HP 53131A PSU repair.

[pizzigri]

Hi everyone,
so I managed to get a used 53131A built by HP around 1997. I got it from an internet seller that gave it to me for 300 Euro. The front window was badly scratched, and even if the seller sold it as working, he did not have images of the unit on. It was incredibly sticky and scratched, but it still had calibration stickers on, so at least I could imagine that it was not tampered with. I tried turning it on, and although it did work (sorta), the VFD was DIM. So at least now i know why it was so cheap. I took it apart. Inside, it was incredibly dirty, and the fan was slow and very noisy. Completely disassembled the unit and cleaned everything. Now it looks almost new (I even managed to polish the front window plastic and it's less scratched...
But, I have a problem with the Delta PSU. It is a DPS-43DL-2 rev. B1. I have found in an old EEVblog thread a schematic,
this one:

PSU problem? Maybe you need a diagram PSU. 

but it dos not look to be very helpful to solve my problem.
So, as I said everything seems to work, but I noticed overheating stains on the PSU PCB, and what seems to be charred copper traces.
I connected the PSU to the counter outside its chassis and powered it looking at the PCB with a thermal camera. I found that C4, a X7R ceramic cap, is overheating and possibly shorted. The fact is, why is this happening, and why the traces - far from this cap - seems charred from too much current?
The schematic does not help me too much, as I cant seems to reconcile the markings on the C4 cap itself (which is visibly damaged) and the corresponding cap on the schematic published by Jf2014.

Here are a few pics of the problem. Note, I cannot ID the ceramic cap that failed, it says X7R, 471K (or 1171K?) and 1KV. On the schematic, C4 is actually a
 22nF, 630V cap.

Also, what could have caused the corrosion on the traces? And, sinc it is obvious that high temps may have damaged the board, should I replace all caps?
Please help me!
thanks, Franco

[pizzigri]

Strange, I cannot edit my own post above. I'm forced to reply.

I just noticed that there indeed is a C2 near C4 that has a 470pF/1KV rating in the PSU schematic, so maybe that is the cap i have failed in my PSU, although it is clearly marked C4. Can a ceramic cap fail this way? I always though they were near indestructible.

[Manul]

Your capacitor 471 is 470pF @ 1kV. It looks like capacitor C2 in schematic, not C4. It is part of RCD clamp for transformer primary, so generally speaking, some heat dissipation is expected, because all capactors have loss depending on construction and dialectric. Is it excesive I do not know, looks a bit high temperature, but again, might be a normal condition for this particular capacitor and circuit design.

[Runco990]

That capacitor looks a bit damaged.  I'd replace it and not worry about it.  It looks like the cap itself is just failing.  I got a 65" TV with a shorted ceramic cap, replaced it, and have been watching Netflix on it ever since.  There was NO other problem. 

The trace looks like corrosion, NOT heat damage.
I see this sort of damage a lot.  Usually I clean it up, flow fresh solder over it, and seal it with UV cure solder mask.  Although that may not be necessary as long as you tin the damaged traces.

I have one of these counters...  I took the fan out and mounted it back using rubber fan mounts for PC fans.  MUCH quieter.... most of the noise is mechanical, so the rubber mounts make a huge difference.

I keep my counter on a switched outlet so it doesn't run 24/7.

[pizzigri]

Thanks to everyone! I will order now the cap from Mouser, any suggestions on preventive replacements on the board? It is way over 20 years old, and it is obvious that it has sustained heating problems (maybe not on those traces, but under the failing cap, the PCB is yellowed from heat).
I've seen that the resistors fail? Or maybe the smaller caps?
Since I'll be ordering stuff, throwing in additional components seems reasonable!

All my best,

Franco

[DeepLink]

I recommend you to change all electrolytics capacitors in the PSU
These old types have a history of leaking, drying out and have increased ESR
There is also 1 electrolytic capacitor on the main board

I modified mine with a Noctua fan, which is less noisy
Also modified the “soft power switch” to a regular “hard power switch”
The original design has the power supply running as soon as the power plug is inserted (to keep the reference running 24/7)
I use an external 10MHz reference, so do not need the counter to be warm and ready
I think I have some PCB’s left over
Will find the schematic, layout, BoM and post it here on EEVBlog

[pizzigri]

SO I replaced the cap with a better Vishay part and it works flawlessly.
I measured the temp again of the cap and it is now around 45-50C°, however the PS as a whole runs quite hot - I have a friend with a 53181 which I convinced to open up to compare and his PS runs exactly as hot, with the same temps.
SO, the fan is quite important I suppose. My fan is loud, at least compared to my E3631 PS although definitely not as loud as the 34410 which is like standing by a jet engine. And I cannot replace the DMM fan since it still has valid cal stickers sealing it.
Does anyone have a source for the Notcua fans? I'd like to buy a couple to replace as soon as cal runs out on my DMM and on the 53131, it's so bad that I regret selling the 34401 and still use the 5385 counter because they're so quiet.
I checked all electrolythic caps and they're fine - just to be on the safe side I placed a BOM on Mouser and will add it to whathever i need to buy in September, but for now all seems good.
Yeah I saw the cap on the board, I'm actually more concerned with all the tantalums scattered on the MB though.

Franco