for the 3.3V rail as well. While the 5V rail is indeed specified for up to 30 Amps, the 3.3V rail is specified for only 14 Amps max. Why? Because the 3.3V rail uses a mag-amp circuit, so that's only how much it can do.

Furthermore, you see that 5V * 30A = 150 Watts and 3.3V * 14 Amps = 46.2 Watts. Thus if you add the 3.3V and 5V rail powers you get 196.2 Watts. The label, however, says you can get only 160 Watts max from the 3.3V and 5V combined, and that's because of that shared transformer winding tap for the two rails.

On the other hand, some newer group regulated PSUs have a "7V rail" tap on the main transformer instead of a 12V rail, and the 12V rail is derived by adding the 7V tap on top of the rectified (DC) 5V rail for a total of 12V. This means that any current pulled on the 12V rail must also go through the 5V rail rectifier. For PSUs like that, you may also see a 5V + 12V rail combined ratings.

And last but not least, the power/current numbers on the label are irrelevant if the rectifiers are not rated adequately to support what the label claims for each rail. Also, the primary side is another limiting factor to how much power the PSU can do.

For the Taks PSU you posted in this thread, it is a single-transistor forward converter topology (STF for short.) Generally, this topology/design is limited to about 350 Watts continuous. It may do 400W "peak", but certainly not for very long. To me, this PSU looks more what I would expect to see in a 250-300 Watt build. So the values on your label may be slightly fictional.

Quote:
Originally Posted by socketa View Post
Anyway, i was feeling confident about this being one of those PSU's that requires an external load to operate, and whatever it is about the input impedance sensitivity that you mentioned, so connected a hard drive, and used just the 5A fuse wire on the AC Live wire.

Turned it on and it went good.
Turned it off after a couple of minutes, being grateful that nothing went "pop" or "bang"
The voltages fluctuate plus or minus 0.03V

So it's working, finally!
This is why I suggested the heating element in place of the bulbs, as you would have likely seen the PSU work with that too. But since you "jumped the gun" and tried the PSU with only a fuse and it worked... no need for the series bulbs or heating element anymore.

Quote:
Originally Posted by socketa View Post
This is the first PSU that i've had that appears to require an external load to function
Unfortunately, it's not my first one.
There was another thread here, where a chap was troubleshooting his PSU and many people offered suggestions what to check/measure, myself included. Finally, it dawned on me (having read this somewhere else, again on badcaps a very very long time ago) that the PSU may be misbehaving due to low loading, I suggested that, the chap tried it, and his PSU worked.

So I always keep it in mind when testing PSU unloaded that they may not be working simply because they need a bigger load... which is a shame, because when using a series bulb / heating element, you can't have a large power draw from the PSU. That makes it hard or impossible to use the series bulb trick on such PSUs, unless you use something like a large heating element.

Quote:
Originally Posted by socketa View Post
Can you confirm that this PSU has no balancing resistors, because, unless i'm delusional, this PSU has no resistors attached to the middle terminals trace of the primary caps.
No, fortunately you're not delusional.
UNfortunately, this PSU does not seem to have the balancing resistors across the input caps. Usually, they are almost always found next to the primary caps or near the bridge rectifier. The picture you put in post #5 shows no space for such resistors, though:
https://www.badcaps.net/forum/attach...6&d=1586164479

... so that's poor design on the manufacturer's part.

Quote:
Originally Posted by socketa View Post
Would it be a good idea to add the resistors on the underside of the board?
Yes, you can add them to the underside of the board if you don't want to drill holes through the PCB to install them there. (Though generally installing through-hole components directly on a bottom of a PCB is considered "cheap" and often frowned upon... but if done well, it can be done safely without issues.) Just make sure to put a layer or two of heatshrink wrap around these resistors if you install them on the bottom/solder side of the PCB... or wrap with electrical tape. Don't leave them exposed, even if the PSU case has a plastic insulation sheet on the bottom.

Last edited by momaka; 10-05-2020 at 10:26 PM..
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Old 10-05-2020, 10:44 PM   #98
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
The Task and the Bestec seem to be significantly inefficient, compared to the Aywun - but maybe that is because the Aywun is gutless?
Yup.

Generally, the bigger / more powerful a PSU is, the more inefficient it becomes at very low loads. It's actually not uncommon even for very expensive and high-end PSUs to get terrible efficiency at very low loads (like for example, a PC that only draws 30-50 Watts, and the PSU being rated for 500 Watts or more.)

With the Aywun being a gutless wonder, it will get better efficiency at low loads. But try all of those PSUs with something like a PC that draws around 120-150 Watts. The Aywun efficiency will likely not be stellar... if not outright appalling (lower end of 70%, I'd guess.) Meanwhile, the Bestec and this PSU might do something like 75 to almost 80% with the same load, despite appearing to be more inefficient at low loads.

In general, PSU efficiency is a bit like a bell curve.
- At very low loads, efficiency is low
- At very high loads (usually near the PSU's power rating, or slightly above it, if it is build well), the efficiency will also drop sharply past a certain point (and for PSUs with questionable protections, this is a sign to STOP loading further, or the PSU will likely get damaged / blow something.)
- At approx. 1/3 to 2/3 of the power rating on the label, the PSU will have optimal efficiency... provided the PSU label is accurate, of course.

Quote:
Originally Posted by socketa View Post
Also, bearing in mind that the big pink 0.1 ohm resistor that i lost, has being replaced with one that is 0.2 ohms
That's fine.
This will just lower the power limit where over-power protection (OPP) kicks in on the primary side... which may not be a bad thing for this PSU, all things considered.
Just keep an eye on it after using it with a real PC (or similar) load for the first time. If it seems to have ran extremely hot and discolored the PCB, you may have to find a 0.1 Ohm resistor (or just another 0.2 Ohm resistor to put in parallel )... but this is very unlikely to be the case.

Anyways, congrats on getting it working!

Last edited by momaka; 10-05-2020 at 10:47 PM..
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Old 10-10-2020, 01:02 AM   #99
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Default Re: Task TK-940TX-DF Blowing fuse

Thanks for your clarity of input and assistance - so congrad's to you as well.
During the course of this, i've done a lot of thinking, and learned a lot, in more ways than one

Was the creation of the bulb trick the result of your own thinking/reasoning?

I'm not comfortable about ZD8 acting as protection for the 5VSB MOSFET though - should i remove and check to see if it's appropriate?
The zener voltage might be too high, and if the voltage spikes at the drain, the MOSFET could be damaged.
Any idea as to what voltage it should be, or how to calculate it?
I think that it's a similar setup as in the attachment that i've included - My PSU has the same 2Mohm resistor that is between the +ve DC and the zener

Any thoughts as to which primary capacitor balancing resistors to use, in respect to resistance values and wattage?
I've looked at quite few PSU schematics, and noted that values range from 100k to 330K, with 150k and 330k being the most common.


Do i have to replace the damaged NTC thermistor with one of the same value, or is there some wiggle room here?


Quote:
until the discharge/balance resistors across the caps discharge them
Since there is no balancing resistors in this PSU, but the PSU does discharge (you could measure and hear the point when it's discharged - it aligned with the squeak sound), how do you think that could be discharging?


Quote:
those orange "102" ceramic caps by the bridge rectifier (C43 and...?) Remove them permanently and don't put them back on. They aren't really needed
What was their function?


Quote:
Have removed all of the primary DC components and thoroughly cleaned the backside of the board - but the bulb still glows (no surprise there)
Quote:
That is not possible.
Yes it is, if the bridge rectifier is installed the wrong way around.


Would the PSU have a 5VSB critical cap, and, if so, what circuity configuration would i be looking for, in order to find it?
(maybe there's one in the attachment that i've included)

5VSB Mosfet rotection zener.png

Last edited by socketa; 10-10-2020 at 01:29 AM..
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Old 10-10-2020, 03:59 PM   #100
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Default Re: Task TK-940TX-DF Blowing fuse

Whoops! You already explained that the zener voltage must be close to, but less than, the maximum MOSFET gate voltage
And since the 5VSB voltage is not coming from any chip that needs to be protected, then drive voltage is not a concern, right?
I removed the zener, and it has "18" with "2" underneath that's printed it - so that means 18V 2W right? - So it seems to be more ideal to find another zener that's closer to the maximum MOSFET gate voltage of 30W.
Or, realistically, what's the likelihood of the MOSFET gate-source voltage going above 18V, since i measure only 1V across the zener?

P.S there is another 12V fan on the other half of the case and the PSU starts when that's plugged in - so no extra load is required for this PSU to start

Last edited by socketa; 10-10-2020 at 04:14 PM..
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