Keyboard columns 1, 3 and 5 not working - shorted
Posted: Fri Oct 13, 2023 1:28 pm
Hello! First time caller... I have a problem with three of the 'columns' on the keyboard of my Issue One ZX81...
The columns with keys 1, 3 and 5 aren't working (so also 6, 8 and 0; Q, E and T; A, D and G, etc.). When I push them, the screen flickers briefly, same as it does when you push a key, but nothing gets printed, so I think "something" is happening but not the key being pressed correctly.
The keyboard is a modern replacement and looks to be in a good condition and was working until recently (although the fault did only seem to occur after it had been on for a few minutes, until recently). During the investigation of the problem, I managed to blow up the ULA by putting it back in the wrong way (with the notch at the bottom, assuming it would be aligned with the socket and was stupidly too lazy to check!) — I only get a white screen with that in, so I replaced it with an vLA which gives me back a working machine, but the same keyboard problem.
I then got my multimeter out and measured the resistances across RP3 between the common end and each of the pins feeding the 5-way keyboard connector: the working second and fourth columns have the usual 10kΩ but the faulty columns are only reading 3.3kΩ. So I thought RP3 was faulty and ordered a replacement then changed it over, getting the same result. I tested the resistances with it soldered in and am getting the same 10kΩ/3.3kΩ values. Before soldering it in, and removing it again and checking, the replacement resistor pack is getting the correct 10kΩ between common and each pin, so that looks OK.
With RP3, the keyboard and ULA removed, I'm getting only 0.2-3Ω between the pins for columns 1, 3 and 5 so something is shorting between them and it's not the ULA or keyboard itself. I don't get this for columns 2 and 4.
I can't spot anything on the board which might be causing this but it's really odd that it's affecting alternate columns: if it was just something like a wire or solder across the tracks, I would expect adjacent ones to short.
Being an Issue One, the tracks between the keyboard and ULA socket take a bit of a circuitous route (http://www.breakintoprogram.co.uk/hardw ... 1/hardware — not mine!), under the ROM and some resistors.
I did find another thread about someone having bits of broken old keyboard ribbon in the socket but I can't see any and it seems odd it would be alternate pins.
That said, I think my next step is probably to desolder the keyboard connector and see if it's that as I can't see anywhere else where there might be shorts. Given the age of the board, however, I'd appreciate some places to start looking as I'm worried about damaging something else (as I've done with the ULA!).
The columns with keys 1, 3 and 5 aren't working (so also 6, 8 and 0; Q, E and T; A, D and G, etc.). When I push them, the screen flickers briefly, same as it does when you push a key, but nothing gets printed, so I think "something" is happening but not the key being pressed correctly.
The keyboard is a modern replacement and looks to be in a good condition and was working until recently (although the fault did only seem to occur after it had been on for a few minutes, until recently). During the investigation of the problem, I managed to blow up the ULA by putting it back in the wrong way (with the notch at the bottom, assuming it would be aligned with the socket and was stupidly too lazy to check!) — I only get a white screen with that in, so I replaced it with an vLA which gives me back a working machine, but the same keyboard problem.
I then got my multimeter out and measured the resistances across RP3 between the common end and each of the pins feeding the 5-way keyboard connector: the working second and fourth columns have the usual 10kΩ but the faulty columns are only reading 3.3kΩ. So I thought RP3 was faulty and ordered a replacement then changed it over, getting the same result. I tested the resistances with it soldered in and am getting the same 10kΩ/3.3kΩ values. Before soldering it in, and removing it again and checking, the replacement resistor pack is getting the correct 10kΩ between common and each pin, so that looks OK.
With RP3, the keyboard and ULA removed, I'm getting only 0.2-3Ω between the pins for columns 1, 3 and 5 so something is shorting between them and it's not the ULA or keyboard itself. I don't get this for columns 2 and 4.
I can't spot anything on the board which might be causing this but it's really odd that it's affecting alternate columns: if it was just something like a wire or solder across the tracks, I would expect adjacent ones to short.
Being an Issue One, the tracks between the keyboard and ULA socket take a bit of a circuitous route (http://www.breakintoprogram.co.uk/hardw ... 1/hardware — not mine!), under the ROM and some resistors.
I did find another thread about someone having bits of broken old keyboard ribbon in the socket but I can't see any and it seems odd it would be alternate pins.
That said, I think my next step is probably to desolder the keyboard connector and see if it's that as I can't see anywhere else where there might be shorts. Given the age of the board, however, I'd appreciate some places to start looking as I'm worried about damaging something else (as I've done with the ULA!).
