ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
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Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

ZX81 rebuild

Post by Lardo Boffin »

A while back I bought this poorly motherboard from eBay (sold as untested :lol: ).
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I then cruelly ripped out its heart and used it to make another zeddy whole. I have been feeling a bit guilty about this. :cry:

Since I recently bought an unbuilt zedddy kit but have not been brave enough to make it I thought - I have a schematic, instructions and a bill of materials. Why not rebuild this one? I'm sure (hope) I can get modern bits for most of it, ULA aside obviously. The rf modulator is proving illusive for a reasonable price so I suspect I will just get a panel mount socket instead and fit a ZX8-CCB.

So I have started to remove all its original components (plus I am in serious need of de-soldering practice):-

A few bits gone
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Did a load more tonight:-
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I guess the next stage will be to remove everything, tidy up the holes and go over it with a magnifying glass and continuity tester!

Meanwhile I will need to start to identify modern components to buy. I have some bits off Sell My Retro etc. - keyboard connectors, a Z80, a crystal and new power sockets etc. Just the vast majority to go! Baby steps!

Will be a while before I have time to move this forward but am looking forward to the challenge. :D
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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Paul
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Joined: Thu May 27, 2010 8:15 am
Location: Germanys west end

Re: ZX81 rebuild

Post by Paul »

Thumbs up!
That is a challenge!
Please keep us informed.
If I were you I would first not add RAM and test it with an external RAM pack.
When it works I would add a larger static RAM on the PCB.
16k,32k or even 56k.
Kind regards Paul
In theory, there is no difference between theory and practice. But, in practice, there is.
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1024MAK
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Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK »

@ Phil - Good luck with the rebuild :D

Before trying a modulator or a composite video arrangement, temporarily connect a ZX8-CCB to the rebuild board. Use one that has already been set up for the ULA that you are using on a known working board (that had the same ULA chip fitted during the set-up) ;)

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
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Andy Rea
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Re: ZX81 rebuild

Post by Andy Rea »

Cool job, its the unsoldering that boring, but now you've go that out the way... Skies the limit

Andy
what's that Smell.... smells like fresh flux and solder fumes...
Moggy
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Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 rebuild

Post by Moggy »

Have faith in yourself Lardo and go for it! :geek:
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin »

Thanks Mark - will def test video before final assembly.

Still have two sockets to remove and then a bare naked board! Time to start finding and ordering stuff.
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
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin »

Finally a completely bare board. Apart from a few clumsy scuffs not looking too bad! I have already used the photos to brush off some unwanted solder flakes.

Tomorrow's job is to start to find where I can get all the general bits from.
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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
makeit2day
Posts: 32
Joined: Sat May 10, 2008 2:31 am

Re: ZX81 rebuild

Post by makeit2day »

What kind of desoldering tool did you use for your project. You did a nice job
Lardo Boffin
Posts: 2160
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin »

makeit2day wrote: Wed Oct 04, 2017 12:28 am What kind of desoldering tool did you use for your project. You did a nice job
One of these:-

https://m.ebay.co.uk/itm/Solder-sucker- ... Ciid%253A1

There is a video on https://youtu.be/76gyKWRI7dY

I suspect that the fact this was a hand built zeddy (judging by the number of dry joints) probably helped the removal!
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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1024MAK
Posts: 5102
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK »

Clues as to the method of soldering are:-

No signs of flux on the joints of normal components (excluding some types of plastic connectors, odd size components and sensitive components), most of the joints all looking very uniform and unused pads full of solder indicates it was flow soldered in a solder bath. Despite the automation, if the PCB or components have contamination or dirt, you can still get dry or incomplete solder joints.

Professional and consistent joints, but with signs of flux, any unused pads free of solder - hand soldered by someone who is proficient.

Inconsistent solder joints, with signs of flux (sometimes lots of flux). Often there are joints with too much solder, or too little solder - indicates that either the board has been worked on since production, or it was hand soldered from new.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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