Following on from the previous post where I’d removed the SID and desoldered the PLA, and with a Commodore 64 that was still exhibiting a fault, I moved onto the next stage of the diagnostics, removing and testing the DRAM.

This was a fairly straightforward task, and mostly covered in the video. At the end, it appeared that I was still having issues with the address lines on the CPU, so the next stage was to remove the MOS logic chips. These have a reputation for failure.

The MOS chips have a different numbering system from the 74xx standard I’m used to. Thankfully there is plenty of information on these online. I removed the following chips:

• U8: 7406 Hex Inverter
• U13 and U25: 74LS257 Multiplexor
• U15: 74LS139 Demultiplexor
• U27: 74LS08 Quad AND

Any non-MOS chips I left in for the time being.

I’d discovered a few weeks ago that the TL866II+ Eprom Programmer I use is able to test logic chips. I ran them all through the tester and they all passed, with the exception of one of the 74LS257 chips. I’ll be replacing them all with non-MOS chips.

After all that, I was still seeing spurious signals on the address lines of the CPU. It’s difficult to test in-situ, so the next step will be to desolder the CPU and test on a breadboard using a NOP test circuit.