Keyboard columns 1, 3 and 5 not working - shorted

[Mince]

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!).

[1024MAK]

I can’t see how the keyboard connector would cause the symptoms you describe (assuming you have indeed confirmed that there is no old membrane tails left in it or indeed any other conductive debris).

When you say you did your testing, you said that this was with the keyboard disconnected, did you disconnect both the five way and the eight way membrane tails?

If yes, then the input circuit to the ULA from the keyboard is comprised only of the five way keyboard connector, the resistor pack RP3, the ULA, the ULA socket and the PCB tracks/board.

Apart from the ULA, which chips are socketed on your board?

Mark

[Mince]

When I did the continuity testing (to find the shorts between 1, 3 and 5), I did it without RP3 installed, nor the ULA (vLA), nor the keyboard with both connectors. That's what I was confused about, as there didn't look anything else involved.

Anyway, I have found the problem and it was the keyboard connector — well, not it itself but the way it was installed: I noticed that one of the shorts seemed to stop when I pushed the connector in one direction. After removing it, they all went away. There was nothing particularly wrong with the connector, however.

On the board side of the connector (i.e. the side opposite to where you push the ribbon in), the metal of the connectors between the pins and the leafs in the socket is exposed and I think some solder had dribbled down through the holes and was touching the tracks on the board. Because of the way the tracks go diagonally under the socket, I think the positioning was such that the underside of the first leaf touched the third track, then the third leaf the fifth track:

I've cleaned up the excess solder and resoldered the connector without it in a position that leaves a bit of clearance between it and the board by wedging some PVC tape in it, whilst I resoldered it. I suspect I reflowed the solder and bent the connector over, when I found it a bit wobbly, at some point, and messed it up. I'm not sure why it used to only happen after a few minutes but I guess something expanded or flexed.

It all appears to be working now, so thanks for looking!

[1024MAK]

Heat expansion maybe.

Anyway, glad to hear that you are now sorted

Mark