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Old 07-23-2020, 04:47 PM   #41
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Default Re: Task TK-940TX-DF Blowing fuse

I'm pretty sure that i've removed all the components that i just put back on.
Have included a photo, just in case you can spot any that i've missed, or should have removed
But the bulb is still glowing.

So i tested the AC and CD sides separately at the rectifier circuit board holes, and noticed that 1MΩ again that i previously couldn't find (so it wasn't due to my body resistance, although coincidentally that what it roughly measured while holding onto the test leads normally) it was hidden under the X capacitor. So the AC side looks normal, and i've put the capacitor and coil back on.
And there is no short on the DC side.
I thought that the 1MΩ might not be due to a resistor, because i didn't see it on a schematic that i was looking at (attached below) - but it obviously does (and shouldn't there be one of these in the schematic?)
What's the function of that resistor, as i'm picking that it's not a discharge resistor for the primary capacitors, as it looks like R1 and R2 in the schematic would do that job.

And as any short arising (which neither of my meters is showing) across the DC side, looking at the the schematic, could something be going on with those two transistors that is causing a short when the power is switched on? (assuming that my PSU is similar enough to that - hey, it's got two BJT transistors on the primary, and has an LM339, which looks like it's for the 5VSB)

Quote:
Did you try removing/unwinding/cutting the the isolating tape on the transformer? A lot of traffos hide the fuse in there. Usually it's a square box fuse, kind of looking like a small X2-class safety cap.
Unfortunately, no fuse in this one - thanks for the tip though.




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Last edited by socketa; 07-23-2020 at 05:11 PM..
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Old 07-25-2020, 12:45 PM   #42
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
I'm pretty sure that i've removed all the components that i just put back on.
Have included a photo, just in case you can spot any that i've missed, or should have removed
But the bulb is still glowing.
Hmm. So bulb is still glowing even with the new bridge rectifier removed?

Looking at the first picture, I see two more components that may be playing tricks on us: 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.

I also see that there is a lot of black sooth under where the fuse was and the area nearby. You might want to scrape/clean that with some IPA, as the sooth can be conductive.

And while looking there, I see that one of the green disc components next to the fuse holder (looks like two NTC thermistors) has a crack in it. Check and maybe also post part number here, just to confirm what it is.

Lastly, I see a small blue ceramic cap (not the large one) next to the big white resistor in the 5VSB circuit, is still installed. If I recall, that cap is part of the snubber on the 5VSB. Remove it as well.

Quote:
Originally Posted by socketa View Post
And there is no short on the DC side.
I thought that the 1MΩ might not be due to a resistor, because i didn't see it on a schematic that i was looking at (attached below) - but it obviously does (and shouldn't there be one of these in the schematic?)
What's the function of that resistor, as i'm picking that it's not a discharge resistor for the primary capacitors, as it looks like R1 and R2 in the schematic would do that job.
The 1 MOhm resistor is there to discharge the X2 cap(s) on the AC side. Otherwise, if you disconnect the PSU from the wall and it so happens that you do this while the AC line voltage is at its highest peak, the X2 caps will be fully charged... or at least until the big electrolytic caps on the DC side start discharging, after which point, the small charge stored in the X2 caps will flow through the bridge rectifier and discharge the X2 cap. But if the above happens, there could be a period of possibly several second where if you touch the metal prongs on the PSU plug after unplugging it, you might get a slight shock. Also, the 1 MOhm resistor is there to provide a current path in case the AC is suddenly disconnected and there is still a current flowing through the common-mode chokes. Inductors don't like sudden interruptions in current, so they always need to be provided a safe path in case of sudden disconnection. The 1 MOhm resistor provides a function for that as well.

Last edited by momaka; 07-25-2020 at 12:48 PM..
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Old 07-26-2020, 01:54 AM   #43
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Hmm. So bulb is still glowing even with the new bridge rectifier removed?
No.
Sorry, should have mentioned that.

So i've removed those other components and got rid of the sooth (i thought that that may have being burn marks into the board from so much soldering/heat, because it wouldn't come off with isopropyl alcohol and a cotton bud - so as you suggested, i went over the board scratching it off with the test probes (perfect for the job!) - it came off like crusty flakes.
But the 60W bulb still comes on as seen in the photo below:
Well done on noticing the cracked NTC thermistor - it's part number is sck054.

P.S. i found an app that looks like it sorts out the Firefox RAM issue - it's called FireMin "-It calls the Windows function EmptyWorkingSet over and over again in a loop to free up memory. Calling the function removes as many pages as possible from the working set of the specified process. "
It also make a DNS request, which i block with the firewall.
So far, so good - have noticed a significant improvement
There is also CleanMem Pro, and Memory Fox Next plugin
For me, Firefox was stalling and crashing as soon at RAM got to around 2GB (i have 3.25GB out of 4B available, because of Windows 7 32bit RAM limitation)

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Old 07-26-2020, 07:44 AM   #44
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
No.
Sorry, should have mentioned that.

So i've removed those other components and got rid of the sooth
...
But the 60W bulb still comes on as seen in the photo below:


Let's keep knocking.
Try also removing the big white resistor.
If bulb is still glowing, remove bridge rectifier and test out of circuit or swap into another working PSU (that you don't mind testing on... or non-working PSU would be fine too, as long as it isn't one that blows fuses.) IDK how it would be possible for this board to burn bridge rectifiers, but there are just no other current paths.

If bulb still glows after the above, check voltage across each 200V cap while PSU is plugged in (be careful there!) and report back what you get.

Also do you have a 18-20V laptop adapter?

Quote:
Originally Posted by socketa View Post
P.S. i found an app that looks like it sorts out the Firefox RAM issue - it's called FireMin "-It calls the Windows function EmptyWorkingSet over and over again in a loop to free up memory. Calling the function removes as many pages as possible from the working set of the specified process. "
It also make a DNS request, which i block with the firewall.
So far, so good - have noticed a significant improvement
Cool, thanks for sharing the info.
Currently running with FF ESR on my XP 32-bit machine with 2 GB of RAM, so haven't had issues running out of RAM anymore. I still use Opera 12.18 for the majority of browsing that allows it, though.

And, oh yeah, I always disable DNS Service from Windows Services. Otherwise, my custom HOSTS file doesn't really work. I do this both for Win XP and 7.

There's a bunch more other stuff that I disable. I always forget the specific ones until I see them, but Automatic Updates is probably the first one on that list.

Quote:
Originally Posted by socketa View Post
For me, Firefox was stalling and crashing as soon at RAM got to around 2GB (i have 3.25GB out of 4B available, because of Windows 7 32bit RAM limitation)
Interesting.
What version of FF? And is it ESR or Quantum.
I haven't had issues with Quantum even with 2 GB of RAM. Seems to run better on dual core (or better) CPUs.
I only use ESR for Win XP, because Quantum just won't work there.
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Old 07-26-2020, 10:26 PM   #45
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Default Re: Task TK-940TX-DF Blowing fuse

Took off the white resistor - Bulb still continiously Glowed.
Removed the rectifier and put it in a good PSU, and it goes well - bulb briefly lights
then dims out in about a second, in line with the primary caps charging up.
And the fan starts up when i jump-start it
So it really looks like the rectifier is not the problem

Quote:
Also do you have a 18-20V laptop adapter
No, but I do have a 16V 4.5A laptop adapter though.

Quote:
What version of FF? And is it ESR or Quantum.
FF 48.0.2 - the last non-ESR version for socket A processors. (although i'm currently using Windows 7)
But it (as well as Basilisk, Palemoon, and even k-Meleon 76.2) returns no response when clicking on Reddit's "View entire discussion" button - for example, on this page:
https://www.reddit.com/r/Amd/comment..._cool_n_quiet/
Does it work for you, with your Opera 12.18?

Some time ago i did trial Quantum, (can't recall it ever crashing or struggling), but i couldn't completely deactivate all of it's "phoning home" attempts.

Last edited by socketa; 07-26-2020 at 10:31 PM..
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Old 08-05-2020, 12:39 PM   #46
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Default Re: Task TK-940TX-DF Blowing fuse

So did you try measuring the voltage across the two big caps when the PSU is plugged in and the bulb glowing?

Quote:
Originally Posted by socketa View Post
No, but I do have a 16V 4.5A laptop adapter though.
Yes, that will be fine. Let's see what results you get above for the voltage across each cap, though. Then we will see where to go from there.

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Originally Posted by socketa View Post
FF 48.0.2 - the last non-ESR version for socket A processors. (although i'm currently using Windows 7)
But it (as well as Basilisk, Palemoon, and even k-Meleon 76.2) returns no response when clicking on Reddit's "View entire discussion" button - for example, on this page:
https://www.reddit.com/r/Amd/comment..._cool_n_quiet/
Does it work for you, with your Opera 12.18?
No, it didn't work for me in Opera 12.18
In general, Opera 12.18 tends to not work well with new websites. But for older/simpler websites like BCN, it is a lot more efficient (can open 20-40 tabs, and RAM usage is still very small, not to mention pages are very responsive.) So I just use it alongside FF.

I tried the above link with Firefox 52.4 ESR Portable on Windows XP, and had not problems clicking on the "View Entire Discussion" button.

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Originally Posted by socketa View Post
Some time ago i did trial Quantum, (can't recall it ever crashing or struggling), but i couldn't completely deactivate all of it's "phoning home" attempts.
Anything after FF24 is questionable to me.
For example, all of the ESR and non-ESR versions of FF after 40 have been trashing my CPU usage in random intervals. Not sure what's going on "under the hood" on those browsers, so I just simply don't trust them.

On that note, I use Quantum on PCs that don't have any personal info. I couldn't care less if the PC is tracked or even hacked. I keep an image for those and can just wipe and copy over if needed. So far, I haven't needed to do that.

Also, I built a test PC last week to test an old socket 754 board. I put an Athlon 64 3400+ and 2 GB of DDR RAM. The 3400+ (2.4 GHz version, 512 KB L2) is in the top 5 CPUs for s754 platform (if not top 3, only slower than the 3700+ and 4000+ mobile.) And while I was able to view 480p content on YouTube without stutter (with FF 52.4 ESR Portable) once the video loaded, it wasn't a smooth experience. For example, minimizing and maximizing windows (especially FF) while I had a music video playing in YT would often produce audio stutter or pause. This doesn't happen on my P4 HT CPUs. So with that said, I think single-core CPUs really are just not a viable option for online browsing anymore. Even a slow dual core CPU will do many times better. Of course, this is where the old FF architecture falls short, as it can't really take full advantage of multi-core CPUs. And that's another reasons I just had to switch to Quantum on my newer PCs. I don't like it... but there aren't that many alternatives really. Chrome? -HA, not a snowflake's chance in hell!

Last edited by momaka; 08-05-2020 at 12:46 PM..
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Old 08-06-2020, 01:37 AM   #47
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Default Re: Task TK-940TX-DF Blowing fuse

Just in time - i was getting close to removing the two transistors
Quote:
So did you try measuring the voltage across the two big caps when the PSU is plugged in and the bulb glowing?
No - but i have just measured it now.
Across the +ve of one cap and the -ve of the other cap, as soon as the switch is flicked there 55-60V DC and then it quickly goes to 40V, and over the course of about a minute it drops in (roughly) exponential decay towards 21V (by then it's falling at a slow rate of 0.01V/s)
Just across the outer cap there is an initial voltage of 19V that instantly drops to 13V and then, in the same fashion as above, falls to 12.6V after about a minute
And ditto across the inner cap 30V > 22V > 10.4V

Quote:
Anything after FF24 is questionable to me.
Sounds good to me - i was reluctant to go to FF30, and then only used nightly

Thanks for the info on the 754.

Quote:
all of the ESR and non-ESR versions of FF after 40 have been trashing my CPU usage in random intervals
That doesn't happen to me with FF48.0.2 Windows7 Ultimate 32bit Intel Core2 Duo CPU E7500 @ 2.93GHz - i keep an eye on it with SysTrayMeter in the taskbar.

Chrome? - wouldn't surprise me if that's what will be used when TPTB lure gullible folks and technophiles into being AI'ed

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Old 08-07-2020, 08:19 PM   #48
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Just in time - i was getting close to removing the two transistors
Which two transistors? I thought all the ones on the primary side are gone.

Quote:
Originally Posted by socketa View Post
No - but i have just measured it now.
Across the +ve of one cap and the -ve of the other cap, as soon as the switch is flicked there 55-60V DC and then it quickly goes to 40V, and over the course of about a minute it drops in (roughly) exponential decay towards 21V (by then it's falling at a slow rate of 0.01V/s)
Just across the outer cap there is an initial voltage of 19V that instantly drops to 13V and then, in the same fashion as above, falls to 12.6V after about a minute
And ditto across the inner cap 30V > 22V > 10.4V
That's a very strange result.

It looks almost as if there is some active device somewhere pulling the voltage down to that level. Still, I assume this is with all primary transistors removed, so this shouldn't be happening.

Is your bridge rectifier installed correctly? - i.e. + and - polarity going properly to the big caps? With no primary transistors and no 5VSB transformer, there should be no current path to conduct across the two big caps. Keep removing components until you get somewhere. Clearly there must be something that we're missing that I can't seem to see in the pictures.

Once you get a result after removing/re-installing something, repeat once or twice more to confirm which component or part is causing the change in symptoms. In other words, we need repeatability. If you remove a component and the circuit stops (or starts) doing something new, then re-inserting same component should restore back old behavior. If not, check everything that you're doing to make sure you're not introducing an error somehow.

Quote:
Originally Posted by socketa View Post
That doesn't happen to me with FF48.0.2 Windows7 Ultimate 32bit Intel Core2 Duo CPU E7500 @ 2.93GHz - i keep an eye on it with SysTrayMeter in the taskbar.
Well, that E7500 is quite powerful CPU (relatively speaking, compared to P4's and single core Athlon XP/64 chips), so indeed won't see it hog CPU resources (it would probably be down in the 10-20% CPU usage when it happens.)

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Old 08-07-2020, 11:49 PM   #49
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Which two transistors? I thought all the ones on the primary side are gone.
There is two small BJT transistors (but ignore that, because i figured out that i have no idea what they are for)
Quote:
Is your bridge rectifier installed correctly? - i.e. + and - polarity going properly to the big caps?
Yes.
There is obviously a short, or low resistance happening somewhere, so why can't i find the it using the multimeter? - that would make this a lot easier.
I get 1Mohm ohm across the AC legs of the installed rectifier (that 1Mohm resistor) which makes sense, and across the DC legs i initially get 30Mohms that falls to 9Mohms over the course of a few minutes and keeps falling. Does that sound normal? (shouldn't there initially be low resistance and then it be rising as the primarys slowly charge up?)
What's the odds that the replacement primary caps are bad (or were destroyed after fitting them and turning it on), as well as the original ones?
I just removed one of the primary caps, and the bulb didn't glow.
Then put it back in, and then the bulb glows.
Swapped it for one of the original ones and the bulb glows
Swapped the other one with the original (so that both of the original ones were back in), and the bulb glows.
I measured the original caps with the ESR meter and got readings of 0.037 and 0.041 ohms

Last edited by socketa; 08-08-2020 at 12:46 AM..
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Old 08-17-2020, 10:34 PM   #50
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
There is two small BJT transistors (but ignore that, because i figured out that i have no idea what they are for)
Well, if they are on the primary, then I guess it doesn't hurt to remove them too, just in case (though I don't see how they can make the bulb glow when they are not connected in any way that will allow so much current to pass through them.)

Quote:
Originally Posted by socketa View Post
Yes.
There is obviously a short, or low resistance happening somewhere, so why can't i find the it using the multimeter? - that would make this a lot easier.
Multimeter only measures DC resistance and based on a low-voltage test.
Parts can fail such that they only short-circuit or pass current with normal circuit voltage.
Active circuits will also only pass current with normal circuit voltage (i.e. diodes, transistors, etc.)
So that's why the issue is not showing itself.

Quote:
Originally Posted by socketa View Post
and across the DC legs i initially get 30Mohms that falls to 9Mohms over the course of a few minutes and keeps falling. Does that sound normal? (shouldn't there initially be low resistance and then it be rising as the primarys slowly charge up?)
Capacitance on the primary caps is rather small. Likely they charge so quickly that the "low resistance" is possibly not captured by the multimeter. I find this to be the norm with slower multimeters and capacitors of 1000 uF or less.

Of course, to verify that something else is not interfering with this measurement, then it's best to repeat the same experiment with the caps out of the circuit.

Quote:
Originally Posted by socketa View Post
What's the odds that the replacement primary caps are bad (or were destroyed after fitting them and turning it on), as well as the original ones?
Unlikely.
You'd need reverse voltage to kill them. You can take either the old or the new caps and put them back into another test PSU, then see if they work there. If they work and your cap meter is also reading the capacitance and ESR correctly, then the caps are fine.

Quote:
Originally Posted by socketa View Post
I just removed one of the primary caps, and the bulb didn't glow.
Then put it back in, and then the bulb glows.
Swapped it for one of the original ones and the bulb glows
Swapped the other one with the original (so that both of the original ones were back in), and the bulb glows.
I measured the original caps with the ESR meter and got readings of 0.037 and 0.041 ohms
That further suggests there is some part of the circuit after the caps that is the issue.

If you have the tools, remove the main PS (big) transformer.
After this, test with bulb, and continue removing parts from the primary until it stops glowing and you're left with nothing but the primary caps, bridge rectifier, and balancing resistors between the two caps.

Do it one part at a time so you know which part made the bulb stop glowing. First start with any components that are directly connected to the + or - of the primary caps (again, with the exception of the bridge rectifier and balancing/bleeder resistors.)
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Old 08-19-2020, 07:02 PM   #51
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Default Re: Task TK-940TX-DF Blowing fuse

Ok, thanks for the explanation.
Have removed the transformer, and the bulb still glows.
Will continue on as you described, and will post back when it bulb ceases to glow.
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Old 08-20-2020, 05:27 PM   #52
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Default Re: Task TK-940TX-DF Blowing fuse

Well i removed all of the components, two at a time, that were connected to the DC legs of the rectifier, and the bulb still glowed, so then i removed the green NTC thermistors that were noted earlier, and the bulb didn't glow.
The one that is cracked, (and a chunk of it fell off of it when removing it) measured 42 ohms and the other one 9 ohms - according to the datasheet it seems that they should be in the Mohms range, so they were shorting out the AC input - that's why i couldn't find the short, because i was testing further into the circuit.
Maybe you meant "test" when you said "check"?
The board is a mess now, with traces lifted in several spots, courtesy of my 'practising' on it, and have lost that big resistor, so there is no way that i would risk using it now.
The initial reason why i wanted this PSU is no longer valid because i thought that i had to use a particular board with more than 2GB Ram for the internet; but the issue was with firefox (the current computer that i'm using has 3 GB of RAM and if FF pulls up to around 2GB it starts locking up the page with a circular throbber on top, and sometimes it just crashes - i'm going to get around to trying that version of Opera that you suggested)

What sort of design is this PSU, and would you rate it on par with one of those ISO or Hyena power supplies, as opposed to a Liteon or Hipro?
Is a 5VSB IC controller preferable to what this PSU has?
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Old 08-22-2020, 05:52 PM   #53
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Well i removed all of the components, two at a time, that were connected to the DC legs of the rectifier, and the bulb still glowed, so then i removed the green NTC thermistors that were noted earlier, and the bulb didn't glow.
The NTC thermistors are on the AC side, so when you remove them, AC no longer gets to the bridge rectifier... hence why the bulb stopped glowing.

Need to remove components on the DC side of the bridge rectifier to find out what causes this. If nothing, at least in the end you'll end up with a bag full of components for re-use.

Quote:
Originally Posted by socketa View Post
The one that is cracked, (and a chunk of it fell off of it when removing it) measured 42 ohms and the other one 9 ohms - according to the datasheet it seems that they should be in the Mohms range, so they were shorting out the AC input - that's why i couldn't find the short, because i was testing further into the circuit.
The cracked one is reading high-resistance and is consistent with how one would fail. After all, these are just temperature-dependent resistors whose resistance goes down with an increase in temperature. So like other resistors, their failure mode is high-resistance or open-circuit.

The one that is measuring 9 Ohms might be OK, depending on the room temperature. SCK 054 should be about 5 Ohms at 25C ambient and higher resistance with colder temperature (or lower resistance with higher temperature.) Your multimeter may also add an Ohm or two...

So yeah, these aren't the issue of your PSU and they are not shorting out the primary - not unless you solder them accidentally in a spot meant for a Y2-class cap.

Quote:
Originally Posted by socketa View Post
Maybe you meant "test" when you said "check"?
Sorry, yeah I use those two interchangeably sometimes.

Quote:
Originally Posted by socketa View Post
The board is a mess now, with traces lifted in several spots, courtesy of my 'practising' on it, and have lost that big resistor, so there is no way that i would risk using it now.
Can't be as bad as this, can it?
https://www.badcaps.net/forum/showpo...&postcount=206

Quote:
Originally Posted by socketa View Post
What sort of design is this PSU, and would you rate it on par with one of those ISO or Hyena power supplies, as opposed to a Liteon or Hipro?
I'd say this PSU is closer to the LiteOn and HiPro PSUs, since it uses a single-transistor forward design for the main PS, which is a little newer than the cheapo PSUs that use half-bridge (like the ISO and Hyena, IIRC.)

Quote:
Originally Posted by socketa View Post
Is a 5VSB IC controller preferable to what this PSU has?
Yes, since it offers more protections and generally won't burn out when overloaded. 2-transistors 5VSB designs don't take it kind to bad caps or over-loading.
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Old 08-22-2020, 07:40 PM   #54
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Default Re: Task TK-940TX-DF Blowing fuse

Ok, my initial intuition was correct, in respect to you not mentioning those NTC thermistors again.
i didn't read the datasheet carefully; as i should have being looking under the zero power resistance column - thanks for the explanation - i get the gist of these now.
Will continue on, over the next day or so, as i'd like to find the culprit.

Quote:
Can't be as bad as this, can it?
Thanks for the encouragement
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Old 08-29-2020, 05:48 PM   #55
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Default Re: Task TK-940TX-DF Blowing fuse

Have removed all of the primary DC components and thoroughly cleaned the backside of the board - but the bulb still glows (no surprise there)


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Old 08-29-2020, 08:00 PM   #56
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Default Re: Task TK-940TX-DF Blowing fuse

That is not possible.

How are you wiring the incandescent light bulb? Across where the fuse goes? Also, are you sure it's an incandescent light bulb and NOT LED or flourescent? Bridge rectifier still OK (showing two diode voltage drops when red multimeter probe is on [-] leg and black probe on [+] leg, with power disconnected from PSU of course.)

Looking at this picture...
https://www.badcaps.net/forum/attach...9&d=1595749921
... how about bridge the fuse and wire the bulb between where the brown wire and red wires attach instead (that is, separate connection between brown and red wire, then put incandescent bulb across there.)

All I can think of is that there is a mistake in the wiring of the bulb if bridge rectifier is OK. May also remove the two blue Y2 caps as well, but I doubt that will do the trick.

This is a single layer PCB and I don't see any solder bridges across traces, so it really isn't possible for the bulb to be glowing still.

Also, you said you removed everything from the primary side, but keep the high-resistance balancing resistors across the two electrolytic caps still in the circuit.

PCB doesn't look that bad at all, by the way. Still serviceable.

Last edited by momaka; 08-29-2020 at 08:07 PM..
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Old 08-30-2020, 03:18 AM   #57
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Default Re: Task TK-940TX-DF Blowing fuse

As you can see, in post #43, the bulb is wired across the fuse holder
Yes, the bulb is an old type incandescent filament type, that heats up
I reinstalled R9, the 1.5Mohm resistor, that was across the -ve of one primary cap and the +ve of the other (AKA, the outside terminals).
Looking at other PSU schematics, it seems that the usual balancing resistors are something along the lines of two 330K resistors, one across each capacitor - but not looking like that in this PSU - so could it be that there is only one balancing resistor?
The bulb still glows after bridging the fuse holder and connecting it between the red and brown live wires.
The rectifier tests as you say it should with a double diode reading one way and nothing the other way.
The two blue Y2 caps are not shorted (not even high resistance on the Mohms range) so is there any point in removing them?

I checked the fuse holder that i installed, to make sure that i didn't do something silly - but that's on correctly, and not in contact with any other component
Also replaced the primary caps, with correct polarity orientation, with the other ones that i previously had in there; and still the bulb glows. (and managed to lift another trace while doing so)

Last edited by socketa; 08-30-2020 at 04:01 AM..
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Old 08-30-2020, 01:28 PM   #58
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
I reinstalled R9, the 1.5Mohm resistor, that was across the -ve of one primary cap and the +ve of the other (AKA, the outside terminals).
Looking at other PSU schematics, it seems that the usual balancing resistors are something along the lines of two 330K resistors, one across each capacitor - but not looking like that in this PSU - so could it be that there is only one balancing resistor?
No, there should always be two balancing resistors of equal value. 300-500 KOhms will do (but again, both resistors have to have equal rated values.)

That alone can't cause the bulb to glow, though.

Quote:
Originally Posted by socketa View Post
The two blue Y2 caps are not shorted (not even high resistance on the Mohms range) so is there any point in removing them?
I suppose not, because without the bridge rectifier, you already confirmed the bulb is not glowing.

Quote:
Originally Posted by socketa View Post
The bulb still glows after bridging the fuse holder and connecting it between the red and brown live wires.
The rectifier tests as you say it should with a double diode reading one way and nothing the other way.
...
I checked the fuse holder that i installed, to make sure that i didn't do something silly - but that's on correctly, and not in contact with any other component
Also replaced the primary caps, with correct polarity orientation, with the other ones that i previously had in there; and still the bulb glows. (and managed to lift another trace while doing so)
Welp, I don't know what else there is to do at this point. I just can't explain what's eating up the power on that board. Maybe Nikola Tesla was right, and there is a way to transfer power wireless-ly.

On a more serious note, do you have something with a powerful heating element in your house? Perhaps a kettle or hair dryer or a loose heating element from a stove/oven? If yes, get the one with the highest power you can find (1 KilloWatts or more would do nicely), but also not something that is too high and would overload the breaker/plug/wires you're testing the PSU with. Then connect that heating element / device in place of the incandescent bulb, power it up, and let it rip. HINT HINT HINT: you may want to do this outside. Most likely you will end up blowing up the bridge rectifier and big caps if there is still a short-circuit on the PSU board. So if you have a set of crappy no-name primary caps from a junk PSU, you may want to put those in the PSU instead. Also put a bridge across the NTC thermistors if you'd like to save them (and to not limit current in any way to the heating element.) Extra credit: put multimeter to measure voltage across the primary caps while doing this experiment. If whatever is causing the short on the primary burns out before the bridge rectifier and the rest of the wires, then you will see a healthy ~150-170V DC across each cap (or 330-340V between [+] and [-] on the bridge rectifier.) The reason to use a big heating element is so that much more power can be passed through the primary side. So in the worst case, the heating element will fully power on, and the breaker in your house won't trip and you won't end up with melted plugs anywhere (except perhaps for melted traces on the PSU board.)

Last edited by momaka; 08-30-2020 at 01:34 PM..
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Old 08-30-2020, 08:11 PM   #59
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Default Re: Task TK-940TX-DF Blowing fuse

Could it be possible that that 1M ohm resistor, that's between the AC input rails, is faulty, and only shorts out when high voltage is applied?
Would it be OK to remove it and test again with the bulb?

Yeah i have a spare stove element that i can use.
How long should i "let it rip" for? Until the caps/rectifier explode or one of those voltages shows up on the meter?

It really doesn't look like this PSU has those two balancing resistors that you speak of - the only components that are connected to the fat trace where the primary caps connect together, are two MOVs. Apart from that there is no tee off.
You can clearly-enough see this in the last photo that i posted - (moving from right to left) there is the two holes where the two cap legs have gone through, there is a blob of solder, an empty hole that was originally empty, and then two holes for the MOVs. That's it. Not even any jumper! Can't even see two same value resistors on the photos of the board, that have a yellow multiplier.

Last edited by socketa; 08-30-2020 at 08:21 PM..
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Old 08-31-2020, 01:11 PM   #60
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Default Re: Task TK-940TX-DF Blowing fuse

Quote:
Originally Posted by socketa View Post
Could it be possible that that 1M ohm resistor, that's between the AC input rails, is faulty, and only shorts out when high voltage is applied?
No, resistor failure mode is always open-circuit or higher resistance.
Yes, you can remove it and do the bulb test again, but it shouldn't change anything.

Quote:
Originally Posted by socketa View Post
Yeah i have a spare stove element that i can use.
How long should i "let it rip" for? Until the caps/rectifier explode or one of those voltages shows up on the meter?
Maybe a minute or two or 5 tops.
Something should pop/melt at that point if it's faulty / weak conductive path. If you do have a hard short-circuit on the primary of the PSU, though, the heating element will just run at full power (so beware how hot it can get.)

I usually do this with a 1200 Watt toaster oven, since it's all closed up and don't have to worry about heat from the heating element melting anything.

Quote:
Originally Posted by socketa View Post
It really doesn't look like this PSU has those two balancing resistors that you speak of
That's poor design.
With two caps in series like this, balancing resistors of equal value should always be installed. Otherwise, one cap can take on a larger voltage than the other (due to differences in internal impedance and also capacitance), which can result in a cap failure over time.

Maybe I gave this PSU more credit initially than it deserved. If the manufacturer didn't get this right with the balancing resistors, who knows what else they got wrong / goofed up.

Last edited by momaka; 08-31-2020 at 01:12 PM..
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