Actually, 99% of probelms in old PETs are due to the
terrible IC sockets
used. I find it best to replace the lot with good turned-pin ones. That
may well cure this fault, if not, at least you know that's _not_ the
problem (nothing worst that trying to trace logic faults on a PCB with
bad connections!).
Tony, believe in me: Turned pin sockets are an unnecessary spent money
The actual sockets are very good for anything. When the pet was made,
the socket technology was awful, but the nowadays sockets are good enough to
rival the turned pin "garry" sockets. Believe me!