Quadra 840av boot problems...

[cab3]

Hello,

I got gifted a quadra 840av but it will only boot after several tries but then works fine...
I've replaced the capacitors and bought a new p-ram battery but it didn't seem to make any difference... the board looks like it is in great condition no corrosion on the traces...

Does anyone know whats going on with this machine? Could it be the power supply? I've noticed I get better chances of booting when I let it run for a few minutes and then turn it of and on very quickly..

 

[MacTech68]

The symptom you describe is indicative of a failing electrolytic capacitor.

If you are CERTAIN you've replaced all the ones on the logic board, then it probably is in the power supply as you suspect.

Whilst you can replace all the caps in the PSU, you could check the +5 volt standby and see if it's running. It could be that the only problem is the 5 volt standby power supply which is a second small power supply on the same boards as the main power supply. You might be able to trace a failed cap in the PWM more easily.

A small amount of heat applied may make finding the culprit easier, if you can SAFELY run the power supply whilst disassembled. IIRC the 8500 PSU was not easy to run disassembled, but I can't recall if the 840AV was as bad.