Sinclair ZX80 / ZX81 / Z88 Forums

Thanks

The black bars and the SYNC signal come and go, so maybe a bad socket connection.

While the SYNC is missing I get:

A14 looks like data
A15 Nothing

/MREQ looks like a nice square wave, so yes it looks like it's pulsing.

Hi Mark Thanks for your help again. The fault has changed because I noticed lots of muck on the back of the board, especially lots of brown hardened flux between solder joints. After trying a few cleaners that didn't seem to help, I pulled out all the ICs and washed the whole board in hot water (It ...

Yes, and thanks, I'm very pleased

There doesn't seem to be a drop on IORQ and I'm getting a 16k ohm reading between CPU pin 20 (IORQ) and VCC, which isn't there on working machines.

I swapped the CPU and now have an active M1, but still no picture.

My ZX80 that was working, has lost it's picture. I think this machine was built from the kit, because every IC is socketed.

Just checking a few things with my oscilloscope and limited knowledge, I believe the video SNYC is missing.

I've also found that on the Z80 CPU, M1 is always high.

FIXED!

I removed IC20 and found that it still had the 70 ohm short between Pin 3 and Pin 7 (Earth). Replaced it, and the clock voltage jumped to 1.4V and the text is back

For those interested and following this, I've found that there is a 70ohm reading between IC20's Pin3 and earth on this faulty machine but 160K ohms on the working, so it looks like something is pulling the clock voltage down.

Hi again I've just fixed one of these with the same issue, it turned out to be IC9 74LS165N. As I mentioned in the other topic, my donor ZX80 that I borrowed the CPU and ROM from that was working fine, suddenly also lost it's text. So I replaced IC9 on that one, thinking it would fix the issue, but ...