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Old 25th Jun 2022, 8:07 pm   #30
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 4,678
Default Re: Dead commodore 4040 disc drive

If it is a 6502, which I doubt.

The circuit diagrams I've found for the 4040 show that the 'interface' processor is a 6502 at location UN1 but that the drive processor is a 6504 (28 pin package) at UH3 (roughly in the middle of the PCB). It's the latter chip you need to test.
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