ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
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Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

ZX81 rebuild

Post by Lardo Boffin » Sat Sep 30, 2017 11:36 pm

A while back I bought this poorly motherboard from eBay (sold as untested :lol: ).
IMG_7083.PNG
IMG_7083.PNG (1.48 MiB) Viewed 571 times
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
IMG_7079.JPG
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IMG_7080.JPG
IMG_7080.JPG (2.8 MiB) Viewed 571 times
Did a load more tonight:-
IMG_7081.JPG
IMG_7081.JPG (3.53 MiB) Viewed 571 times
IMG_7082.JPG
IMG_7082.JPG (3.3 MiB) Viewed 571 times
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
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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Paul
Posts: 880
Joined: Thu May 27, 2010 7:15 am
Location: Germanys west end

Re: ZX81 rebuild

Post by Paul » Sun Oct 01, 2017 8:10 am

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
Posts: 1801
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Sun Oct 01, 2017 9:35 am

@ 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

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Andy Rea
Posts: 1268
Joined: Fri May 09, 2008 1:48 pm
Location: notts UK

Re: ZX81 rebuild

Post by Andy Rea » Sun Oct 01, 2017 11:31 am

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

Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

Moggy
Posts: 1513
Joined: Wed Jun 18, 2008 1:00 pm

Re: ZX81 rebuild

Post by Moggy » Sun Oct 01, 2017 1:05 pm

Have faith in yourself Lardo and go for it! :geek:

Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Mon Oct 02, 2017 9:44 pm

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.
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Tue Oct 03, 2017 9:35 pm

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.
IMG_7119.JPG
IMG_7119.JPG (3.04 MiB) Viewed 437 times
IMG_7123.JPG
IMG_7123.JPG (3.18 MiB) Viewed 437 times
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

makeit2day
Posts: 22
Joined: Sat May 10, 2008 1:31 am

Re: ZX81 rebuild

Post by makeit2day » Tue Oct 03, 2017 11:28 pm

What kind of desoldering tool did you use for your project. You did a nice job

Lardo Boffin
Posts: 658
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Wed Oct 04, 2017 7:24 am

makeit2day wrote:
Tue Oct 03, 2017 11:28 pm
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!
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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1024MAK
Posts: 1801
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Wed Oct 04, 2017 8:53 am

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

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