Mac IIci: Video voltages low when first powered up.

[Stephen_Usher]

I've a recently fixed (battery and cap damage) IIci which has the symptom that the voltages on the RGB lines on the video connector are very low when the machine is first powered up but they increase over a period of about a minute to the correct values. If the machine is shut down then the degree to which the voltages are low when powered up again is determined by how long the machine has been switch off for. Feels like some sort of capacitor issue.

The PCB has had all the electrolytic capacitors replaced but ti don't see any between the RBV, the RAMDAC and the video connector.
All I can think of is possibly the power supply has a low voltage when first powered up but the +5V seems stable.

Anyone seen this before?

The next thing I'm going to try is replacing the capacitors in the PSU but seeing as the voltages look OK I'm not putting much hope in this fixing the problem.

 

[Byrd]

Likely the PSU caps, you could try rigging up an ATX PSU to your IIci to test the theory first. IIci PSUs aren't much fun to work with hence the suggestion

 

[Stephen_Usher]

I can't really rig an ATX PSU up at the moment.
I've had the PSU apart once to see if I could see any obvious cap juice but I couldn't. I should have the replacement parts either tomorrow or Wednesday so I can do the full recap and see.

 

[Stephen_Usher]

I've recapped the low voltage side of the PSU. They were all leaking even though they were Nichicon. Well, all but one cap, C23, which the posting giving the capacitor specs said was 47uF 16V and actually it should be 100uF 35V, so I had to put the original leaking cap back.

Unfortunately the problem remains the same and the power rails are clean and correct, which suggests something on the motherboard itself.