[BBC-Micro] One more BBC B to fix
mjfoot at paradise.net.nz
Tue Dec 12 07:55:49 GMT 2006
With lots of help from Peter Coghlan, I now have this Model B working again.
The faulty part was IC63, the RAM chip for bit 2 when in 32K mode. What was
happening was that the machine would only boot when in 16K mode and the +5V
lead near the ram was unplugged. When a graphic mode was selected, the text
would appear yellow and the colour blue could not be chosen.
Peter rightly diagnosed that the VideoULA was always seeing bit 2 as being
set. IC63 was the problem and due to leakage was causing bit 2 to stay high
and was also corrupting the data bus causing all sorts of random behaviour.
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
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