Replacing caps on HP 54522a SMPS

[Tim T]

One of the 2200uF 50v electrolytics on the secondary side of the SMPS in my HP 54522A scope decided to blow the other day, and on inspection other caps were leaking as well. The power supply is a Computer Products XL130-3630E from 1993, no schematic/specs can be found, the HP service manual just says to replace the entire unit, which of course is not much of an option today. So my thinking is to change out all the electrolytics on the secondary.

Not having worked on SMPS previously, a bit of guidance on cap selection would be appreciated, this appears to be a complex topic with many conditional statements. Would using caps marketed as “suitable for switching power supplies”, such as Nichicon UPA series, be appropriate for the entire secondary, or are there some traps where a low ESR/high ripple current rating should be avoided. I suspect any general purpose 105C rated electrolytic would be OK on the primary side if deciding to replace them as well.

Thanks,  tim

[rjmusto]

Hi Tim,
I would not worry too much about it, just make sure you use good quality caps - cheap ones aren’t a good idea.

Also check the main switching transistors are not short circuit and the all the main rectifiers are still good. I would also switch it on first time with a light bulb in series with the mains (an old fashioned incandescent type). That way you won’t create smoke if there is still a fault.

Ralph

[Tim T]

Thanks Ralph.

The leaking cap resulted in a dead short of -5.2v supply to ground, blasting a nice hole in the PCB that will be fun to repair.
I was able to power down the scope within a few seconds of the failure, neither the mains line fuse or the fuse on the power supply board failed, hopefully no other damage happened.

So yes, dim bulb tester and variac will be used when powering up the SMPS after recapping.

tim

[BrokenYugo]

Generally speaking, more ripple current rating=more better. Meet or exceed installed temp and voltage rating.