Help me with keys 1-5! (new membrane etc)

[Erm]

Hello,

I have a ZX81 problem. Keys 1-5 do not work.

Keyboard membrane works, and is new. Multimeter connected to the far left of the right k/b socket (the longer one) and to the far right of the left k/b socket (KB0) indicates continuity when button '1' is pressed, which is what it should do. It's a pokemon keyboard so I am able to verify that there is a connection between the PCB and the keyboard via the useful exposed contacts on the back.

Diodes seem to be ok (according to my multimeter).

The affected 1-5 keys go through D1, up to the edge connector and to pin 1 of the Z80 (all ok, there is continuity). From there it goes to pin 18 of the ROM, also fine. And I've re seated the chips and checked actual continuity to the legs on the chips not just the solder underneath.

Resistor pack RP3 seems to only measure about 6k instead of 10k, but all the other keys work so I wouldn't have thought that was the issue?

I've swapped all chips with others and also tried it with a RAM pack plugged in. Always the same result, even though I don't absolutely know 100% that all of the replacement chips are ok, I wouldn't have expected the same 1-5 key problem with each chip if you know what I mean.

Any ideas? Thanks.

[1024MAK]

The only chips involved with the keyboard are the high address lines from the Z80 MPU (A8 to A15) each via a diode, and the ULA.

If only keys 1 to 5 don’t work and all other keys do work, then the problem has to be the row select line for these five keys.

The row select, originates as A11 from the Z80 MPU. The A11 output from the Z80 MPU must be working, otherwise the computer would crash.

So the problem is either diode D1, a dry solder joint, a broken conductor (PCB track) or poor contact. Have you checked inside the keyboard connectors for bits of old membrane or bent/dirty contacts?

Grab a piece of thin sold core wire, then use it to briefly bridge across from the contact of the 8 way keyboard connector (the contact that goes to D1 anode) and any of the contacts in the five way keyboard connector. Does this result in a keystroke being registered by the computer?

See the schematic (a copy is here).

Mark

[Erm]

It really was that simple in the end! Despite multimeter showing good connections, I suppose something must have been wobbly. I removed the wonky keyboard connector, re-soldered, fiddled around some more and all is ok now. Thanks.

[1024MAK]

Great to hear that you are up and running

Now go and have fun with your Zeddy

Mark