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I recently got an Apple iic with a fault; in Basic any last registered keystroke will register continuously until a new one is made, then that will register continuously. I only have the one machine and can't trouble shoot by swapping parts - what is the most likely culprit here? See attached video. Thanks
Try unplugging the keyboard AFTER it boots. (I don't believe it will boot up without it) See if the repeat characters stop. If so, you will need to troubleshoot your keyboard. If not, then you will need to troubleshoot your keyboard encoder chip and/or the traces around it.
A note of caution... Unplugging the keyboard isn't so bad, but plugging it back in under power is. Turn off the computer before plugging it back in.
Don't unplug your keyboard while the machine is powered on.
You only have one Apple IIc, so why risk it?
In a IIe when this happens it is because one single keyswitch is patially making. And when you press a second key it repeats that second key.
Very likely it is the same issue in a IIc because the architectures are similar.
Check every key doing that repeat trick. There should (or could) be one keyswitch that doesn't do anything. Also, what is the first key that is registered automatically on turn-on? was it the comma? I'd start there and desolder that switch. If the problem disappears then you've found the culprit.
I agree with baldrick's general approach, but before I desoldered anything I'd first do a really thorough cleaning of the keyboard and try some De-oxit on it first.
Thanks all - I was actually able to return the machine to the seller as it was supposed to be working when I got it. I didn't want to do any work to it after that so I didn't get a chance to determine exactly what was wrong, but it seems it was probably the encoder chip or possibly a faulty keyswitch causing the fault.