Sinclair ZX80 / ZX81 / Z88 Forums

I've got an issue one board that has been composite modded.

It had a socketed Z80 and was giving a white screen on power up. I've swapped the Z80- for another chip and now I get a K on screen for 1/2 a second, it then goes blank. Any idea what this could be?

Blank as in, back to a all white screen?

Normally if the machine can make it to displaying the K, everything must be working.

Was this with the keyboard connected? If yes, try disconnecting the keyboard.

Mark

Keyboard disconnected.

No blank as in a completely black screen as if no signal

citizenfish wrote: ↑Tue Jun 29, 2021 6:55 pm Keyboard disconnected.

No blank as in a completely black screen as if no signal

Oh dear, that’s pointing to a faulty ULA. But also check that the +5V supply is within specification (4.75V to 5.25V). Also visually check the ULA for dry or cracked solder joints, and if the ULA is in a socket, that all socket contacts are good.

Mark

Turned out this was a faulty composite mod. The board had been previously modded and looked a bit of a bodge. I replaced with a Rude dog Retros unit and now have a lovely clear K.

This ZX81 fixing is addictive. One more to go

citizenfish wrote: ↑Fri Jul 02, 2021 12:50 pm Turned out this was a faulty composite mod. The board had been previously modded and looked a bit of a bodge. I replaced with a Rude dog Retros unit and now have a lovely clear K.

This ZX81 fixing is addictive. One more to go

citizenfish wrote: ↑Fri Jul 02, 2021 12:50 pm Turned out this was a faulty composite mod. The board had been previously modded and looked a bit of a bodge. I replaced with a Rude dog Retros unit and now have a lovely clear K.

This ZX81 fixing is addictive. One more to go

Ahh, you have discovered the warm fuzzy feeling

Thanks for letting us know what the problem was

Mark