ZX81 kit assembly

Discussions about Sinclair ZX80 and ZX81 Hardware
sP1d3r
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Joined: Mon May 16, 2022 9:20 am

ZX81 kit assembly

Post by sP1d3r »

I recently bought a partially assembled ZX81 kit in need of completion.
Unfinished
Unfinished
I think I found the reason that the original owner didn't complete the assembly, the CPU socket had one of the pins tucked under the socket rather than into the through hole and it would appear that desoldering wasn't his forte.

Also several of the components were incorrectly placed, so I desoldered all the sockets and checked the component values and placing.

I've now completed the assembly and I tried plugging in to my Macintosh Performa 5300 TV card and there is a definite signal at UHF 35 but it would appear that the TV card needs a back porch signal because I can't tune the display in.
Assembly complete
Assembly complete
I'll have to try to find something suitable to plug into.
sP1d3r
Posts: 118
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Re: ZX81 kit assembly

Post by sP1d3r »

The picture quality isn't clear and you can't see that I had to file the 28-pin socket a bit:-
Turned pin sockets
Turned pin sockets
Lardo Boffin
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Re: ZX81 kit assembly

Post by Lardo Boffin »

Best way forward with video is one of these:

https://www.ginger-electronic.com/en/re ... -zx81.html


You can fit one non-destructively for testing if you want to keep it original.
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
sP1d3r
Posts: 118
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Re: ZX81 kit assembly

Post by sP1d3r »

Lardo Boffin wrote: Thu Sep 29, 2022 10:56 am Best way forward with video is one of these:

https://www.ginger-electronic.com/en/re ... -zx81.html


You can fit one non-destructively for testing if you want to keep it original.
Fantastic Lardo, that looks like a great module.

I was thinking that I could try my vLa81 with the back porch signal enabled if this would mean that the signal from the modulator would contain it, I'm unsure as to exactly what enabling the back porch means:- would only a composite signal have it or would it apply to UHF output as well.
Lardo Boffin
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Re: ZX81 kit assembly

Post by Lardo Boffin »

I think the vLA81 just produces a composite signal by default? Been a while since I read the instructions!

The basic idea is to bypass the RF module circuitry and send the composite signal (from either the ZX8-CCB or the vLA81) straight to the TV. Its like composite modding a Spectrum - you disconnect a couple of wires and desolder a resistor. Got a link somewhere.

Here - https://sinclairzxworld.com/viewtopic.p ... 798#p21798
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
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 kit assembly

Post by 1024MAK »

sP1d3r wrote: Thu Sep 29, 2022 6:20 pm I was thinking that I could try my vLa81 with the back porch signal enabled if this would mean that the signal from the modulator would contain it, I'm unsure as to exactly what enabling the back porch means:- would only a composite signal have it or would it apply to UHF output as well.
A composite video signal or baseband video signal is a combination of the monochrome video, the control signals (including horizontal and vertical synchronisation signals, front and back porch, etc.) and for colour television/video, the colour picture information in the form of a colour carrier signal.

Because TVs available in the 1980s often did not have a composite video input socket, home computer manufacturers included a modulator. All the modulator does is mix the composite video signal with a very high, or ultra higher frequency oscillator. Hence creating a signal like the signal an aerial would pick up from broadcast TV transmissions.

So, both a composite video signal and a VHF/UHF signal should both have all the control signals if you want a good chance that the monitor or TV will correctly display the video image.

Having said that, some LCD or “modern” colour CRT TVs may show a ZX80 or ZX81 video signal that is without the back porch. It’s rather random if this works at all, or with the composite video input or the VHF/UHF input.

Keep in mind that the ZX80 and ZX81 were designed to be used with monochrome CRT TVs where it was not necessary for the back porch to be correct.

The vLA81 should produce a composite video signal with the back porch as standard.

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

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sP1d3r
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Re: ZX81 kit assembly

Post by sP1d3r »

Many thanks Lardo and Mark,

It'll have to wait until next week for the moment because of other things that need doing.

I'd like to try this with a ZX8-CCB because then I could test my other ULAs.
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1024MAK
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Re: ZX81 kit assembly

Post by 1024MAK »

I assume you know the differences between the ULAs with respect to the back porch? If not click on the link in my signature.

I have a LCD TV that will work on UHF with a ULA that does not generate the correct back porch. But not via composite video unless I use a ZX8-CCB. I don’t buy them via the container load like Lardo, but I have bought rather a lot. Hence I definitely recommend them for Zeddies that use a 2C158E or a 2C184E ULA… ;)

For use with a 2C210E, I use the circuit described here ;) . However, a ZX8-CCB will also work although the preset (variable) resistors will need adjusting slightly.

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

Re: ZX81 kit assembly

Post by Lardo Boffin »

1024MAK wrote: Fri Sep 30, 2022 12:31 pm I don’t buy them via the container load like Lardo, but I have bought rather a lot.
Mark
True but they only tend to arrive in small containers.

The ZX8-CCB is probably overkill for the later ULAs but I buy to do up and sell on so its worth doing.
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
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 kit assembly

Post by Moggy »

Yeh and you hog all the bloody clickys too! :lol:


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