ZX81 rebuild, take 2

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
Posts: 2155
Joined: Sat Nov 26, 2016 2:42 am

ZX81 rebuild, take 2

Post by Lardo Boffin »

I recently kindly got offered a spare PCB by Trevor_B as he had read about my previous endeavours and wanted to know if I wanted to do another rebuild.

After all the pain of the last one I was tempted to politely decline but as I am getting a proper desoldering station for Xmas I thought it would be a good project to get to grips with using it so I said yes. Big thanks to Trevor!

Here is the board, working status currently unknown but that doesn’t really matter. The important part is that the PCB is in lovely condition which it is. :D

4D88A502-9BAE-4BE5-8EFB-C0FE648CE8E8.jpeg

It also has what looks like a composite mod done.

DF9F5080-9B03-4850-B5C0-D795694BC145.jpeg

With that and the connections to some sort of external keyboard this has clearly had some usage in the past.

So, armed with what I learned last time (leave the original crystal in as modern replacements are flighty at best) I am going to have another go. But after xmas so this is just a placeholder while I gather the parts etc. again and of course a place to say thanks to Trevor_B.

I think this time I will fit 32K of RAM and a vLA81 rather than the 16K I did last time and inspired by The Byte Attic's refurb (viewtopic.php?p=38175#p38175) - gold plated sockets. 8-)
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: 2155
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

It boots to a K when connected to my Chroma 81 but can’t get a picture out of the modulator using RF or composite but it doesn’t really matter - the board is sound. :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
Trevor_B
Posts: 101
Joined: Thu Nov 05, 2020 12:31 am

Re: ZX81 rebuild, take 2

Post by Trevor_B »

..can I point out that none of the mods were mine; it was like that when I got it - honest!

Trevor
Moggy
Posts: 3222
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 rebuild, take 2

Post by Moggy »

If I were to trust anyone with my board Lardo would be among my choices,look forward to seeing another one back in action. :D
Lardo Boffin
Posts: 2155
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

Moggy wrote: Thu Nov 12, 2020 3:10 pm If I were to trust anyone with my board Lardo would be among my choices,look forward to seeing another one back in action. :D
:oops: :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
User avatar
1024MAK
Posts: 5087
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild, take 2

Post by 1024MAK »

Some thoughts:

What crystal :shock:
The Sinclair ZX81 uses a 6.5MHz ceramic resonator.

That ZX81 may not have been composite modified, it may be one intended for use with a French TV. Look inside the modulator ;)

Gold plated sockets :shock: Why, is there a supply of chips with gold plated pins?

Other than that, good luck Lardo :D

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

:!: Standby alert :!:
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Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
Moggy
Posts: 3222
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 rebuild, take 2

Post by Moggy »

Gold plated sockets!!

Hells teeth Lardo I didn't know your build-a-a-tooter business was that brisk!

Are you getting a government covid subsidy for "solder used" or what? :lol:
Lardo Boffin
Posts: 2155
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

1024MAK wrote: Thu Nov 12, 2020 5:09 pm Some thoughts:

What crystal :shock:
The Sinclair ZX81 uses a 6.5MHz ceramic resonator.

Mark
One of these days I will get the terminology right. Maybe.
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: 2155
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

1024MAK wrote: Thu Nov 12, 2020 5:09 pm
Gold plated sockets :shock: Why, is there a supply of chips with gold plated pins?

Mark
I guess I watched one too many episodes of Pimp My Ride (back in the days when I had time to watch such drivel) and wanted to Pimp My Zeddy.
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: 2155
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

This motherboard is proving to be a bit of a nightmare to desolder. There is a lot of solder on the component side (compared to pretty much all the others I have worked with) such as on this RAM chip (after desoldering the pins!):

241ADD8F-02BE-4A81-B014-F07CE05F6100.jpeg

Once removed I had clear the component side with desolder braid, not something I normally use but it was the only way I could find to get rid of the excess.

The other chips were socketed and also had a lot of top side solder so removing them was ‘fun’, especially as I couldn’t really see how bad it was.

Still, it is coming on.
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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