ZX81 with no display

Discussions about Sinclair ZX80 and ZX81 Hardware
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Jeroen1328
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Joined: Wed Dec 19, 2018 8:03 pm
Location: The Netherlands
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ZX81 with no display

Post by Jeroen1328 »

Hello Everybody,


last saturday I got my hands on a broken ZX81 while visiting a computer club.
The man that gave me the ZX81 told me that it didn't produce any video output.

At home I opened the ZX81 (carefull not to break the keyboard membrane, but that failed).
first thing I did was creating a video mod with 2 resistors and a transistor as I don't have a (small) tv at my work space.
after this I applied 5 volt to the output pin of the regulator with a lab power supply as I don't have the original power supply.
Checked the power over the ULA and verified 4,9 volt.
But as told by the man, there is no image at all just a black screen (only a little jump when applying power to the ZX81).

So I started to look on the internet, finding this great forum.

The ULA in the ZX81 is a 2C210E so this should give a back porch.

So I turned on my osciloscoop and started to look for changing signals on the address and data bus, but nothing there.
Checking the reset line on the CPU shows a high signal so system is not kept in reset state.
Then I found that there's no clock signal on pin 35 of the ULA.
So I swapped the ceramic resonator with another one (6 Mhz), just to see If I would get a clock, but no.
So I replaced C5 and TR2 (and put back the original ceramic resonator), but I have no clock pulse.

Anybody that could give a tip on where to look now ?

Regards,

Jeroen
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1024MAK
Posts: 5103
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 with no display

Post by 1024MAK »

Hello and welcome :D

Alas, I suspect that the ULA has developed a fault. The resonator and the circuitry in the ULA form the master oscillator.
If there is no CPU clock output on ULA pin 14 and the power connections are okay (+5V and 0V), then the next step is to try another ULA. But original ULAs are hard to find and expensive from the remaining supplier.

Andy (a forum member on here) does however produce a modern replacement module. See this thread.

SellMyRetro is a site where you can buy spares ;)

Mark
ZX81 Variations
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Looking forward to summer later in the year.
dessony
Posts: 342
Joined: Tue Oct 29, 2013 2:26 pm
Location: Indiana, USA

Re: ZX81 with no display

Post by dessony »

Jeroen1328,

Welcome to this forum.

Transistors sometimes break down. What does their labels show you?

Thank you very much in advance.

DesSony
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 with no display

Post by Lardo Boffin »

If it turns out you need a Z80 see this thread viewtopic.php?f=9&t=3037
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
Jeroen1328
Posts: 2
Joined: Wed Dec 19, 2018 8:03 pm
Location: The Netherlands
Contact:

Re: ZX81 with no display

Post by Jeroen1328 »

@1024MAK
a broken ULA was also the first thing that came to mind, but as there was no clock at the input (pin 35) I was hoping it could be something else.
for the record, there is also no clock on pin 14, but that's not strange as there in no clock in.

@dessony
TR2 is labeled ZTX313, I swapped it with another ZTX313 from a working ZX 48K.

@Lardo
I have some scrap PBX boards with a Z80 on them, so I could use them if needed.
What is this kludge fix I see is on your issue 1 in your signature ?

guess I could check the tracks for cracks, and then order a cpld ULA from Andy.

Regards,

Jeroen.
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 with no display

Post by Lardo Boffin »

A kludge fix zeddy is one with the early bugged ROM - it has maths issues. They were sometimes fixed by placing a PCB over the Z80 to skip the unwanted instructions and correct some issues.

A photo of one can be found here:-

viewtopic.php?f=7&t=2153&start=150#p26265

They are not ridiculously rare but I have bought a lot of zeddies (probably over 30) and only seen 4 now I think.

Edit - I also have had 3 with the ROM bug and no kludge fix so that makes 7 with the bugged ROMs. That makes me wonder if I have bought more than 30 by now. :oops:
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
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