Sinclair ZX80 / ZX81 / Z88 Forums

Hi to all from Ontario Canada.

I have been a Sinclair fan since the early 80 TS, I won a brand new ZX81 in 1981 at a open house where my wife worked ( Nortel Networks ). I got a renewed interest in the ZX upon the death of Sir Clive Sinclair. I searched in my storage area and found my Zeddy still in the box that it came in, so I decided to see if it still worked. I was elated when that farmilar K came up on the old monitor ( green ) screen and all, I had done a mod on the video years ago. So I decided to do some up grades to it, a new improved composite mod, a modern voltage regulator replacement, a RAM upgrade with a 62256 SRAM, now has 16K internal ram, and new CPU a Z84C0008PEG, and finally a new keyboard membrane. I have typed in a couple of programs over the last month, what a joy to do
I started an new project last week, a friend gave me a TS1000 in its box, he told me that it no longer worked. I was eager to tear into it and see if I could fix it, I opened it up and the first thing I seen was a red coloured issue 3 board, I removed the two screws that hold the PCB in and you guesed it a cracked membrane. So I did some checking over and it has the 2C210E ULA date 8249, a Z8400A Z80A CPU dated 8249, a Motorola ZCM38818P ROM dated 8302, and a Toshiba 2016 2Ke Ram. I went ahead and done a composite video mod to see if I could get a screen up, I was happy to see that K cursor. I next did a repair to the keyboard membrane, you know cut the end, I could not see anything else wrong with the membrane, so I reassembled the unit, and pluged it in and up came the K cursor. I decided to type in a ram check program, but to my suprise I had no keyboard function? nothing not a number or letter or keyword anything! Thinking that it was a faulty keyboard, I went ahead and done a RAM upgrade just like in my ZX81, and plugged it in, and after 6 - 8 seconds up come the K cursor. I reassembled it again thinking maybe the original RAM chip was at fault, and tried it again and still had no keyboard function. I got a brand new keyboard in and installed it thinking that this would cure the problem, same result though. I am at a loss to know what the problem is, I have rechecked everthing even tried the jumper wire thing, double checked all the address lines. Can anyone suggest what the problem is? Take care now EH!
Cheers.
RJ

Hello and welcome to our forum RJ!



I hope you enjoy this forum

Quantum wrote: ↑Fri Dec 17, 2021 4:52 amI got a brand new keyboard in and installed it thinking that this would cure the problem, same result though. I am at a loss to know what the problem is, I have rechecked everthing even tried the jumper wire thing, double checked all the address lines. Can anyone suggest what the problem is?

Have you ensured that there is no part of the old membrane stuck in the PCB connectors? A short circuit means the system will not see any key presses.

With the power off, use a multimeter to test each of the contacts on the five way connector to +5V and to 0V. And to each other. Do this with the ULA fitted. If the ULA is in a socket, and you get a strange reading, try carefully removed the ULA. If you have never removed a DIL/DIP chip from a socket before, ask for advice before doing so, as a bent or broken pin needs to be avoided.

Mark

Hi Mark,
Thank you for your message. I am sure that I will enjoy the forum.

I have cleaned and blew out the connectors and can see down in side them with my maglight, I have inspected and tested all the traces and can find no faults. I am begin to suspect the ULA, if I had another one it would be the next step in my mind. What do you think?

Cheers.

RJ

I have an issue three that displays the same problem insofar as known good new membranes and the key shorting method fails to produce anything but works ok when a Memotech external keyboard is attached via the expansion slot, could that be part of the ULA failing? The only reason I have left it alone is because the ULA isn't socketed so I make do with the external keyboard.

Not sure if that's of any use to you?

The keyboard matrix electrically is made up of two parts. The Z80 supplies eight address (A8 to A15) lines via diodes (D1 to D8).

Then when a key is pressed, the signal from one of the eight address lines is connected to one of five input lines.

Each of these five input lines is normally held in a logic high state when no keys are pressed by 10k resistors (RP3 or individual resistors).

The input lines go to the ULA. When a key is pressed, during the time that the relevant address line is logic low, this is when the ULA will detect the key press.

If the part of the ULA that detects the inputs, or which outputs the data to the Z80 is damaged or faulty, then this would cause the problem.

Unless you have an oscilloscope, the only easy way to prove if it is the ULA is to swap it to another board. Then see if the fault follows the ULA.

Mark

I had a zeddy with a dodgy ULA - the moment any key was pressed it crashed out the computer, otherwise it booted ok and would get as far as the menu with a ‘Spand plugged in. Swapped out the ULA and all was well with it - I used a vLA81 to replace it.

Hello,
Thanks for your post and input. I am going to order a vLA81 in the new year.

Take care and stay safe.

Cheers.
Richard.

I finally got a vLA81 ULA for my TS1000 and I installed it. I installed the membrane on the pcb, the installed it in its case. I then hooked up the computer to the monitor and plugged in the power, and up came the K as before. I then tried a couple of keys, but still no response? I was somewhat dismayed. I disassembled the computer again, ran a couple of logic probes on the Z80 and it seemed OK. I then said what the heck is going on, then I decided to replace the Z80 anyways with a new one, after carefully removing and installing the new Z80 I then just hooked up the pcb to the monitor and powered it up, again the K came up, I then power down and installed a new membrane just to the pcb and powered back up. YAHOO it works now So now I will have to try the original ULA in another ZX81 and see if it is defective, I don't want to remove the vLA81 from the board. I now have a updated ZX81, and a TS1000. Thanks to all that helped me.
Cheers.

Richard.