Page 1 of 15

ZX81 rebuild

Posted: Sun Oct 01, 2017 12:36 am
by Lardo Boffin
A while back I bought this poorly motherboard from eBay (sold as untested :lol: ).
(1.48 MiB) Downloaded 864 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
(2.54 MiB) Downloaded 864 times
(2.8 MiB) Downloaded 864 times
Did a load more tonight:-
(3.53 MiB) Downloaded 864 times
(3.3 MiB) Downloaded 864 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

Re: ZX81 rebuild

Posted: Sun Oct 01, 2017 9:10 am
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

Re: ZX81 rebuild

Posted: Sun Oct 01, 2017 10:35 am
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) ;)


Re: ZX81 rebuild

Posted: Sun Oct 01, 2017 12:31 pm
by Andy Rea
Cool job, its the unsoldering that boring, but now you've go that out the way... Skies the limit


Re: ZX81 rebuild

Posted: Sun Oct 01, 2017 2:05 pm
by Moggy
Have faith in yourself Lardo and go for it! :geek:

Re: ZX81 rebuild

Posted: Mon Oct 02, 2017 10:44 pm
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.

Re: ZX81 rebuild

Posted: Tue Oct 03, 2017 10:35 pm
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.
(3.04 MiB) Downloaded 730 times
(3.18 MiB) Downloaded 730 times

Re: ZX81 rebuild

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

Re: ZX81 rebuild

Posted: Wed Oct 04, 2017 8:24 am
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:- ... Ciid%253A1

There is a video on

I suspect that the fact this was a hand built zeddy (judging by the number of dry joints) probably helped the removal!

Re: ZX81 rebuild

Posted: Wed Oct 04, 2017 9:53 am
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.