Apple IIc RAM Repair

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X5900's picture
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Joined: Mar 27 2015
Posts: 14

So I've got a IIc here, starts up with random characters on screen and the built in self test throws out RAM ZP 01000000. I went ahead and replaced RAM #MRD6 and now it shows RAM 00100000. I found this strange and piggybacked MRD5, resulted in RAM ZP 10000000 (MRD7), piggybacked that one too and it just goes up the ladder into the auxiliary bank with *RAM 0.... .

Any suggestions? Should I suspect the MMU?

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X5900's picture
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Joined: Mar 27 2015
Posts: 14
Re: Apple IIc RAM Repair

Well, I've assumed that the fault would just go on after ARD#7, but piggybacked that one too and got a "SYSTEM OK". No random characters either, but the IIc still can't boot from disk, the system starts normally, drive boots up but no matter what disk is in, it tries to read the disk with the same audible pattern, to no avail. I've tried that with 2 different good DISK II drives from my IIe, same thing.

speedyG's picture
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Re: Apple IIc RAM Repair

Hello X5900,

the MMU and the IOU are linked together - and also the IWM Chip is linked to that Bus.
The IOU and the IWM chip are responsible for the access to the diskdrive....
so it may be a problem of the bus where the chips are linked together
(i.e. a prolem with the PCB and the traces somewhere or bad filtering capacitor that does not
eliminate noise from the bus .... ) or one or more chips affect the operation of the other chips....
to me it seems to be resulting from a bad IWM chip.... but that is just a suspect that must get confirmed
by examination with Oscilloscope... )
speedyG

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X5900's picture
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Joined: Mar 27 2015
Posts: 14
Re: Apple IIc RAM Repair

Thank you, speedyG and sorry for being rude, not answering sooner. It was in fact the IWM chip, Sams Repair Guide had some informations on how to test it. I'm trying to get hold of a replacement, which proofs more difficult as I thought.