30 Aug
2011
30 Aug
'11
12:53 p.m.
Hi everyone,
I have an Apple II mainboard I'm trying fix. I really enjoy gettting these
particular machines going, for some reason. Maybe it's that emphatic BEEP
when they finally run up... anyway...
Each to his pwn :-)
This one has a pretty obvious fault, it sinks too much current on a
yet-to-be-determined power supply line. Starting it up, the PSU goes into a
startup/shutdown loop. The seller said he'd tested the PS in another
OK, I assume you know it's the mainboard (you've remvoed all expansion
boards and unplugged the kebyaord cable).
machine and found it worked fine. I gave it a quick
test and it did seem
good, so I fitted another known-good PSU to the machine just to verify -
same symptom with the new PSU. Just as a guess, I'm suspecting caps and RAM
in that order, but it could be anything.
I 've had very little time to play with it so far, but I'm pondering my
next step. I've been considering something like this:
http://damon4.com/Default.aspx?blogentryid=112
but my bench power supply is a bit basic (i.e. crap), voltage only goes
down to 3V or so, and no adjustable max current. Not really any good for
this approach. Also, I'm not sure if the Apple II (US DM Plus, in this
case, 4116 RAM) needs the PS rails to come up in a certain order to avoid
damage. Overall, perhaps a bit too violent for this situation.
My ideas so far:
1. First thing I'll do when I get back to it is discover which PS output is
being overloaded. That might eliminate a few suspects.
Sometimes an analogue voltmeter will 'kick' on all PSU outputs except
one, that one is the one that's shorted.
Alternatively, pull the RAMs (the only thing that cares about multiple
supply rails IIRC) disconenct one PSU output at a time at the 6 pin
connecotr and apply a suitable dummy load to it. If you find an output
that, when disconnected, allows the PSU to come up, you know that's the
one that's overcurrenting.
2. If I'm desperate and step 1 suggests it, remove
all the RAM ICs. Doesn't
take long, and I could also inspect the legs on the things, which seem to
fail with surprising regularity.
3. Panic. Hopefully this doesn't involve removing every IC on the board,
but I'd be willing if that's the safest bet.
IIRC the ICs on an Apple ][ mainboard are all socketed. It doesn't take
that long to pull the lot.
-tony