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From: Clocky on 29 Jun 2010 06:19
Dombo wrote:
> Clocky schreef:
>> Ian McCall wrote:
>>> OK - looks like my breadbox-style C64 has died. I power it up, and
>>> nine times out of ten I get gibberish characters on the screem. Few
>>> things load, either tape or disk. It won't read from the MMC64 I've
>>> been using for ages either, and that also has gibberish characters
>>> on the menu.
>>> Sound likely to be a RAM error to anyone else? Any checks that can
>>> be done, and is the prognosis terminal? Am guessing yes, but you
>>> never know.
>>>
>>
>> Nothing is terminal ;-)
>>
>> It does sound like a RAM problem it it is intermittent, but have you
>> reseated the VIC-II and any other socketed chips? You might also
>> want to check the power supply as voltage ripple can take out the RAM
>> before
>> anything else.
>
> Based on my experience problems with RAM chips are either bit failures
> (symptoms: some characters are might wrong, crashes with some or all
> programs) or a shorted chip (resulting in a dead C64). Reseating chips
> and checking the power supply are good tips, which I would try first
> before looking further.
>
>> A shorted memory chip will get very hot when compared to the other
>> chips so do a feel test to see if any are hotter then the others.
>
> If it is shorted chip (getting very hot) I wouldn't expect
> intermittent symptoms but rather a totally dead C64.
>

Depends entirely on which chip has failed. On the breadbox particularly that
has a number of RAM chips a single shorted chip won't prevent the C64 from
starting up.

>> If any are cooler then the others it may have gone open circuit,and
>> an easy test would be to clip a spare DRAM chip over the suspect
>> faulty one piggyback style and see if it corrects the problem.
>
> If does correct the problem it does tell that the RAM chip has gone
> circuit (have never sen this happen), however if it doesn't correct
> the problem you can't say anything about the state of the RAM chip;
> it might be good or it might be bad.

A RAM test would show which DRAM was faulty in that case, however, my
experience with piggybacking known good chips over suspect ones has been
positive (and I have repaired a lot of 8 bit equipment).
My last repair involved isolating a suspect address line on a 6502 and
piggybacking a known good 6502 over the suspect one to confirm the
diagnosis.
The console worked perfectly with one CPU piggybacking the other. Parallel
processing ;-)


From: Clocky on 29 Jun 2010 06:21
Sam wrote:
> On 28 jun, 16:55, Ian McCall <i...(a)eruvia.org> wrote:
>> OK - looks like my breadbox-style C64 has died. I power it up, and
>> nine times out of ten I get gibberish characters on the screem. Few
>> things load, either tape or disk. It won't read from the MMC64 I've
>> been using for ages either, and that also has gibberish characters
>> on the menu.
>>
>> Sound likely to be a RAM error to anyone else? Any checks that can be
>> done, and is the prognosis terminal? Am guessing yes, but you never
>> know.
>>
>> Cheers,
>> Ian
>
> I think it's the PLA who is bad. I've seen this symptoms before.
>

Yeah, but a bad PLA doesn't correct itself once it has gone bad.


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