ZX81 Keyboard

Discussions about Sinclair ZX80 and ZX81 Hardware
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Bill H
Posts: 163
Joined: Sat Nov 27, 2010 6:05 pm

ZX81 Keyboard

Post by Bill H »

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?
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1024MAK
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Re: ZX81 Keyboard

Post by 1024MAK »

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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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Bill H
Posts: 163
Joined: Sat Nov 27, 2010 6:05 pm

Re: ZX81 Keyboard

Post by Bill H »

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
Bill H
Posts: 163
Joined: Sat Nov 27, 2010 6:05 pm

Re: ZX81 Keyboard

Post by Bill H »

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.
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1024MAK
Posts: 5103
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 Keyboard

Post by 1024MAK »

Good to hear that it’s fixed :D

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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