Dead ZX81.

Discussions about Sinclair ZX80 and ZX81 Hardware
flaviosr
Posts: 10
Joined: Sun May 12, 2013 11:25 pm

Dead ZX81.

Post by flaviosr » Sun Sep 09, 2018 2:09 pm

Dear All,

I have just received a... dead ZX81.
Completely black screen.
I have prepared a checklist (this is the first time I do this):
° I have read a 3D "somewhere" in which it is told that could be a (or many) transistor! I never checked transistors so I am looking for a "resistors check manual for dummies"... :)
° I am going to check also capacitors;
° all resistances seem ok (I suppose that if they do not work they... burn... right?)
° apart these, I have the chips: ZX81, ULA, memory chip and... MOSTEK?
° I thought it could also be a video output issue but I am doubtful! I want to do the AV mod anyway.

Do you have any more suggestions and ideas on my problem? Like being sure that it depends on a specific issue...

Thank you

Lardo Boffin
Posts: 811
Joined: Sat Nov 26, 2016 2:42 am

Re: Dead ZX81.

Post by Lardo Boffin » Sun Sep 09, 2018 2:24 pm

An obvious one but have you turned the brightness up a lot in case it is just a really dark output?
ZX81 iss 1 (bugged ROM, kludge fix, normal)
Iss 3 with ZXVid, another in nearly perfect box
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

flaviosr
Posts: 10
Joined: Sun May 12, 2013 11:25 pm

Re: Dead ZX81.

Post by flaviosr » Sun Sep 09, 2018 2:36 pm

Lardo Boffin wrote:
Sun Sep 09, 2018 2:24 pm
An obvious one but have you turned the brightness up a lot in case it is just a really dark output?
Before connecting the ZX81 the TV has a standard salt and pepper output (no signal).
When I connect the ZX81 it becomes completely black (all screen).
I am sure about the power unit and everything else since... the other ZX81 is ok! :)

Lardo Boffin
Posts: 811
Joined: Sat Nov 26, 2016 2:42 am

Re: Dead ZX81.

Post by Lardo Boffin » Sun Sep 09, 2018 6:06 pm

I guess the fact that the screen changes when switched on means it is trying to do something, otherwise it would just be salt and pepper (nice way of putting it :D ).

Some of the early zeddys had a really dark output.

I recently had a zeddy which switched on (had a later ULA) and only produced a white screen, no cursor. A little chip swapping and I found out it was a dodgy ROM.
ZX81 iss 1 (bugged ROM, kludge fix, normal)
Iss 3 with ZXVid, another in nearly perfect box
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

User avatar
Paul
Posts: 962
Joined: Thu May 27, 2010 8:15 am
Location: Germanys west end

Re: Dead ZX81.

Post by Paul » Sun Sep 09, 2018 7:20 pm

flaviosr wrote:
Sun Sep 09, 2018 2:36 pm

Before connecting the ZX81 the TV has a standard salt and pepper output (no signal).
In Germany we call this ants soccer.
In theory, there is no difference between theory and practice. But, in practice, there is.

User avatar
mrtinb
Posts: 760
Joined: Fri Nov 06, 2015 5:44 pm
Location: Denmark
Contact:

Re: Dead ZX81.

Post by mrtinb » Sun Sep 09, 2018 8:11 pm

In Denmark we call it snow or snow weather.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

User avatar
1024MAK
Posts: 2074
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Dead ZX81.

Post by 1024MAK » Sun Sep 09, 2018 9:12 pm

Thoughts...

If the TV works okay with other ZX81's, do you know the number of the Ferranti ULA (the 40 pin chip that is not the Z80 CPU) in the ZX81 machine(s) that work?

Do you know the number of the Ferranti ULA in the machine that only produces a black video picture?

Can you please post the ULA details on this forum.

There are three versions of the Ferranti ULA (the Sinclair logic chip). They are semi-custom chips produced for Sinclair by Ferranti. Most 'black screen' display faults are due to the ULA.

Sinclair Research designed the ZX81 to work with monochrome " black and white" televisions (TV). The first two ULA designs were not completely compatible with colour televisions. They don't produce part of the video signal that colour TVs use to auto-calibrate the normal black colour signal level. So with some TVs you get a very dark and very poor contrast picture even with the brightness and contrast controls turned to near maximum.

This problem was solved with the third and last version of the ULA.

There is a solution for the problem with the first two ULA versions.

It is also possible that the ULA in machine that only produces a black video picture has a failed video output stage or other fault.

Mark

flaviosr
Posts: 10
Joined: Sun May 12, 2013 11:25 pm

Re: Dead ZX81.

Post by flaviosr » Mon Sep 10, 2018 8:25 am

1024MAK wrote:
Sun Sep 09, 2018 9:12 pm
Thoughts...

If the TV works okay with other ZX81's, do you know the number of the Ferranti ULA (the 40 pin chip that is not the Z80 CPU) in the ZX81 machine(s) that work?

Do you know the number of the Ferranti ULA in the machine that only produces a black video picture?

Can you please post the ULA details on this forum.

There are three versions of the Ferranti ULA (the Sinclair logic chip). They are semi-custom chips produced for Sinclair by Ferranti. Most 'black screen' display faults are due to the ULA.

Sinclair Research designed the ZX81 to work with monochrome " black and white" televisions (TV). The first two ULA designs were not completely compatible with colour televisions. They don't produce part of the video signal that colour TVs use to auto-calibrate the normal black colour signal level. So with some TVs you get a very dark and very poor contrast picture even with the brightness and contrast controls turned to near maximum.

This problem was solved with the third and last version of the ULA.

There is a solution for the problem with the first two ULA versions.

It is also possible that the ULA in machine that only produces a black video picture has a failed video output stage or other fault.

Mark
As I have written above, I tested what I could and I found out that the two capacitors are good (30 vs 22.5 mF and 6 vs. 1 mF... perhaps this one is even "too much").
BUT the two transistors seem dead. Today I am going to bring the ZX81 to a friend that repairs eletronic devices as a job.

Codes on the "black" ULA: 2C184E / 8142 (I suppose the first code is the datacode and the last the code, right?).
I would not like to open the other ZX81 if not strictly necessary (as if a simple switch is needed...).

I attach a photo of the mobo if this can help.
PIC_20180909_152507.JPG
Putting contrast and brightness to maximum level would bring to something on the screen, right? Another way to see if the ULA is doing its job...
I have 4 TVs + one decoder, and one TV is an old CRT one (the one I use for tests)... I hope something can happen...

So now I will get a technical check of the capacitors and transistors and if they are dead or bad working they are going to be professionally replaced.
If nothing is solved I try to change contrast and brightness in the various TVs.
If, again, nothing happens I am going to make the AV mod (that I am going to do anyway).
and if, again, nothing happens, the other ZX81 has to be opened...

Thank you

User avatar
1024MAK
Posts: 2074
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Dead ZX81.

Post by 1024MAK » Mon Sep 10, 2018 11:28 pm

It's very rare for the transistors in a ZX81 to fail. And the values of the two electrolytic capacitors are not very critical. The ZX81 will run with C3 (22uF) missing. C5 (1uF) is part of the reset circuit, so a larger value just means reset after power up takes a little bit longer (fractions of a second). The tolerance on electrolytic capacitors is very wide anyway.

In this post I give some details of the three ULA variations. As you can see, your machine has the common 2C184E version which does not generate a the 'back porch' part of the video signal. So may well fail to produce anything other than a dark picture on colour televisions that are not happy with the non-standard video signal.

So before changing anything, I suggest you try it on as many different televisions as you can.

Mark

flaviosr
Posts: 10
Joined: Sun May 12, 2013 11:25 pm

Re: Dead ZX81.

Post by flaviosr » Sun Sep 16, 2018 9:23 pm

1024MAK wrote:
Mon Sep 10, 2018 11:28 pm
It's very rare for the transistors in a ZX81 to fail. And the values of the two electrolytic capacitors are not very critical. The ZX81 will run with C3 (22uF) missing. C5 (1uF) is part of the reset circuit, so a larger value just means reset after power up takes a little bit longer (fractions of a second). The tolerance on electrolytic capacitors is very wide anyway.

In this post I give some details of the three ULA variations. As you can see, your machine has the common 2C184E version which does not generate a the 'back porch' part of the video signal. So may well fail to produce anything other than a dark picture on colour televisions that are not happy with the non-standard video signal.

So before changing anything, I suggest you try it on as many different televisions as you can.

Mark
I have to admit I measured the transistors the wrong way! Both, transistors and capacitors, are ok! :(
I have cleaned the ROM chip that had "dirty" pins.
And I have tested the ZX81 on all 5 TVs I have:
° LGs (one LCD and one CRT) see C36 but I have a black/dark grey screen;
° Samsungs (2 LCDs) refuse to see C36 and, even if I force them, the image is terrible (the LG ones are uniform);
° the decoder works like LGs.
But I have a new!!! While making these tests obviously I touched the motherboard (my first tests was with closed ZX81) and:
° the ROM chip is at ambient temperature;
° the Z80 if fairly hot (you can leave your finger on it but you feel the temperature);
° the ULA is really hot (you cannot leave the finger on it);
° the side plate is in the middle between the ULA and the Z80 (you can anyway leave the finger on it).

I have also opened the other ZX81 and... it's another complete different version! :(
PIC_20180916_141752.JPG
I have, again, searched over the internet and I have found the motherboard but WITHOUT all that wiring!!!
Do you know why? A Sinclair mod or...?

Anyway I suppose I have to look for a new ULA... what do you think?

Post Reply