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I have a CoCo 2 (MODEL 26-3027 upgraded with 64K of RAM) with a very weird fault. If you run the machine at double speed (POKE65495,0 or POKE65497,0) the O key stops working. However, if you keep the shift key held down and press O, it works... So far I have tried replacing the keyboard, re-seating the 6822 PIA chip, replacing the SAM chip, and replacing the processor. Does anyone have any idea what could be causing this fault?

I discovered that the problem was caused by the 6822 PIA chip failing to operate correctly when the machine was running at double speed. Luckily I had a spare PCB with a SC67331 PIA chip. I desoldered the SC67331 from the spare PCB, and fitted it in place of the old 6822 chip. The machine is now working perfectly.