Non working ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
Posts: 1046
Joined: Sat Nov 26, 2016 2:42 am

Re: Non working ZX81

Post by Lardo Boffin » Sun Oct 06, 2019 11:29 am

If the zeddy puts out a black screen it could be an early ULA. Try turning the brightness all the way up.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
Iss 3 ZXVid
TS 1000 iss 3, ZXPand AY, ZX8-CCB, ZX-KDLX & ChromaSCART
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Various Spectrum 48ks plus a DIVMMC future and SPECTRA

colday
Posts: 13
Joined: Sun Aug 18, 2019 8:28 pm

Re: Non working ZX81

Post by colday » Wed Oct 09, 2019 9:08 am

Well last night I tried the above suggestion, turned up the brightness and contrast but to no avail.

Just a blank black screen.

Occasionally if I pull the power and plug it back in, I get a sort of slightly diagonal set of stripes.

Pressing the keys seems to do nothing and there are no obvious changes on the screen.

What's next on the agenda chaps and chapesses?

User avatar
mrtinb
Posts: 1043
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Non working ZX81

Post by mrtinb » Wed Oct 09, 2019 10:35 am

If you write SAVE"X" and newline, sound output will come on the EAR connector. Try with headphones.

Type SAVE "X" is pressing these keys:

Code: Select all

S shift-P X shift-P newline
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

Moggy
Posts: 1846
Joined: Wed Jun 18, 2008 2:00 pm

Re: Non working ZX81

Post by Moggy » Wed Oct 09, 2019 1:18 pm

It's also possible that the internal RAM is shot. Do you have a known good RAM pack to test with?

User avatar
1024MAK
Posts: 2484
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Non working ZX81

Post by 1024MAK » Wed Oct 09, 2019 3:51 pm

What test gear do you have?

Multimeter - you say you have one. Is it a cheap one or a higher priced model?
Logic probe?
Logic analyser?
Oscilloscope - you say you have one on the way, has it arrived, and what type/make/model is it?

A fault with any of the four/five chips can prevent a ZX81 from working. But alas, a black screen is often the symptom of a failed ULA (IC1).

Using your multimeter first, then if you have it yet, your ‘scope, measure the voltage on IC1 (ULA) pin 16.
If your ‘scope has a suitably high bandwidth and sampling rate (if digital), also see if you can see if the CPU clock input is active on IC3 (CPU) pin 6.

Mark

colday
Posts: 13
Joined: Sun Aug 18, 2019 8:28 pm

Re: Non working ZX81

Post by colday » Wed Oct 09, 2019 9:52 pm

Yes, have a couple of multimeters, the newest is true RMS 6000 count so should be fine.

Goldstar Oscilloscope OS-9020P 20MHZ. Works nicely.

My drawback on the scope is the settings, however I can muddle through with it.

I don't have a logic probe as yet and am not actually 100% what it is, but I'll google it. Same goes for logic analyser.

Had a rubbish night last night so am off to bed imminently as shattered.

Will have a look at pin 16 on the ULA as soon as I can.

Quick question in the mean time, can I use the Speccy ULA in it's place to see if I get an output? Are they the same?

User avatar
mrtinb
Posts: 1043
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Non working ZX81

Post by mrtinb » Wed Oct 09, 2019 10:05 pm

The ULA in the ZX81 and the Spectrum are different.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

colday
Posts: 13
Joined: Sun Aug 18, 2019 8:28 pm

Re: Non working ZX81

Post by colday » Wed Oct 09, 2019 10:07 pm

Arse

User avatar
1024MAK
Posts: 2484
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Non working ZX81

Post by 1024MAK » Thu Oct 10, 2019 3:14 pm

The different ULAs are a large part of what gives the ZX81 and ZX Spectrum their personalities and characteristics.

Mark

colday
Posts: 13
Joined: Sun Aug 18, 2019 8:28 pm

Re: Non working ZX81

Post by colday » Thu Oct 10, 2019 9:58 pm

Ok, gonna try Pin 16 and Pin 6.

What sort of values should I be looking for?

From memory last night, I got something like 3.6v on pin 16 and 4.1v on pin 6.

They are approximate as brain has switched off since then.

Post Reply