Amiga 500 Keyboard

[r.cade]

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?

[Daedalus]

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.

[r.cade]

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

[Zippy Zapp]

Hey I have a similar problem with an A500 keyboard. First I noticed the caps lock flashing 3 times. Which If I understand correctly could mean controller or watchdog timer. But then sometimes it works perfectly. And then sometimes it also is locked up with solid caps lock LED lit. Usually if it is functioning at bootup then it stays functioning.

What did you look for when you fixed this? Did you use an oscope to diagnose?

I noticed the keyboard controller chips are quite pricey. iComp has some but unfortunately their shipping is 4-5x higher then normal to the USA, right now. 43 euros vs normally 9 euros.

Has anyone made a replacement chip? I wasn't sure if the kipper2k keyboard had a replacement or not.

Thanks!

[r.cade]

If you don't have tools or time to diagnose it, just replace the few electrolytic caps on the board and clean up anything they have leaked out. 9/10 times it will fix the problems.

[Zippy Zapp]

Good idea. I'll replace those first and see what happens. I will try reflowing anything that needs it. I do have the tools to diagnose, including an oscope and logic probe I just don't exactly know what the operational flow is and what is supposed to do what. I am still learning those aspects. I did find the Mitsumi A500 schematics on github so I will check that out too.

Hopefully it is not the microcontroller because they are a bit costly.

Thanks!

Edit: After looking at my board, all the caps are mostly ceramic. C10 and a couple others are tantalum, no electrolytics. I will test those tants in case they are failing or shorted.