| Posted By
Gabucino
on 2015-09-06
03:59:15
|  Plus/4 repair
Hi guys!
I've been lurking here for quite some time, contributed some missing software. First time in the forum. I'm repairing the plus/4 of my late grandmother, and need some help... See the pictures (these are already after the cleaning stage):
gabucino.hu/tmp/plus41.jpg
gabucino.hu/tmp/plus42.jpg
gabucino.hu/tmp/plus43.jpg
Apparently the video ram from $0DE8-$0DFF is gibberish, and whatever is after that is repeated at $0C00 which is otherwise unmodifiable. Is a RAM chip bad? If yes, I wonder which?
Also two of the 8501's pins rusted and broke, so I'll have to solder the CPU into a separate socket and also replace the main socket due to its sorry state - for now it's working with a borrowed CPU carefully aligned in 
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Posted By
crock
on 2015-09-06
07:13:38
 | Re: Plus/4 repair
Hello!
The design of the +4 architecture is such that a completely failed RAM usually stops the whole machine as each ram chip is wholly responsible for 1 of the 8 bits. It's unusual (but possible) for a RAM to fail across only part of the address space. The fact that you have repeated blocks makes it more likely that it's an addressing problem.
Also, you appear to be able to run software? Did you manage to load something from tape or disk? If so, it makes me think that RAM from the CPU's perspective is OK, so it's maybe just an addressing problem from the perspective of the TED chip. If you have some pins rusted through, I would try to vary carefully check that the TED is seated and making good contact with the socket.
cheers, Rob
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Posted By
Gabucino
on 2015-09-06
08:05:58
| Re: Plus/4 repair
Oh crap, thanks man, it really was the TED and not even bad contacts but plain bad, I had hoped this wasn't the case. Now it can load even Mercenary, which froze mid-loading before. I haven't seen the TED fail this way before. I have a half-bad TED stashed away, it's better than nothing - guess it's time for him to do some more heavy lifting!
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Posted By
Gabucino
on 2015-10-17
04:09:00
| Re: Plus/4 repair
Here comes another one, now this is certainly due to a bad memory chip, but which one? Top part of the screen should be filled T, rest with F.
gabucino.hu/tmp/plus4badmem.jpg
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Posted By
Chronos
on 2015-10-19
03:07:32
 | Re: Plus/4 repair
I bet on U11. I've replaced it one of my plussies when it displayed an "!" somewhere on the screen, and the problem is gone.. My advice: when you desoldered the module, put a socket into place..
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