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Old 26 September 2019, 14:49   #1
r.cade
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Amiga 500 Keyboard

I have an A500 where the keyboard has failed. It is in good physical condition, but when plugged into the motherboard (tried more than one) the CAPS LOCK is solid, no blinking, and no response from any keys. Is this most likely the CPU on the keyboard?
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Old 26 September 2019, 23:36   #2
Daedalus
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There's a watchdog timer circuit on the A500 keyboard controller PCB. If it doesn't detect activity from the main CPU for a certain length of time, it sends a reset signal, which turns on the caps lock LED. If the watchdog itself gets stuck, the reset signal also gets stuck and the caps LED will be stuck on.

I've come across this once before, and it turned out to be a bad capacitor associated with the watchdog timer. I can't remember now which one it was, but I'm pretty sure it was one of the small ceramic ones.
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Old 03 January 2020, 06:08   #3
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I had some time today so I traced it down with my logic probe. I was getting a garbage input on the reset pin of the keyboard CPU, which I traced to the 74LS27, then traced that to the 74LS123 that seems to have both parts cascaded. Finally traced the bad signal to the C10 capacitor that feeds the EXT/RCEXT to the 74LS123.

Replaced C10 and keyboard is alive again!

This doesn't match my keyboard- mine is a Samsung. However the circuit is similar enough (pin designations are wrong).
https://github.com/BleuLlama/AmigaSc...yb_Mitsumi.pdf

Last edited by r.cade; 03 January 2020 at 06:59.
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