Sinclair ZX80 / ZX81 / Z88 Forums

Moderators note: Topic split out from this topic Z80 cursor but no text. Mark

This is the same problem I have; in fact, I now have it on two machines.

These are original version 2 ZX80's, the first one was a USA model, I removed the diode, changed the A-B-C link and replaced the RF box. At first, I was getting nothing, but then I saw the EPROM was cracked. I swapped the EPROM and CPU from another ZX80 and then found I have a good picture, but no text.

After a few weeks had passed, I put the EPROM and CPU back in the donor machine, only to find that machine has now got the same issue. I've tested the EPROM and CPU in a third ZX80 and found they both work fine.

I'm sure(-ish) I saw someone, somewhere with the same issue, and they replaced IC9, but I've been told that IC9 would not cause that problem.

I have a basic oscilloscope, but I don't know what I should be seeing on the IC9 pins.

cursor is made by inverting the video signal cat xor gate (ic20) commanded by D7.
Characters are made by shift register, IC9. Check if this IC is receiving both clock and load signals.

Thanks for the reply.

I have around 6.4MHz clk on pin 2 and it looks like some data on D7.

Just to add, I removed IC9 and tested it, it works fine. I've installed a new IC and it's the same.

is there any signal on pin 1 (shift/load)? if so check for signals on the outputs of the shift register.
Also check if the bridge for selecting normal/inverse video is ok.

Yes, see what signal you get on IC9 outputs, pins 7 (/Q) and pin 9 (Q).

if your 'scope is a two channel type, you can try to sync to IC9 pin 1 (S/L) for per character or to SYNC (IC19 pin 6 or R32).

If you are getting an output signal from IC9, next up, is to test IC20 pins 4, 5 and 6.

Mark

I had a similar problem with my 80 one in which had no display after removing the CPU to try in another machine then putting it back. No matter how many CPU's I tried the problem persisted.

The answer lay in the socket itself which was of the small hole type where the socket pins are hidden and whilst the sharp points of a multi meter probe showed continuity when pressed hard on the pins and lower pads, I discovered that the sockets pins were so tarnished internally that the press fit of a CPU into the socket meant some of the pins weren't making a proper electrical contact whereas the the points of the probes easily pushed through the tarnishing to make contact and give a meter reading.

I removed the socket, broke open the plastic and indeed the pins were badly tarnished so one new socket later order was restored and the ZX80 was all working again.

I would add at this point that all the above was at the instigation of Mark(who has my grateful thanks) who persisted that I look at socket whereas all I wanted to do was throw the computer in the bin in frustration.

May not be the answer to your issue but worth looking at I would have thought.

Thanks again for the replies. I've been busy trying to fix other things, but now I'm back to these ZX80's

I only have a Hantek 6022BE USB oscilloscope and Hscope software (Which I might not even be using properly), but this is what I've found.

IC9 Pin 1, 3.7V flat-ish looking.
IC9 Pin 2, 1.5V looks like a clock.
IC9 Pin 7, 4V noisy looking.
IC9 Pin 9, nothing.

@Moggy, that sounds plausible, I will see if I can check.

I've tested both the ROM socket and the CPU socket. I tested them with the chips in place, making sure there is a circuit from the chips leg to the underside of the board.

RetroByte wrote: ↑Wed Jan 20, 2021 5:05 pm I've tested both the ROM socket and the CPU socket. I tested them with the chips in place, making sure there is a circuit from the chips leg to the underside of the board.

I too had a circuit showing when the nice sharp probes were pressed against the legs of the chip and the underside but the tarnish inside the socket clips was just enough to stop the CPU working until I swapped out the socket.
Remember too that the CPU has connections on top of the board. Have you checked that?

Are the sockets they type where you can easily see the tops of the socket pins when the chip is removed or the very small hole type where all you can see is the hole? I found that the latter type tarnishes very easily.

May not be the answer I know but worth pursuing as it saved my ZX80 from the scrap pile.