68kMLA Classic Interface
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| Click here to select a new forum. | | Mac Classic II Troubleshooting Help | Posted by: armadsen on 2017-01-31 20:17:07
I have a Mac Classic II that I picked up for next to nothing recently. When I power it on, the fan starts up, but the screen stays blank. The floppy drive makes a little noise every few seconds. Otherwise, nothing happens.
My first thought was bad caps, as I know that's a common problem in these. The caps on the logic board didn't show any obvious signs of leaking (no pooled electrolyte, no obvious corrosion of surrounding pads and component leads). I replaced the capacitors with new ones anyway. I also reheated the solder joints for the connectors and wires on the analog board. (I'm not sure if these suffer from the common cracked solder joint problem that e.g. the Mac Plus does, but I thought it wouldn't hurt.) I also replaced the PRAM battery, which judging by the date on it, was original (no leaking, though).
Unfortunately, replacing the capacitors and reheating those solder joints produced no change in the behavior of the machine when I power it on.
Doing some research, I see it's common for bad caps to cause vertical lines and a failure to boot. In my case, the screen remains entirely blank, looking exactly as it does when powered off. Other than the weird, repetitive floppy drive noise, there's no other obvious activity.
I'm looking for ideas? Has anyone seen this particular behavior in a Mac Classic II? Any tips for next things to check?
(I'm an EE, and plenty handy with test equipment and a soldering iron, so I'm not afraid to work on the machine. Just trying to learn from others that have gone before.)
| Posted by: armadsen on 2017-02-01 14:19:23
A little more information:
I measured the voltages coming out of the power supply. With the logic board connected (but floppy and HDD disconnected entirely), the voltages are around 11.2V and 2.2V. With the logic board disconnected, the 5V rail is about where it should be ~4.9V. This seems to suggest that either the logic board is drawing too much current pulling the 5V rail down, or the power supply is sagging under a normal load. I haven't measured current drawn by the logic board yet, but will do that when I get my hands on a schematic.
Any common/obvious causes of a low 5V rail on a Classic II?
| Posted by: dochilli on 2017-02-02 13:16:02
May be leaking caps in the analog board? In this case it smells like fish.
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