AT PSU produces video artifacts

[rattlesnake]

I recently put together a retro build that uses a particular AT PSU that fits in this case (LPX).

The model is HPN-100. I'm attaching pictures.

The issue is that after a few seconds of being on, I start to get strange artifacts. The image looks wavy. First it's fine and then it starts to pick up. After about 60 seconds it looks like this:

Does anybody know what kind of failure this is?

Can the caps be faulty? Which ones should I put more emphasis on?
What's the potentiometer for? What else should I check?

I'd prefer not to recap it entirely unless it's unavoidable.

I thought it might be the monitor (it's a CRT) or the motherboard, but this issue does not happen when I use a different PSU.

Any advice is welcome.

[stj]

replace all the small caps. everything 220uf or less.
make a list of them and tell me what you can buy.

[rattlesnake]

Here's what I found out.

I was mistaken about this particular PSU. It turns out that I get artifacts with all my AT PSUs, but with one they are only barely noticeable.
I changed to a different CRT monitor and I don't see the artifacts. An LCD doesn't show them either.

So it must be either the motherboard or the CRT itself.

The CRT is a DELL E773c.

Next: I tested all the tantalum capacitors (most are 10µF 16v) on the motherboard and they were fine, capacitance around 11µF, ESR around 1-2 ohm. But the single electrolytic capacitor (330µF 16v) was almost completely dry (it measured 500pF!) so I changed it. That didn't make a difference in regards to the video artifacts with the DELL CRT but it's a good thing I found it.

So here's the most interesting part.

If I wait about 3-4 minutes the picture on this DELL CRT starts to look normal (of course, with this motherboard and this PSU).
Finally it's almost perfect, almost imperceptible.

Once the picture looks OK, if I touch or rub the S3 video chip on the motherboard the picture becomes wavy again.
Can this mean that the video chip needs to get warm to filter some frequencies? Can the video chip be to blame?

Considering that a different CRT and an LCD worked fine with this PSU and this motherboard, I think the problem is the CRT. Namely, capacitors on the board.

Maybe there's an incompatibility of the on-board video card with the CRT, or it's just the CRT itself. But it's funny that if I touch the S3 chip and rub it, the picture becomes wavy again, and only after some minutes it becomes almost perfect again.

What do you guys think?

Also, please tell me what the white potentiometer that can be seen inside the PSU is for.

[sam_sam_sam]

Here's what I found out.

The CRT is a DELL E773c.

Next: I tested all the tantalum capacitors (most are 10µF 16v) on the motherboard and they were fine, capacitance around 11µF, ESR around 1-2 [/I]

If I were doing this repair I would replace every single capacitor that the ESR value was higher than 1.2 or if you compare it to a new one I would not consider leaving it if the difference is more than 0.5 ESR higher than a new capacitor does because the 10uf capacitors have a higher failure rate than less say a 470uf capacitor does

And depending on your capacitor tester 11uf might indicate a capacitor that might not be very healthy

[momaka]

That's not video artifacts I see. More of a signal distortion or interference somewhere.
If this is only happening on the CRT monitor you are using, check if the CRT works OK with other computers / video sources too. If OK with other sources, then it might be something on the motherboard or video card of the computer in question that is causing this issue.
Going by your description so far, the issue seems to be not present when the equipment is cold, then happens after some time has passed, then disappears when everything is warm?? If so, that's more likely to be a bad solder joint somewhere.

It's not uncommon for CRT monitors to develop bad solder joints, so it may be worthwhile to take a look at everything on its mainboard and neck board.

As for the original AT PSU... it looks a bit on the skimpy side, but it's not a total disaster or gutless wonder. I would say just "a-OK" for an AT PC. In particular, the output filtering looks quite alright for an AT PSU (I see lots of large caps and PI coils.) However, the 12V rail "rectifier" (2 diodes on a bracke... er. DOAB) is a cheesy thing to do... but I suppose given the low 12V rail requirements of an AT PSU, it'll do fine. Still, it's a sign of cheapness. Same goes for the red film cap on the AC input serving function as EMI/RFI filter. Capacitors in that spot not to be X1 or X2 class -rated. This here is just a regular film cap... so again, a bit of cheapness there going on. But at least the EMI/RFI common-mode choke looks normal.
BTW, the PSU shouldn't be able to cause this type of interference. However, it still might be worthwhile to check the capacitance of that red metal film EMI/RFI suppressor cap on the input filter. If it's gone open circuit or low capacitance, it will be spewing extra EMI/RFI noise into the AC line. The CRT monitor's input EMI/RFI filter and main filter cap should be able to deal with that, nevertheless. On that note, I suppose it might also be a good idea to check the CRT monitor's main/big input 400V (or 450V?) cap. If bad or going high ESR, that could be another source of the picture signal "waviness".