Thanks for the response.Failed electrolytic capacitors can have numerous different failures, here are a few:
- Short Circuit
- Open Circuit
- Out of spec capacitance (too low or too high)
- High ESR
- Turning into a resistor
- Electrically leaky, passing DC instead of blocking it
You need to replace all of the capacitors, not just some. Replace the rest of the capacitors and try to boot the board again. You also should clean the board well with something like CRC Lectra Clean. IPA is junk and usually just smears things around and leaves white streaks.
I'm not familiar with the 7500, but it may need a ROM SIMM to boot. It also may need a good PRAM battery, there are several mac models that won't boot or have erratic behavior with no PRAM battery installed.
Short Circuit, Turning into a resistor, Electrically leaky, passing DC instead of blocking are different degrees of the same thing. If they were significant enough to affect performance, a voltage droop at the capacitors' positive terminals would be measurable: no voltage droop was measured. Open Circuit, Out of spec capacitance (low), High ESR may vary in magnitude, but they all are equivalent to reduction in capacitance. I doubt that the design could not tolerate the complete removal of any one capacitor. Out of spec capacitance (high) would improve power rail filtering/bypassing. I did obtain the additional needed caps and all have now been changed. That did not fix it.
Changing all capacitors may make sense to prevent future failures, but changing only bad capacitors would be needed to return the board to operational. In Defense, space and commercial aerospace, IPA is specified for soldering rework cleanup.
No ROM SIMM was installed in the 7500/100 system, and it operated properly in that same configuration.
The PRAM battery is new, and it measures 3.66V both installed and out of circuit.
Thanks for the response.The 7500 should not need a PRAM battery. Only early 7500s would need a ROM SIMM; most of them don't.
Thanks for the response. I followed the guidance in the link you supplied. There was no change.It's been a moment since I restored mine but I seem to remember holding a reset button for about a second every time I make any changes to the machine, before powering it on. Much more about it here though https://68kmla.org/bb/index.php?threads/powermac-7500-100-dead.19371/
There is one more data point that someone here may recognize. When I have (a) memory stick(s) installed, I get 3 seconds of white noise 1 second after power up. When I remove the memory, I get 1 second of white noise 1 second after power up.