K - then black screen

Discussions about Sinclair ZX80 and ZX81 Hardware
Post Reply
citizenfish
Posts: 30
Joined: Sun Apr 11, 2021 5:31 pm

K - then black screen

Post by citizenfish »

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?
User avatar
1024MAK
Posts: 5104
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: K - then black screen

Post by 1024MAK »

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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
citizenfish
Posts: 30
Joined: Sun Apr 11, 2021 5:31 pm

Re: K - then black screen

Post by citizenfish »

Keyboard disconnected.

No blank as in a completely black screen as if no signal
User avatar
1024MAK
Posts: 5104
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: K - then black screen

Post by 1024MAK »

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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
citizenfish
Posts: 30
Joined: Sun Apr 11, 2021 5:31 pm

Re: K - then black screen

Post by citizenfish »

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
Lardo Boffin
Posts: 2161
Joined: Sat Nov 26, 2016 2:42 am

Re: K - then black screen

Post by Lardo Boffin »

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
:D
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
User avatar
1024MAK
Posts: 5104
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: K - then black screen

Post by 1024MAK »

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 :mrgreen:

Thanks for letting us know what the problem was :D

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Post Reply