HP 54520A PSU repair

[SoundTech-LG]

Good plan...  measure everything you can, otherwise you are flying blind.

The CNY21 will be difficult, expensive, or both. Take a look at Vishay CNY65, or 66 series. Measure for correct dimensions. Cheap to try.

[akimpowerscr]

The CNY21 optocoupler is neither hard to find nor too expensive.

https://www.ebay.fr/itm/CNY21-Optocoupleur-T-H-T-OPCNY21-/170798599099

If you want to fix your oscilloscope, don't behave like a homeless man ... don't try to save a few euros on spare parts.

You risk introducing new problems that could make the diagnosis even more difficult

Buy only the original part, not the so called equivalents.

[sv3ora]

Hi,
today I removed a few electrolytics on the secondary side and I noticed that the pcb traces near them have signs of leaked electrolyte (attached). There is also electrolyte in other places near some other electrolytics on the secondary and it stinks fish when desoldered, this is electrolyte. In some caps the electrolyte can be seen below the cap body at the black rubber.

So there is definitely some leakage under the silicone. I bet these have to be carefully examined and replaced, what do you think?
Do you think this could cause this 2 minutes on, then BOOM problem?

[mistamidget]

I had exactly this problem this weekend with my HP 54510B DSO. I was using it to look at an rs232 data stream and it threw an error message and started to smell very badly.
I turned if off immediately and having a sniff around I found the power supply was the cause. Measuring the supply voltages I saw the -12V was not present and the lm240t (A8) was very hot.
Using the reversed engineered circuit from Thierry Magis referred to previously I found that -12V and +3.5V had 10ohm resistance between them which turned out to be exactly the same corrosion under C27 that your red arrow points to.
So, I scrapped out the circuit board between the two tracks and removed all the white silicon and bingo the carbon I dug out was the 10 ohm resistor.
I reassembled using all the original capacitors and the power supply is now perfect and my scope is fully functional again.
I think the electrolytic's are not leaking but the silicon causes condensation to be trapped and cause the corrosion.
I removed all the silicon where it touches the pcb so it doesn't happen again.
Hope this helps.

[sv3ora]

Finally, I fixed mine by replacing all the electrolytics in the PSU. Now the PSU seems to be working fine.
Lesson to learn, even if this is not always obvious, you should first check all the electrolytics UNDERNEATH for leaked electrolyte before you try anything else in these old SMPSUs, unless there is a visible trace of damage in another component.

Thank you all for the help, I leave the thread open for other people to post if they have similar problems with this PSU.

[wje]

I wish I had found this thread a week ago. I had exactly the same problem, -12 reading only 1v or so, some mystical low-ohm reading between -12 and ground even with every cap that could have shorted removed. Then I found the bad cap on 3.5, same carbon buildup, bingo. I sure spent a lot of time tracing, removing, replacing. At least it was educational. BTW, my final diagnostic tool was to use a delicate procedure... back-feed the -12 with a bench supply capable of supplying 5A at 12v and waited to see what caught on fire, or at least got hot. Seconds later, that pesky cap identified.
Bill