Sinclair ZX80 / ZX81 / Z88 Forums

HI all - I have a perplexing issue, at least to me. I have a ZX81 where the keys 1 - 5 are not working. I pulled it apart, trimmed the membrane etc and they still don't work. If I use my multimeter I can tell that they are working though. With cover off and no power I touch the row and column solder points on the motherboard where the membrane connects and press the 1 key and I get continuity, don't press, no continuity. All else works - it starts, does memory test, all other keys work etc. I swapped ULA to see if that was an issue but no go. I have done the 16k internal mod on it (62256 chip wired to use 16k) and it appears to pass the memory test (it takes a bit to start up) and I changed the jumper from L1 to L2 so it would work.

Anyone have any idea where else to look?

Diode D1 and it’s connections. Also test between it’s cathode (the end with the stripe) and Z80A pin 1. And between it’s anode and the relevant pin of the eight pin keyboard ribbon connector.

Mark

1024MAK wrote: ↑Wed May 19, 2021 8:50 pm Diode D1 and it’s connections. Also test between it’s cathode (the end with the stripe) and Z80A pin 1. And between it’s anode and the relevant pin of the eight pin keyboard ribbon connector.

Mark

That what I was thinking but I wanted to get a second opinion

1024MAK wrote: ↑Wed May 19, 2021 8:50 pm Diode D1 and it’s connections. Also test between it’s cathode (the end with the stripe) and Z80A pin 1. And between it’s anode and the relevant pin of the eight pin keyboard ribbon connector.

Mark

Thanks Mark that was it. The D1 connector for the keyboard was no longer connected to the diode. I just had to put a drop of solder to bridge them.

Good to hear that it’s fixed

Mark