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TonyDuell · 25th Jun 2022, 9:07 pm

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.

25th Jun 2022, 9:07 pm	#30
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,215	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.