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Old 04-28-2021, 04:02 AM   #41
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Default Re: Compaq PDP-121P - Baked MOV

I just missed out on editing my previous post. So ignore that one, and use this one instead; as i had further thoughts, connected more dots, and corrected and tidied it up.

Quote:
Getting something like a 12V car light bulb or 12V halogen light bulb (anywhere from 10 to 35 Watts will do) and put that on the 5V rail only.
Seems like this PSU might not not 5V heavy (as previously thought)?
It's rating are:

+3.3V _____ 15A
+5V_____ 11A
+12V _____ 5A
+12.8V_____ 7.5A
-12V_____ 0.15A
5.05V Aux_____ 3A

Hence that's why adding the car bulb didn't have any effect, because i'm fairly sure that the problem is with the 12V rail (see below).

It was after that, that i plugged the 20pin ATX connector into the test motherboard, just to see what would happen.
One useful thing that came out of that, was seeing correct voltage on the 12V rail

So, in retrospect, it's no surprise that 23W bulb (and only the 23W bulb), had no effect on starting the PSU when connected to the 5V rail

I just realized, and figured out, that [email protected] is the same as [email protected], or 2.5A - so that 23W 12V bulb, when placed on the 5V rail, should have worked if it required more loading for the PSU to start;
but, as i've discovered, the problem appears to be with the 12V rail, not the 5V rail
I didn't think to try it on the 12V rail at that time

After getting no joy when adding a wide range of resistors to the 5V rail, the PSU now starts if i attach a 3W 74ohm load resistor to the 12V rail to draw close to 2W, and i arrived at that value by trying decreasing resistances until the PSU started and the voltage hit it's highest value, which is 10.64V
The other rails are well with spec and stable
When i plugged the PSU ATX into the test motherboard, i got 11.8V out of it; so i know that it can do it, so it's odd that the max voltage that i can draw out of the 12V rail is 10.64V.

I removed the 25V 470uf Taicon PW cap that was on the 12V rail
It measures , according to the 328 transistor tester, 454uf and 0.42 ESR (or 0.83 ESR using a dedicated ESR tester)
The datasheet says that it should have an impedance of 0.273, so if "impedance" = "ESR" then it's out of spec.
And when i compared the 12V rail with a good PSU 12V rail, on an old oscilloscope (probably not doing it right though), without changing the previous settings, i could notice significant ripple compared to the other PSU.
It's still a mystery to me as to why i can get 11.8V out of it when plugged into that test motherboard ATX, but not when loading the 12V rail with any resistor. Obviously something's happening within the motherboard that's causing the 12V rail to come into spec, that i can't yet replicate.
Even though the other rails are good, might it be that they need extra loads to push up the 12V? - which is a bit different (voltage is too low) from the scenario that you mentioned:
Quote:
If 12V or 3V3 are too high, you may have to put a small load on those as well
Anyway, before removing the cap, i plugged it into the Compaq computer with some 5A fuse wire and got an LED code of over voltage condition, and it wouldn't start
That was no surprise

While removing the cap, i noticed that this has a +12.8 rail as well, for the 4pin CPU plug.

This PSU had an 8A fuse in it. P=IV = 1840W. That seems to be a bit high for a PSU that's rated at 220W.
I have a 5W ceramic fuse, so i think that there would be no problem replacing the original fuse with that. Correct?

Last edited by socketa; 04-28-2021 at 04:25 AM..
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Old 04-29-2021, 05:51 PM   #42
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
I removed the 25V 470uf Taicon PW cap that was on the 12V rail
It measures , according to the 328 transistor tester, 454uf and 0.42 ESR (or 0.83 ESR using a dedicated ESR tester)
The datasheet says that it should have an impedance of 0.273, so if "impedance" = "ESR" then it's out of spec.
Actually, the datasheet says 0.065 (looked at the wrong column), and i meant 0.083 instead of 0.83
and it now measures , according to the 328 transistor tester, 454uf and 0.2 ESR (or 0.080 ESR using a MESR-100 ESR tester)
(Possibly due to a change in temperature)
I trust the MESR ESR meter more, as it lines up better with the datasheets and other newer capacitors that i brought

So i replaced it with an OST RLP 1000uf (actually 860uf) that has ESR 0.027
But now the PSU wont start unless i lower the 12V load resistance (i used 47ohms instead of 83 ohms)
And when it starts, the 12V rail voltage is still too low, as before

Last edited by socketa; 04-29-2021 at 05:56 PM..
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Old 04-29-2021, 11:13 PM   #43
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
[Edit]
i just realized, and figured out, that [email protected] is the same as [email protected], or 2.5A
Close...
Dropping the voltage in half will drop the power by 4x. So a 40W bulb running on half of its rated voltage will draw only about 10 Watts.

Quote:
Originally Posted by socketa View Post
- so that 23W 12V bulb, when placed on the 5V rail, should have worked if it required more loading for the PSU to start;
but, as i've discovered, the problem appears to be with the 12V rail, not the 5V rail
I didn't think to try it on the 12V rail at that time
Well, that's an interesting result then.

Generally, when a group-regulated PSU (like this one) has a low-voltage reading on one rail (in this case the 12V rail), then loading the other rails (5V and 3.3V) should work to raise the voltage on the rail that was reading low. Hence why I suggested you put a load on the 5V rail only.

Quote:
Originally Posted by socketa View Post
When i plugged the PSU ATX into the test motherboard, i got 11.8V out of it; so i know that it can do it, so it's odd that the max voltage that i can draw out of the 12V rail is 10.64V.
...
It's still a mystery to me as to why i can get 11.8V out of it when plugged into a motherboard ATX, but not when loading the 12V rail with any resistor. Obvioulsy something's happing within the motherboard that's causing the 12R to come into spec, that i can't replicate.
If that's the test motherboard where you had the 4-pin CPU connector unplugged, then you probably got a higher voltage on the 12V rail out of it, because the chipset, RAM, and standby circuits on the motherboard are all powered from the 3.3V and 5V rails... thus drawing more power from those rails only. The 3.3V/5V-"heavy" cross-regulation is likely what's bringing the 12V rail up higher.

Quote:
Originally Posted by socketa View Post
I removed the 25V 470uf Taicon PW cap that was on the 12V rail
It measures , according to the 328 transistor tester, 454uf and 0.42 ESR (or 0.83 ESR using a dedicated ESR tester)
The datasheet says that it should have an impedance of 0.273, so if "impedance" = "ESR" then it's out of spec.
Yes, impedance spec @ 100 KHz is equivalent to ESR.
Cap may not necessarily be out of spec, though - you also need to consider the ambient temperature too.

And isn't there something else filtering the 12V rail too? 470 uF seems way too low. I think you might be looking at the -12V rail cap. 12V rail should have at least 1000 uF (but almost always more for PSUs that have a 4-pin 12V CPU connector.)

Quote:
Originally Posted by socketa View Post
And when i compared the 12V rail with a good PSU 12V rail, on an old oscilloscope (probably not doing it right though), without changing the previous settings, i could notice significant ripple compared to the other PSU.
Did you have the proper bypass caps on your probe when you did that test? (IIRC, it's in Intel's ATX spec sheet, in the section about power supplies and PSU voltage/ripple specs.)
And were both PSUs' covers on and secured properly? A PSU with the cover off may induce a lot of noise on the scope, so that's something to always watch out for.

Quote:
Originally Posted by socketa View Post
Even though the other rails are good, might it be that they need extra loads to push up the 12V?
That's what I'm thinking and that's generally how group-regulated PSUs work too.

Quote:
Originally Posted by socketa View Post
This PSU had an 8A fuse in it. P=IV = 1840W. That seems to be a bit high for a PSU that's rated at 220W.
Indeed it is.
Fuse is original?
Normally, a PSU like that should only come with a 5 to 6.35 Amp fuse.

Since you live in a country with 220/230/240V AC, even a 1.5 Amp fuse would technically work for you, as 1.5A x 230V = 345 Watts. Of course, you do have to factor in PSU inefficiencies. So let's say the PSU is doing 70% eff. when loaded to the max at 200 Watts. Then that means it will be drawing about 290 Watts from the wall... hence the need for slight over-rating (in addition to also have a bit of over-head so that it doesn't blow randomly from surge current from the mains caps.) But when the PSU is moved over to a country with 115V, the current will be twice as high and the PSU will be slightly less efficient too. In such a case, a 3 Amp fuse would be the minimum.

As these PSUs are manufactured for both 115V and 230V AC operation in the same factories, the manufacturer generally just installs the fuse that will work in both occasions - hence going with the bigger fuse.

That said, I agree with you that I too don't see the reason why they went with an 8 Amp fuse. 5-6 Amp would be more than reasonably adequate with enough headroom for the PSU's inefficiency. So probably worthwhile to downgrade that fuse, IMO.

Quote:
Originally Posted by socketa View Post
I have a 5W ceramic fuse, so i think that there would be no problem replacing the original fuse with that. Correct?
You mean 5 Amp fuse?
If yes, correct.

Quote:
Originally Posted by socketa View Post
In case anyone's interested, this PSU's max power ratings are
+3.3V _____ 15A
+5V_____ 11A
+12V _____ 5A
+12.8V_____ 7.5A
-12V_____ 0.15A
5.05V Aux_____ 3A
Should have put that in the beginning so we wouldn't have to guess if it's an older 5V-heavy design or not.

And from that label, I think I know now what's happening here...

That "12.8V" rail is actually the 12V rail that is produced by the PSU and is connected to the 4-pin 12V CPU connector. Meanwhile, the "regular" 12V rail is just a regulated rail off of the 12.8V rail... so the lone 470 uF cap you saw on the 12V rail may be normal. And the low voltage may or may not be normal. Better to just check the voltage on the 12.8V rail. If that one checks out, then we need to worry about why the "regular" 12V rail is reading low.

Quote:
Originally Posted by socketa View Post
I trust the MESR ESR meter more, as it lines up better with the datasheets and other newer capacitors that i brought
Yeah, the MESR meter should be better.

My GM328 tester consistently reads much higher ESR on caps when doing the testing between pins 1 and 3. So that's why I always test my caps between test pins 1 and 2... which also isn't 100% accurate and seems to under-report ESR. Probably something to do with the firmware, but I haven't felt like messing with it. In any case, it's still more than helpful for finding bad caps. I rarely use it to check caps against their datasheets. Rather, I use it to just see if a cap is starting to fail (will read an Ohm to a few Ohms of ESR... which is what it usually takes to make PSU circuits misbehave.)

Quote:
Originally Posted by socketa View Post
So i replaced it with an OST RLP 1000uf (actually 860uf) that has ESR 0.027
But now the PSU wont start unless i lower the 12V load resistance (i used 47ohms instead of 83 ohms)
And when it starts, the 12V rail voltage is still too low, as before
OK, seems like we are not getting too far with these load tests.
And it seems the PSU is no longer blowing fuses.

So perhaps try connecting the PSU without the series bulb(s) (i.e. directly in the wall) and give it a good load on the 12.8V rail, as this appears to be a more modern PSU meant for 12V-based PCs. And to make sure the other rails are loaded too, perhaps now try the HDD on there too (that is, in addition to whatever load you have put on the 12.8V rail, which I would suggest something in the range of 10-20W 12V car/auto bulb.)

If the PSU still refuses to regular, remove both primary caps and test them. Perhaps one of them went open or partially open when that MOV blew. That is the only thing I can think of right now as to why the PSU is not regulating, aside from possibly more/other bad caps on the output filter.
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Old 04-30-2021, 08:12 PM   #44
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Default Re: Compaq PDP-121P - Baked MOV

Thanks for the corrections. Appreciated.

In case i (or someone else) look back on these threads (which i do), i'm just putting this in as a reminder, about what happens when you use 12V bulbs on a lower voltage rail:
P = VČ/R
if R is constant, then P will be directly proportional to VČ
Then, if voltage is halved, what will the power be?
V X V = P
0.5(V X V) = 0.5P
VČ = 0.25P

Seems that there is a problem with 12.8V rail supply:

I plugged the 24 pin in into a good motherboard, and the board starts using the power button, and runs (12.8V rail = 12.22V); but it wont start if i plug in the 4 pin CPU connector.
If i connect a hard drive (but not the 4 pin CPU connector) the board starts, but then shuts down as the drive starts to spin up.
Also checked the Taicon (probably a PW, as it's the same colour as the other Taicon on the 12v rail) 1000uf, 12.8V PSU rail cap, and it's not bad - 872uf 0.044 ohms (i just de-soldered the +ve leg, and tested it, because it's got tough glue on it and it's in a tricky spot)

Unplugged from motherboard, and then loaded the PSU rails:
3.3V rail_____21W 12V bulb
12.8 rail_____5W 12V bulb
Hard Drive
12V_____ Unloaded

Result:

Starts, but shuts down when HD is spinning up. (and shuts down faster if there is a 73ohm resistor on the 12V rail)
if 12.8V rail is then unloaded, it starts and doesn't shut down : 12.8V rail = 11.86V
if 12.8V rail is then loaded with the 5W 12V bulb, and HD is removed, it starts and runs : 12.8V rail = 12.06V
if 12.8V rail is then only loaded with the 5W 12V bulb, and HD is removed, and the 12V rail has 74 ohm resistor connected. 12.8V rail = 11.82V

Most of the time that the 12.8V rail is loaded with the 12v 5W bulb, it can take quite a few trys at shorting the PS-ON pin to ground untill the PSU starts up

If it's loaded with the 23W 12V bulb, it won't even start, nor does the fan twitch, maybe just a slight, but brief, glow on the bulb.

I also checked the resistance of the rails after about 10 seconds of connecting the probes:
3.3V___8 Kohms
5V_____1.1 Kohms
12V____11 Kohms
12.8___233 ohms (so that doesn't look like a problem - only 0.7W)
5VSB___265 ohms

Quote:
Did you have the proper bypass caps on your probe when you did that test? (IIRC, it's in Intel's ATX spec sheet, in the section about power supplies and PSU voltage/ripple specs.)
And were both PSUs' covers on and secured properly? A PSU with the cover off may induce a lot of noise on the scope, so that's something to always watch out for.
No, and no.
And thanks for that - indeed, you do recall correctly
Quote:
Measurements shall be made with an oscilloscope with 20 MHz bandwidth.
Outputs should be bypassed at the connector with a 0.1 μ F ceramic disk capacitor and a10 μ F electrolytic capacitor to simulate system loading
So those caps would just go from the probe ground, to the +ve part of the probe?
And the one that i have is 60MHz.
I'll try it again when i put it back together

Quote:
If the PSU still refuses to regular, remove both primary caps and test them. Perhaps one of them went open or partially open when that MOV blew. That is the only thing I can think of right now as to why the PSU is not regulating, aside from possibly more/other bad caps on the output filter.
The primarys are good; even replaced them with others that tested good - still no change

Plugged it back into the Compaq computer, and still the "over voltage condition" LED sequence is displayed, and it wont start regardless of whether i plug in the 4 pin CPU connector or the hard drive
I can see the voltages start to come up when i press the power button, and none of them is over-voltage, but they get quickly shut down by the motherboard.

Last edited by socketa; 04-30-2021 at 08:48 PM..
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Old 04-30-2021, 09:33 PM   #45
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Default Re: Compaq PDP-121P - Baked MOV

When using incandescent lamp as a load, it will have high inrush current that can trigger power supply over current protection circuit, as filament heats up the resistance will rise. An incandescent light bulb with a tungsten filament has a positive temperature coefficient.
The cold resistance of the incandescent lamp will be about 10 ~15 lower than Hot resistance (when filament heats up the resistance will rise).
You can easily test cold resistance with Ohm meter, the hot resistance can be calculate using Ohms laws.
30W 12V will have Hot resistance of 4.8 Ohms, so cold resistance will be < 1 Ohms.

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Last edited by budm; 04-30-2021 at 09:45 PM..
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Old 04-30-2021, 09:49 PM   #46
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Default Re: Compaq PDP-121P - Baked MOV

So with all of the above tests... did you still have the series incandescent bulb(s)? If yes, remove it and try the PSU without. I think the fact that the PSU can turn on (sometimes) and not blow up means it's ready to test without a series current limiting. Then let's see how the PSU behaves then. If it still trips, check the NTC thermistor, as perhaps that could have gone high impedance?? When the PSU does not start (but also when it starts), can you measure the DC voltage of the caps on the primary side and post what values you get here? (Basically, I'm looking for voltage before the PS-ON signal is jumpered to ground and then when the PS-ON is jumpered to ground.) I'm still trying to determine if this regulation thing is a primary-side issue or something else.

Quote:
Originally Posted by socketa View Post
if R is constant, then P will be directly proportional to VČ
Then, if voltage is halved, what will the power be?
V X V = P
0.5(V X V) = 0.5P
VČ = 0.25P
Yup, good old power/square law, basically.

Quote:
Originally Posted by socketa View Post
So those caps would just go from the probe ground, to the +ve part of the probe?
Yup, right at the connector where you are testing the voltages.

Quote:
Originally Posted by budm View Post
When using incandescent lamp as a load, it will have high inrush current until the filament heats up.
The cold resistance of the incandescent lamp will be about 10 ~15 lower than Hot resistance (when filament heats up the resistance will rise).
Indeed.
Halogen bulbs are even worse in that regard - a high-power one (35W+) can downright trip circuits at nominal rated voltage applied.

On the other hand, high-power halogen bulbs are great for series power limiting (or lack thereof), particularly on PSUs with APFC, because the low cold resistance will make it easier for the APFC to start up.

Last edited by momaka; 04-30-2021 at 09:51 PM..
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Old 04-30-2021, 10:21 PM   #47
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
So with all of the above tests... did you still have the series incandescent bulb(s)?
Nope - i'm quite aware that doing so will leave less voltage for any downstream load.

I'll check the NTC thermistor, and the voltages across primary caps, and report back
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Old 05-01-2021, 03:34 AM   #48
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Default Re: Compaq PDP-121P - Baked MOV

Removed the NTC thermistor, and it measures 4 ohms
There is a 330Kohm resistor across the live and neutral which is good, and is what is measured across live and neutral

With the 23W bulb on the 3.3 rail, before i ground PS-ON there is 159.3V and 153.8V across the primary caps
When i ground PS-ON and the PSU is running, there is 156V and 152V across the primarys
If i then add the hard drive while it's running (easier than adding it before grounding PS-ON, then having to jiggle the paper clip for several minutes to get it to fully start), there is 155V and 151V when it's running

23W bulb on 3.3V, and 5W bulb on the 12.8 rail : there is 155V and 151V
Ground PS-ON and PSU running: 155V and 153V
Add a hard drive hard drive to the above : 159V and 156V (but, as before, the PSU doesn't start when PS-ON is grounded, and still 159V and 156V)

And this PSU has balancing resistors across the primary caps - which is a good feature

So that all looks OK, right?

Last edited by socketa; 05-01-2021 at 04:05 AM..
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Old 05-01-2021, 11:08 AM   #49
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Default Re: Compaq PDP-121P - Baked MOV

I have to ask the question isn’t the voltage of 12.8 a little bit high for a computer switching power supply

Most of the time I have seen 11.8 to maybe 12.2 but this is if you are lucky

Because the computer switching power supply tester I have if the voltage is over about 12.2 it starts flashing over voltage
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Old 05-07-2021, 06:23 PM   #50
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by socketa View Post
With the 23W bulb on the 3.3 rail, before i ground PS-ON there is 159.3V and 153.8V across the primary caps
When i ground PS-ON and the PSU is running, there is 156V and 152V across the primarys
If i then add the hard drive while it's running (easier than adding it before grounding PS-ON, then having to jiggle the paper clip for several minutes to get it to fully start), there is 155V and 151V when it's running

23W bulb on 3.3V, and 5W bulb on the 12.8 rail : there is 155V and 151V
Ground PS-ON and PSU running: 155V and 153V
Add a hard drive hard drive to the above : 159V and 156V (but, as before, the PSU doesn't start when PS-ON is grounded, and still 159V and 156V)
OK, that's good. Looks like the primary caps are holding up voltage alright.

The NTC thermistor seems OK too. Just had to ask, in case it had gone higher than normal resistance. After all, sometimes that can happen when an MOV blows, causing the fuse to blow too (and sometimes the fuse might not blow, but the NTC might instead.)

So as for the issue at hand... I'm still not sure what's causing this. The fact that you can get it to run when you play around with the PS-ON signal for a while tells me either something on that circuit is dodgy, or perhaps one (or both) of the optocouplers are not 100% good after the PWM IC blew up. So if you have any spare parts from other PSUs, perhaps start to swap these around.

Also, when you are able to get the PSU to start, what voltage does the PG (Power Good) signal measure? If it's logic level "low", that means the PSU is not "happy" about the state of the output voltages and so won't signal the PC to turn ON yet. On the other hand, if PG does go high, then that means the PSU thinks its output voltages are OK and "ready".

Quote:
Originally Posted by sam_sam_sam View Post
I have to ask the question isn’t the voltage of 12.8 a little bit high for a computer switching power supply
Yes... but in the case of this power supply... let's just call it an HP/Compaq peculiarity. IIRC, the HP DC5000 computers also had something like with their power supplies when I recapped a bunch of them for an office about a decade ago (or thereabouts.)

The 12.8V rail is only used for the CPU VRM high side and nothing else. I suspect HP did this so that they could feed this rail directly to the CPU VRM high side (for which 0.8V higher voltage probably won't make much of a difference, if not help a tiny bit in terms of drawing less current) while also being able to use this rail to linearly regulate it down to 12V (so only 0.8V maximum Vdrop in the worst case) for the HDDs and the rest of the components on the board. I'm not sure if they did this to get a more stable 12V rail (since many PSUs on which they did this were from the Pentium 4 era) or just to save on filtering components. But either way, it's an HP/Compaq -specific thing.

Quote:
Originally Posted by sam_sam_sam View Post
Because the computer switching power supply tester I have if the voltage is over about 12.2 it starts flashing over voltage
Yeah, that's not really a good tester, then - those testers should only warn of over/under-voltage when the 12V rail goes above 12.6V or below 11.4V, respectively. Both of these values represent 5% deviation, which is the max allowable for ATX spec on the 12V, 5V, 3.3V, 5VSB, and (I think) -5V rails. For the -12V rail, it's 10%, so regulation can be quite loose there.

Last edited by momaka; 05-07-2021 at 06:25 PM..
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Old 05-08-2021, 02:27 PM   #51
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Default Re: Compaq PDP-121P - Baked MOV

Quote:
Originally Posted by momaka View Post
The fact that you can get it to run when you play around with the PS-ON signal for a while tells me either something on that circuit is dodgy, or perhaps one (or both) of the optocouplers are not 100% good after the PWM IC blew up. So if you have any spare parts from other PSUs, perhaps start to swap these around.
Thanks,
i do have such spare parts; so will try that when i get back home in a week or two.

Quote:
Originally Posted by momaka View Post
Also, when you are able to get the PSU to start, what voltage does the PG (Power Good) signal measure? If it's logic level "low", that means the PSU is not "happy" about the state of the output voltages and so won't signal the PC to turn ON yet. On the other hand, if PG does go high, then that means the PSU thinks its output voltages are OK and "ready".
Pretty sure that PG logic level goes high when the main rails are up and running - as i took note of that when i tried it on that test motherboard in post #38
I did notice that the rail voltages started to come up on the compaq motherboard; but i didn't check what the PG voltage was doing - i'm assuming that it would briefly be high before the main rail voltages were shut down by whatever was shutting them down (maybe some PSU controller IC?, or whatever is on the compaq motherboard that is recognizing "overvoltage")
The voltages didn't reach their normal spec voltages, so it seems odd that the LED sequence on the front panel indicated "over voltage"

I'll also try this PSU on a good motherboard, and see if it will start up.
And i'll record the voltages if it does.

Last edited by socketa; 05-08-2021 at 02:29 PM..
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Old 05-11-2021, 09:00 PM   #52
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Default Re: Compaq PDP-121P - Baked MOV

Thanks for your reply I sort-a lost track of this post and found it again
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Old 05-21-2021, 10:44 PM   #53
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Default Re: Compaq PDP-121P - Baked MOV

Am back home now,
Plugged the PSU into a well-working computer (dell dimension 2400, which has a 20 pin ATX, and 4 pin CPU connector), and also connected the HD, DVD, and Floppy (i.e swapped all of the connectors over)
This particular board normally starts as soon as the power is plugged in.
But all that i hear is a click in the PSU fan, and it shuts down pretty much instantly.
So, as suggested, i changed over the three optocouplers; but that didn't make any difference.
And there is a similar blinking front panel LED.
There is 4 diagnostic LEDs on the back panel, but none of them light up
So, it looks like there is some faulty condition within the PSU that is causing the motherboard to shut down the PSU (probably by pulling Power_Good low)
I watched the voltage of Power_Good when i plugged the PSU in, and it went from 0 to about 3V when the fan clicked and then back to zero, probably cutting out before it has a chance to reach it's normal value, (or maybe my meter's response time isn't fast enough)

Apparently the blinking amber light means:
Indicates system has power, but the POWER_GOOD signal is not yet active.

Maybe it is one of the other output caps
Is there some chip on motherboards that shuts down the PSU if ripple, or ESR, is too much?

Last edited by socketa; 05-21-2021 at 10:58 PM..
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Old 05-22-2021, 04:38 PM   #54
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Default Re: Compaq PDP-121P - Baked MOV

i compared the resistances of these two PSUs

_______Good PSU___Faulty PSU
12V_____0.85K_____12K
5VSB____9.7K______260ohms
PG_____ 1.35K_____ 1.25K
5V_____362ohms____1.2K
3.3V____27ohms____10K
-12V____3.65K_____11.86K
PSON___9K_________3.6K
12.8V___N/A_______234ohms

Anything look obviously out-of-whack?

Last edited by socketa; 05-22-2021 at 04:42 PM..
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Old 05-23-2021, 08:10 AM   #55
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Default Re: Compaq PDP-121P - Baked MOV

Yes “ out-of-whack ”
12 volt ( is ?able )
5 volt SB***very low resistance not good
5 volt***something is very wrong with this resistance value on the bad power supply
3.3 volt***something is very wrong with this resistance value on the bad power supply
12.8 volt***very suspicious what this reading is about on the non working power supply
-12 volt***something is very wrong with this resistance value on the bad power supply
PSON***something is very suspicious what this reading is about on the non working power supply

PG to me might be close enough to the work power supply

All others are about are very low resistant and should be investigated very thoroughly

How were these resistance measurement made plus meter lead and negative meter leads were where on this switching power supply board

Your reading should be very close or the same as the good one if both switching power supply are exactly the same model number and have the same exact ic chip on both boards

Last edited by sam_sam_sam; 05-23-2021 at 08:33 AM..
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Old 05-23-2021, 11:19 AM   #56
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Default Re: Compaq PDP-121P - Baked MOV

The power supplies are not the same, hence N/A (non applicable) for the 12.8V (it doesn't have a separate rail for the CPU 4 pin connector)
i compared them just in case someone saw something that was terribly wrong with the faulty one
i know that PSUs can have quite different values for minimum load resistors.

Last edited by socketa; 05-23-2021 at 11:23 AM..
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Old 05-23-2021, 12:41 PM   #57
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Default Re: Compaq PDP-121P - Baked MOV

yes but the values less 1k ohm are an issue
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Old 05-23-2021, 05:05 PM   #58
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Default Re: Compaq PDP-121P - Baked MOV

There is only two values below 1K
One is on the 5VSB and it's capacitor tests good (and i even changed it)
The problem arises when the computer grounds PS_ON and then (i suspect) that something on the computer motherboard shuts down the main rails as they are starting up - So i don't think that there is any problem with the standby rail.
The other value that is below 1K is on the 12.8V 4-pin CPU rail - which i'm not sure is acceptable or not; as a lower resistance might be normal for an independent CPU rail
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Old 05-29-2021, 11:18 PM   #59
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Default Re: Compaq PDP-121P - Baked MOV

I've now measured all of the rail capacitors that are on the the main board and they don't appear to be bad (forgot to put the other AUX rail measurement in, but it tested good as well)
While turning the PCB over, the black wire to that heavy (filter?) coil broke off (i assumed that they would be soldered on to the coil very well, and could handle the movement of me turning the PCB over and back over again)
This coil is between live and an AC terminal of the bridge rectifier
It's a long shot, but could broken stands that were going to the coil result in the power supply not getting, (and maintaining under load) enough voltage? Hence the computer shutting it down.

Can i cut the yellow plastic tape, and reattach the wire to the coil? (hopefully there is no windings inside of it)
Attached Images
File Type: jpg Rail Capacitors.jpg (438.1 KB, 10 views)
File Type: jpg Output.jpg (681.6 KB, 9 views)
File Type: jpg Filter coil.jpg (894.4 KB, 10 views)

Last edited by socketa; 05-29-2021 at 11:24 PM..
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Old 05-30-2021, 01:24 PM   #60
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Default Re: Compaq PDP-121P - Baked MOV

Would it be OK to remove it (i'm guessing that it's passive PFC) and replace it with a jumper?
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