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ZX81 repair help needed - strange fault!

Posted: Thu Mar 26, 2020 8:05 am
by vossi
Hello,
I hope to get some help here - I normally repair Commodore or Atari - Sinclair is new for me ;)

One of my two zx81 shows the "K" but the second pixel from right is a vertical line (white because of the reverse K).
If I press any button I get these verical stripe over the whole screen and they roll fast vertical over the screen.

I socketed the rams and the rom and replaced all chips with tested working chips from my second zx81.
No change!

What to hell could be defective that it starts but has this crazy fault?

greets
Christian

Re: ZX81 repair help needed - strange fault!

Posted: Thu Mar 26, 2020 10:54 am
by Lardo Boffin
Did you swap the ULA for a known working one?

I have had zeddies which were fine until a key was pressed at which point they had various failures.

Re: ZX81 repair help needed - strange fault!

Posted: Thu Mar 26, 2020 12:21 pm
by 1024MAK
I assume you tried the ULA and ROM from the faulty ZX81 in your known good ZX81?

On your faulty ZX81, was this fault present before you carried out any work on it?

Have you carefully visually examined the board, paying particular attention to the tracks near the chip pins. You are looking for broken/cracked tracks, dry joints and solder splashes. It looks most likely like a data line is being read as logic low by the system.

Mark

Re: ZX81 repair help needed - strange fault!

Posted: Thu Mar 26, 2020 12:59 pm
by vossi
Hi, I tested all chips in the working zx81.
I stored the faulty zx81 about 3-4 years ago in working condition. I replaced at this time the old ULA to the new model 210E for a better picture.

When I now powered on the zx81 the first times I was able to press a 0 or some other keys - all chars appear with a black vertical line. After some chars the machine hangs and has a frozen picture.
After changing the ULA I have the now present effect...

I check the traces and esp. the datalines...

Re: ZX81 repair help needed - strange fault!

Posted: Thu Mar 26, 2020 5:50 pm
by vossi
Thanks for the help.
I found the fault. The ULA-socket was bad.
Christian

Re: ZX81 repair help needed - strange fault!

Posted: Thu Mar 26, 2020 7:33 pm
by 1024MAK
Fully working now?

If yes, well done :D

Oh, and welcome to the forum :D

Mark

Re: ZX81 repair help needed - strange fault!

Posted: Wed Apr 22, 2020 6:53 pm
by Jearland
vossi wrote:
Thu Mar 26, 2020 5:50 pm
Thanks for the help.
I found the fault. The ULA-socket was bad.
Christian
Hi Christian, forgive the daft question, I'm new to this work! How did you diagnose the fault was the socket? I have a potential problem which I think is perhaps the fault of the socket but how do I test that? hope you can help.

Re: ZX81 repair help needed - strange fault!

Posted: Fri Apr 24, 2020 9:59 am
by Paul
My guess: Desolder the old socket and solder in a new turned pin socket.
If you don't have good equipment it's much easier to destroy the old socket and desolder pin by pin.
Kind regards Paul

Re: ZX81 repair help needed - strange fault!

Posted: Fri Apr 24, 2020 9:42 pm
by 1024MAK
The cheap and horrible sockets used in the early 1980s computers are known to cause problems.

As well as ZX80, ZX81, TS1000 computers, Acorn Atom and BBC Micro computers are also affected.

With the machine disconnected from the power supply, use a multimeter on either the continuity mode or on a low resistance range to test between each pin of the chip in the socket and the chip or component that is the source or destination of the signal line. For a 40 pin chip, that’s 40 tests to do...

Mark