Though I haven't done nearly as much troubleshooting as claudio has with his DVM I have a slightly different (read MORE DEAD) symptom on this slot load Mac 400.
First this was a discard since it didn't work, but it looked so nice I grabbed it. So I don't know anything about the history, etc. It's been years since I have fooled with macs. I used to have a printshop so we had 6 or 8 then. But that was a while ago.
The symptom is DEAD, except for the green light. At least the +5V works. Heh.
No noise, no hdd spinning, but there is a definite KerChunk! as the HV starts up. So flyback seems OK.
No video, no boot, no activity at all. Nice green light though.
I took the bottom off and checked for power at the HDD power connector.
+5 and +12 right where they are supposed to be. I checked both with the drive power connector plugged into the drive and with it unplugged. About the same. Not spinning though. I took the drive out and connected it to a PC power supply and it spun right up. Which leaves the question as to why the thing doesn't spin in the iMac with correct voltages! No idea. In this condition I would expect the voltages to be low, dragged down by the PS not being bueno and unable to handle the current. But the voltages are correct, or darned close.
I did disconnect the data cable to the drives and power on the iMac, no difference. This was on the off chance the drive was being powered off by the mac logic board. But that is seemingly not the case.
Two Mac techs that I talked to said "that would be the PAV board". And I don't dispute that except for the no spin situation. But that is perhaps the next thing to try except for my not knowing the history of this iMac. It is possible the firmware/OS-10 situation could be at work here except I would think that would be eliminated by my unplugging the drives and trying a boot. Would this be that? I would think some amount of HDD data would be required for that syndrome. But maybe not?
Since I can hear the HV starting up I think that eliminates (well mostly) a monitor problem or a flyback problem. I did plug it into an external VGA monitor, nothing. Which is about what I expected.
FWIW I have done component level troubleshooting for 40 years, even on macs. I used to fix the Mac+ width coil burning problem years ago and I have changed several IWM chips, even a couple of SWIM chips. So I know how to work around HV and CRTs. But I haven't taken the top off this one yet.
What should I look at now? I could buy a PAV board off ebay and try it, but the fact that I have a no spin situation and correct voltages and apparent HV seems to say that's not the problem, but I could be wrong. There are two types, the one with the square box on the yoke (for tray load) and the one with the rounded box on the yoke (like this one, slot load), my understanding is there are several revs of the rounded model but they are all compatible with this particular iMac. Right?
This green light of death problem seems to be happening more and more lately. I wonder if Apple has a secret warranty for this?
What should I try next?
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