Indeed, there does seem to be some variability about the condition of the power supplies, or electronics in general across all the SGIs. I have an old early Indigo 1 prototype with a working PSU, but a newer (mass production) Indigo 1 with a dead supply.
It is hard to say exactly what the causes are, although all electrolytic capacitors age and have a lifespan to them. I've attached a good document on the subject called "Capacitors Age and Capacitors Have an End of Life," from Emerson, who produce Uninterruptable (back-up) Power Supplies. In general, temperature is the biggest factor at reducing their life.
However it should be noted that 15 years is considered a long service life for an electrolytic capacitor. Versus our SGI machines:
- Indigo 1 - up to 28 years old
- Indigo 2 - up to 27 years old
- Indy - up to 27 years old
- O2 - up to 24 years old
- Octane - up to 22 years old
- Fuel - up to 18 years old
- Tezro - up to 17 years old
(Dates as of time of post, just based on rough Wikipedia history timelines rather than real research
)
It's amazing and a testament to good engineering that these devices are still running! But at this point even the Tezro can fall past the end of the expected service life for some of the electronics so I suppose I am not surprised my PSU isn't doing too well