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 Post subject: Faulty c64
PostPosted: Tue Oct 16, 2007 12:52 am 
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Joined: Tue Oct 16, 2007 12:33 am
Posts: 2
Hi all,
don't know whether this is the right forum, anyway...
I'm trying to repair a c64 that's driving me crazy.
It's not the first time I do this kinda things, this should be the fifth
board I got right.
The facts:
At first the board gave just a blank screen.
I resocketed all the RAM chips and put some 41256, perfectly
working and (for now) grounded pin 1 (A8).
I changed the faulty 8701, I've found it checking the chips
on my full socketed test board.
Now as I boot the board I get a screen with a lot of colors:
there are stable raster bar on the border and sometime even
in the background (timed and coloured differently from the border).
Characters are present but the charmap look wrong (but stable, they
do not change). Sometime I get a black background with a big sprite
in it (looks like that mode you can get by setting two bits on VIC,
ECM and MCM I guess...).
All socketed chips were tested so PLA, VIC, the 3 ROM, CIAs,
and 6510 are known to be fine. I'vent inserted the SID yet, as it's not
necessary for the board to work.
I've got a multimeter and a little digital scope... if you have any hints...
Hope you can help me!
Thank you in advance for you attention.
Bye!


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 Post subject:
PostPosted: Tue Oct 16, 2007 10:23 am 
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Joined: Thu Jan 12, 2006 1:52 am
Posts: 203
Location: Denmark
Sounds a bit like the VIC gets chip selected when it shouldn't.
Check the 74LS139 (U15) and check for possible shorts between A8-A11.

Still.. it can be a RAM error, so you should also verify the operation of the two 74LS257 (U13 and U25). There's a 7406 connected to U13's /OE.. Check that one too.. the 7406 are prone to failure after 20 years of operation ;-)


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 Post subject:
PostPosted: Tue Oct 16, 2007 7:59 pm 
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Joined: Tue Oct 16, 2007 12:33 am
Posts: 2
It was U15! Just resoldered and it works like hell.
Another c64 back to life.
Thank you very much and shame to me... I didn't even
think to check VIC CS...
I used the scope to compare the pin signal of a working
unit and the one of the broken. The working unit hardly
put it low where the one in the broken unit looked like a clock!!!
See you and still thanks!


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 Post subject:
PostPosted: Tue Oct 16, 2007 10:50 pm 
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Joined: Thu Jan 12, 2006 1:52 am
Posts: 203
Location: Denmark
Well, you're welcome ;-)


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 Post subject:
PostPosted: Fri Oct 19, 2007 10:49 pm 
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Joined: Wed Jan 11, 2006 12:22 pm
Posts: 845
Impressive error tracking skills there, Devia! 8)


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