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HP/Agilent 54825A Oscilloscope Power Supply Issues
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G0HZU:
Thanks! The 3 caps that failed are all the same value at 10uF 16V and they are really tiny as shown in the image below. The other caps are physically bigger/longer but I agree that I should really replace them as well. It's difficult to access and read some of them so I wondered if there is a known list of cap values for this PSU that could be ordered as a kit.

The three failures were all in the same part of the PSU as circled in the second image below. I'm a bit nervous to fully take the PSU apart because some of the heatsink and grounding arrangements look to be quite critical in terms of alignment and my mechanical/assembly skills aren't that great.
tautech:
Yep them tiny little mothers generally fail first.  :horse:
If you're not confident with your rework skills at least replace all the easy ones and by then you might even feel confident to tackle the rest.

TBH I feel very uneasy giving advice to the RF Guru you are and imagined you'd done your fair share of this sort or work but maybe you haven't.
G0HZU:
In this case I'm nervous to take the PSU fully apart because I'm not sure if I'll be able to put it back together with the same thermal resistance behind all the various semiconductors that attach to the side walls of the PSU. I'm not sure if there's a special thermal interface there and I'm not sure if there needs to be a certain pressure from the clippy heatsink things that press against the semiconductors. If I get this wrong I could make the PSU overheat and this may cause significant damage.

I have repaired loads of stuff over the years and experience has taught me when to be cautious if something looks like it might be critical in terms of the assembly. It probably is fairly easy to take the PSU apart and maybe the heatsink clips and the thermal resistance are easy to manage. If anyone reading this can advise then maybe I'll take the plunge and take it fully apart.

G0HZU:
I should have taken an image of my PSU to show the additional 11 wire interface to the PSU. This looks to be using an 11 way Molex connector. However, the image below is of a similar supply and the circled 11 way connector pokes out of the end of the PSU and this connects to the main acquisition board in the scope where the connections can be probed with a DVM or scope.

This is where I was able to find the alarm/status flag from the PSU. Does anyone have any more info about this 11 wire interface? The scope seems to be able to use it to possibly isolate some of the more delicate parts of the scope if the PSU reports an alarm. I kind of reverse engineered some of the other connections. One of them allows the PSU to be started if it is grounded via a resistor. There's also some ground connections and I recall there is a 15V line there (even in standby?) and 5.1V and -5.3V and a few others. When I fit the properly rated 105degC 10uF 16V caps to the PSU I'll try and take a few images of this interface.

I'm really pleased and relieved that the scope is working again. I've been meaning to investigate this issue for several weeks now but kept putting it off. In the last couple of days it had reallygot worse. I had a mini crisis over it today because it had virtually stopped working at all and I need it for some research work in the next few days. Since the repair it's still powered on and still working fine and I've tried a few more power cycles. It works every time now.

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