There are tech and electronics savvy members who occasionally visit who could tell you more about testing the ps with multimeters and such.
What I can say is that the P600 ps appears to be common to it, the IIvx, the Centris and Quadra 600, and the 7100. If it is the ps, it ought to be reasonably easy to find a replacement for it. As these are not high demand machines (most find them ugly), there are plenty of parts about.
I'd try fixing it first, however, and on the repair side of things, I would advocate a low-tech approach in the first instance, since there are things that anyone can do. I more or less routinely clean the logic board of any machine older than around 1995 that comes in the house these days because of the frequency of capacitor failure. The latter can be hard to spot, and though ultimately the caps will need to be replaced in the machine, cleaning is necessary anyway as part of the replacement process, so why not give it a go and see if it works, I say to myself. And it generally does work; my only failures to cure problems to date are an SE/30, which did not respond either to washing or to capacitor replacement, and a 9500, which is just plain dead. My 30 or so 68k machines are otherwise working fine. I just use warm-to-hot water under moderate pressure from a handheld shower, spray for a good few minutes, allow the board to dry thoroughly for a couple of weeks or more, and reassemble.
Over the winter, I gave my Quadra 650 logic board — and power supply too — the same treatment after it had begun to act up. It had become progressively erratic and in the end would not boot. So I thought I'd try giving it a good cleaning. It was first disassembled, which provides opportunity to clean the case thoroughly. Treatment of the logic board is fairly self-explanatory. In the case of the ps, I just took the top off and hosed it out. Masses of dirt came out of the thing. I then shook the water out, oiled the fan and put it somewhere where it had plenty of chance to dry and a couple of weeks later I reassembled the Quadra, plugged it in and it works perfectly again. It's scrap electronics; what have I got to lose?
What I can say is that the P600 ps appears to be common to it, the IIvx, the Centris and Quadra 600, and the 7100. If it is the ps, it ought to be reasonably easy to find a replacement for it. As these are not high demand machines (most find them ugly), there are plenty of parts about.
I'd try fixing it first, however, and on the repair side of things, I would advocate a low-tech approach in the first instance, since there are things that anyone can do. I more or less routinely clean the logic board of any machine older than around 1995 that comes in the house these days because of the frequency of capacitor failure. The latter can be hard to spot, and though ultimately the caps will need to be replaced in the machine, cleaning is necessary anyway as part of the replacement process, so why not give it a go and see if it works, I say to myself. And it generally does work; my only failures to cure problems to date are an SE/30, which did not respond either to washing or to capacitor replacement, and a 9500, which is just plain dead. My 30 or so 68k machines are otherwise working fine. I just use warm-to-hot water under moderate pressure from a handheld shower, spray for a good few minutes, allow the board to dry thoroughly for a couple of weeks or more, and reassemble.
Over the winter, I gave my Quadra 650 logic board — and power supply too — the same treatment after it had begun to act up. It had become progressively erratic and in the end would not boot. So I thought I'd try giving it a good cleaning. It was first disassembled, which provides opportunity to clean the case thoroughly. Treatment of the logic board is fairly self-explanatory. In the case of the ps, I just took the top off and hosed it out. Masses of dirt came out of the thing. I then shook the water out, oiled the fan and put it somewhere where it had plenty of chance to dry and a couple of weeks later I reassembled the Quadra, plugged it in and it works perfectly again. It's scrap electronics; what have I got to lose?