Zx81 from death to life??

Discussions about Sinclair ZX80 and ZX81 Hardware
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RS232
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Joined: Fri Apr 05, 2019 8:57 pm

Zx81 from death to life??

Post by RS232 » Fri Apr 12, 2019 10:51 pm

Greetings all. I bricked my zx81 after successfully doing both a composite mod and transformer mod for cooling and tv output. After de soldering 22uf cap and replacing i got no signal output. Replacing same effect.. then removing that i accidentally damaged a tiny bit of the board. Now i have two problems how to solder back a 22uf cap on a working trace and why i still get no signal. 9v supply works. Zx81 open heart surgery - medical professionals welcome to aid me with some solution.. i'm novice at these things thanks! :x

Sadly i dont own a multi-metre.. newbie in electrics
Advice appreciated morning the death of an 8 bit :-(
Attachments
20190412_215824.jpg
Blank screen output
20190412_214105.jpg
It started so well
20190412_215405.jpg
Where it went so wrong
Last edited by RS232 on Tue Apr 16, 2019 10:23 pm, edited 4 times in total.

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

Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by Lardo Boffin » Sat Apr 13, 2019 9:25 am

Is that a lifted pad on the last photo? Can’t quite tell.

Did you make sure that the cap had the correct polarity when you put it back in?
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

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RS232
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RS232 » Sat Apr 13, 2019 12:05 pm

Hi well i went with the fact the long leg on the 22uf cap was positive facing the pluss sign on C3. It is an issue 3 zx81 in red. Yes lifted pad in pic
Attachments
20190413_110414.jpg
Where 22uf used to live

Moggy
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by Moggy » Sat Apr 13, 2019 2:44 pm

If memory serves that 22uf cap is just a filter for the original regulator and with a very clean supply the zeddy would work without it and I don't think it adds anything to that modern switching reg you have fitted.

A more accurate and proper response may come shortly when Mark sees this thread. I would recommend you follow all and any advice he gives, as from personal experience I have found him to most knowledgeable with a wonderful way of explaining complicated things very simply.

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RetroTechie
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RetroTechie » Sat Apr 13, 2019 9:18 pm

It's always possible that some component was damaged during handling of the board, in-between powerups to test things, ESD damage etc. But for the NMOS IC's in a ZX81: highly unlikely.

If not that, following method is practically foolproof: start from last configuration that you KNOW 100% sure to be working (not assume to be working), retrace EVERY step / modification done since then (no matter how small or seemingly 'safe'), and double-check or reverse that change. Until you arrive back at your previous working configuration.

Lifted a pad? See what it connected, and re-connect those things. A solder blob short-circuited 2 traces? Clean up & check that short-circuit removed and each track still connects what it connected before. Etc etc.

In the rare case reversing your steps doesn't work, treat as a regular repair. As if machine just arrived at your place in unknown condition. In order:
  1. Do a thorough optical check of the board
  2. Check supply voltages
  3. Check clock signal(s)
  4. Find out if CPU is working or not
  5. RAM
  6. ROM
  7. Audio / video output (for the ZX81, basically the ULA :) )
What measuring tools do you have available? Scope or a simple logic probe would be very useful.

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RS232
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RS232 » Sat Apr 13, 2019 9:37 pm

RetroTechie wrote:
Sat Apr 13, 2019 9:18 pm
It's always possible that some component was damaged during handling of the board, in-between powerups to test things, ESD damage etc. But for the NMOS IC's in a ZX81: highly unlikely.

If not that, following method is practically foolproof: start from last configuration that you KNOW 100% sure to be working (not assume to be working), retrace EVERY step / modification done since then (no matter how small or seemingly 'safe'), and double-check or reverse that change. Until you arrive back at your previous working configuration.

Lifted a pad? See what it connected, and re-connect those things. A solder blob short-circuited 2 traces? Clean up & check that short-circuit removed and each track still connects what it connected before. Etc etc.

In the rare case reversing your steps doesn't work, treat as a regular repair. As if machine just arrived at your place in unknown condition. In order:
  1. Do a thorough optical check of the board
  2. Check supply voltages
  3. Check clock signal(s)
  4. Find out if CPU is working or not
  5. RAM
  6. ROM
  7. Audio / video output (for the ZX81, basically the ULA :) )
What measuring tools do you have available? Scope or a simple logic probe would be very useful.
Thank you for this advice. I actually own no measuring tools and have little idea how to use them. I saw a bunch of YouTube vids and ran with it. I thought id learn on the go.. i guess i need to learn about micro electronics nand gates and do something about my eyesight - the tracks on this board are hard work to see with the naked eye.

It is going to be impossible to reverse the composite mod as i removed the original +5 cable going into the box completely. I cannot see any solder blobs but it could have happened will check.

The fed i get no signal at all.. compared to a corrupt display might mean the board is solvable? That it might not be a bliwn chip?

I know i'm one brave slightly crazy novice electrician so very very grateful and appreciating hints and tips

Thanks!

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RetroTechie
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RetroTechie » Sat Apr 13, 2019 10:04 pm

When did you last get an image on screen from this ZX81? What mods where already on the board @ that time? What modification(s) did you do since then?

A multimeter (voltmeter + something for checking connections between points on the board) is the very least you should have, I think. Modding a 35+ year old home computer without even that, is du... ehm, brave! ;) :lol: Even a $5 multimeter from eBay would be useful for jobs like this.

Beside that: get a good lamp (and/or flashlight), and a magnifying glass. Holding flashlight against the back of the board & look through from other side, is a good way to check traces, find small cracks in traces, and so on. But also check with light coming from same side as from where you're looking. Holding board or light in different angles may help to spot things that are difficult to see.

Lardo Boffin
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Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by Lardo Boffin » Sun Apr 14, 2019 9:15 am

If you have a reasonable quality smart phone take a close up photo of an affected area and then pinch and zoom on the image. I have a cheap little set of clip on lenses for mine (£4 from my local garage when bought with fuel) which helps.

Needs to be done in good light but often shows things that poor eye sight (which includes me) cannot otherwise see.
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

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RS232
Posts: 42
Joined: Fri Apr 05, 2019 8:57 pm

Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by RS232 » Sun Apr 14, 2019 9:48 am

Great tip :shock:

Lardo Boffin
Posts: 933
Joined: Sat Nov 26, 2016 2:42 am

Re: Help my zx81 mod has gone wrong!! 8 bit death

Post by Lardo Boffin » Sun Apr 14, 2019 10:29 am

I started out in the ‘brave’ camp. :D

You really need a multi-meter. So many uses!

Check the output voltage of the PSU you are about to plug into your computer. I had one running at over 16V so it would have caused problems if used.

Check the voltage at various pins - are they getting 5V?

A continuity tester (if it buzzes there is a connection) for checking for shorts or breaks.

Resistance tester - if doing the +3 sound fix you end up replacing about 8 resistors. Useful to know you are using the right ones. I know they are colour coded but coming back to poor eyesight...

Mine cost less than £10.

Some also come with transistor testers and other goodies.
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

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