I used to sell apple ]['s and repaired them, but
seldom down to the component level. Except for
the 74LS125 on the floppy drive. Users were
always frying that by plugging the ribbon cable
backwards, something you really had to work hard to do.
Anyhow, I like the idea of checking all the caps.
At 07:10 PM 8/29/2011, you wrote:
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...
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
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.
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.
I'm sure this has been discussed before, but at the moment Google isn't
being helpful with discussions on this list. Can anyone refresh my
memory/point me at resources/make a sensible suggestion or two? Admittedly
I'm at a really early stage without a great deal of info to put forward,
but sometimes just discussing these things helps clarify the approach
before I really dive into it.
Cheers,
Mike
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