Older computer PS capacitor replacement strategy

[KiwiME]

I have a small collection of about 10 old computers, dating from 1983 to 1997, all in very tidy condition. Silicon Graphics, SUN, IBM, HP and Apple. These are kept mostly for display only, but I insist that they all work correctly and test them at least yearly. Obviously they all have switch-mode power supplies.

The very few power supply failures to date have been EMI suppression caps and 5V smoothing caps, not primary smoothing caps or the multitude of other small caps found throughout. The EMI caps started failing only after I moved to a 230 VAC country a decade ago, no surprise there.

The last Y-cap failure, on my IBM AT, was spectacular. Oddly there is no internal fuse in this PS and it arced continuously until I pulled the plug - leaving the room filled with thick smoke. (Fuses added now.)

It's far too much work and expense to fully re-cap all these power supplies and related monitors so my first step when testing is to always supply power via a portable RCD (GFI) to catch Y-cap failures.

I do have time to tackle a couple of units each winter with preventative maintenance so I'm thinking of this simple strategy:

1) Replace sealed EMI (IEC) filter box (if installed,) and all Y2 and X2 caps.

2) Only replace primary and secondary smoothing caps if there is leakage or signs of swelling.

The rationale is that:
a) EMI receptacle / X2 and Y2 caps are sensitive to our mains which can get up to 245 VAC and may fail dramatically. They may also quietly fail "open" leaving no EMI filtering.

b) primary cap failure should blow a fuse, so no need for preemptive replacement.

c) secondary cap failures will trigger built-in overvoltage and overcurrent protection, so no need to worry about those either.

What say the technical experts?

[PeteS in CA]

Re: Older computer PS capacitor replacement strategy

I'd agree that your first priority is close to correct. I'd widen it a bit to include things that could fail catastrophically or do significant damage:

* Power line filters, X & Y caps;

* Metal oxide varistors (if your X & Y caps have deteriorated due to voltage surges, so also have the MOVs);

* Fans (check and replace as needed, not systematically).

Regarding the primary and secondary filter caps, failure may not be visually obvious or sudden. If you have the equipment, I'd suggest checking the ripple on your +5V and +12 outputs. If you see significant 100 Hz ripple, you've got deteriorated filter caps on the primary side. If you see significant switch frequency ripple (20-40 KHz on the older computers), you've got one or more deteriorated output filter caps and/or a deteriorated output inductor core (you would have to get a new, same type, core and wind a new inductor, duplicating the way the windings are arranged and the number of turns for each winding - both factors are very important).