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Old 24th Oct 2017, 10:02 am   #5
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: Nascom repair - Looking for clues please

My prime suspect in microprocessor systems of this vintage is almost always the RAM chips. They're by far the most unreliable semiconductor part. Glancing at a NASCOM circuit diagram, it seems to use 2102 chips for both the working and video RAM. They're still reasonably available. If you're stuck, I think I've got a couple of dozen in my arcade spares box (Atari Super Breakout uses them).

The 81LS97 data buffer on the output of the working RAMs is also a suspect. If any of these are in sockets, try swapping them around and see if the symptoms change when running your test program.

The working RAMs would be easy enough to substitute for something newer and more reliable like a 6116, since the circuit doesn't explicitly make use of the 2102's separate data inputs and outputs. However, the video RAM would be more difficult to substitute and is probably better kept original without a significant redesign.

Chris
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