On my livestream last night, I attempted to recap the Delta SMP-120EB power supply that was in my Performa 600. And after initially forgetting to reconnect the mains power connector, I measured 4.97V on the +5V standby output... yay!
However, my joy quickly faded when I tried to power up the Quadra 650 logic board with this power supply (chosen because it doesn't have leaky electrolytic capacitors), and I could not get it to power up. Knowing that tantalum capacitors can fail shorted, I measured the resistance of the voltage rails on the logic board, and sure enough, had 12ohms between the +5VDC (not standby) and ground.
So I tried the power supply on my IIvx logic board, and it powered up for a second or two and shut off. I again measured resistances on the logic board, and found that I had 15ohms between the +5VDC (not standby) and ground. OK, seems something might be shorted on that board, I'll figure out what when I recap it.
Next up was my Performa 600 logic board, which I didn't want to use because the electrolytic leakage is worse on it than the IIvx logic board. The power supply did the same thing, and I then again measured 15ohms between the +5VDC (not standby) and ground. This is beginning to look suspicious - three logic boards shorted in the same way, what are the odds?
So finally I decided to check my "parts" IIci logic board. Given that it was battery bombed, I dared not plug it in. However, I measured 18ohms between the +5VDC (not standby) and ground.
At this point, I'm beginning to wonder if that resistance measurement is normal. However, I have exhausted all of the logic boards in my possession that use this style power supply, and ideally would measure the same resistance on a booting logic board that has already been recapped.
To help me figure out where the problem is:
However, my joy quickly faded when I tried to power up the Quadra 650 logic board with this power supply (chosen because it doesn't have leaky electrolytic capacitors), and I could not get it to power up. Knowing that tantalum capacitors can fail shorted, I measured the resistance of the voltage rails on the logic board, and sure enough, had 12ohms between the +5VDC (not standby) and ground.
So I tried the power supply on my IIvx logic board, and it powered up for a second or two and shut off. I again measured resistances on the logic board, and found that I had 15ohms between the +5VDC (not standby) and ground. OK, seems something might be shorted on that board, I'll figure out what when I recap it.
Next up was my Performa 600 logic board, which I didn't want to use because the electrolytic leakage is worse on it than the IIvx logic board. The power supply did the same thing, and I then again measured 15ohms between the +5VDC (not standby) and ground. This is beginning to look suspicious - three logic boards shorted in the same way, what are the odds?
So finally I decided to check my "parts" IIci logic board. Given that it was battery bombed, I dared not plug it in. However, I measured 18ohms between the +5VDC (not standby) and ground.
At this point, I'm beginning to wonder if that resistance measurement is normal. However, I have exhausted all of the logic boards in my possession that use this style power supply, and ideally would measure the same resistance on a booting logic board that has already been recapped.
To help me figure out where the problem is:
- Can anyone measure the resistance between +5VDC (pins 2-4) and ground (pin 5) on a working, booting logic board of one of these four types?
- Does anyone know if there are schematics for the IIvx/Performa 600 or even the Quadra 650... or at least know which capacitor(s) are the main filter caps for the 5V rail?
- Does anyone know of a starting place to look at on the Delta SMP-120EB?
