(..) or there is some bad contact somewhere...Yes, possibly there's a bad contact that's okay when you don't touch anything. When you hit keys, the keyboard presses down on the motherboard (which flexes slightly) -> contact goes bad -> reset / hang etc.
To confirm: power up with keyboard not resting on top of the mainboard. Then use something non-conducting (!) to push lightly in different places on the mainboard. If that causes similar effects: check the solder work or try to find cracks in circuit board traces.
I think it must be something like that, and there is some corrosion/dirt on some parts of the motherboard.
However, it's not a loose connection - if it was, YAKF wouldn't work, and it does.
I just took the keyboard apart and found that some of the switches didn't work. Gave them a quick clean with contact cleaner, and they work now - except that the machine doesn't repsond to them. I think that is a bad contact, though.
Any key which does work, is just causing a reset (which is what is changing the colours). The screen doesn't go black, though, which implies that it's resetting the CPU but not the GPU, or something.
When booting YAKF, on the "MSX system version 1.0" screen, if I press a key (e.g. SPACE), then it does this quick reset thing. As soon as the Konami screen appears, if I press SPACE, then it goes to the game menu, and then starts if I press SPACE again.
In other words, the keys are treated correctly when it's running from the cartridge, but from the firmware it just causes a reset.
Any ideas? Thanks to all who have helped so far!
Just a little update in case it helps others...
the MSX is now working perfectly again. It turned out there was a broken track between the GPU and the 8255 on the A7 line - that was why the right button wasn't working. Very surprised that YAKF worked at all!
I've now repaired two HitBits and one NMS8245.
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