Zx81 composite mod with backporch-Advice

Discussions about Sinclair ZX80 and ZX81 Hardware
rmbrowngr
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Re: Zx81 composite mod with backporch-Advice

Post by rmbrowngr »

As it is a day I cannnot go out because of storm Barbara here in Greece, I've finally got around to opening up my ZX81 it fit a composite mod. Here's some pictures of opening up my first computer.

I'm curious to know about the serial number on the outside of the box.
IMG_4356.jpg
By the looks of these it confirms I got it November 1982.
IMG_4357.jpg
IMG_4358.jpg
1980 circuit board
IMG_4359.jpg
IMG_4360.jpg
ZX81 (purchased November 1982), my first computer.
ZXpand+, ZX8-CCB.
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1024MAK
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Re: Zx81 composite mod with backporch-Advice

Post by 1024MAK »

I can see some of the date codes, definitely made in 1982. Looks like week 6 or later, so likely spring 1982.

Oh, and it looks the all the chips are in sockets. It’s very unusual for factory made boards to have the RAM in sockets.

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rmbrowngr
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Re: Zx81 composite mod with backporch-Advice

Post by rmbrowngr »

And the reluctant keyboard connector looks intact.
IMG_4361.jpg
Thinking of mounting the ZX8-CCB on top of the rf modulator. I saw this done in a youtube video.
ZX81 (purchased November 1982), my first computer.
ZXpand+, ZX8-CCB.
Lardo Boffin
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Re: Zx81 composite mod with backporch-Advice

Post by Lardo Boffin »

You can also mount them quite neatly underneath the motherboard:

178775C0-9C32-4EA5-8CF6-44EBA3DC676C.jpeg

And then just run the brown wire round to the RF can:

28D85BBE-4870-41E9-82CD-ECF1144163B6.jpeg

This way if you find you need to adjust the pots after fitting the keyboard it is much easier to do so as you won’t have to detach the motherboard from the top case.

I find it easier to adjust the pots when there is a reasonable amount of text on the screen.
ZX80
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rmbrowngr
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Re: Zx81 composite mod with backporch-Advice

Post by rmbrowngr »

Something is not right.
picture.jpg
Here's my poor soldering:
top.jpg
bottom.jpg
I cleaned up with 95% alochol.
Gave the power socket a quick spray with contact cleaner as it was not making good contact.
Checked the 5v power line with my ancient 80's cheap meter, seems ok.
Had a good visual look to see if I made solder bridges, don't see any.

Any ideas?
Last edited by rmbrowngr on Sun Feb 05, 2023 4:42 pm, edited 1 time in total.
ZX81 (purchased November 1982), my first computer.
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Paul
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Re: Zx81 composite mod with backporch-Advice

Post by Paul »

Could you please add a wire to the second GND Pad of the PCB?
Mine didn't work without both connected.
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Lardo Boffin
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Re: Zx81 composite mod with backporch-Advice

Post by Lardo Boffin »

I always disconnect both of the two wires going into the RF modulator:

910E306E-E55E-4A0A-950F-8F9C30C4C796.jpeg
Last edited by Lardo Boffin on Sun Feb 05, 2023 4:48 pm, edited 1 time in total.
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
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Re: Zx81 composite mod with backporch-Advice

Post by Lardo Boffin »

Also the position of the pots can have a serious effect on the picture. My start point for adjusting for all but the earliest ULAs is:

9F07CB90-A2BC-43F8-9770-160185A67B43.jpeg

Take a photo of where it is now so you can always revert if necessary!
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
rmbrowngr
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Joined: Thu Dec 09, 2021 2:58 pm

Re: Zx81 composite mod with backporch-Advice

Post by rmbrowngr »

Lardo Boffin wrote: Sun Feb 05, 2023 4:44 pm I always disconnect both of the two wires going into the RF modulator:
First thing i did.
ZX81 (purchased November 1982), my first computer.
ZXpand+, ZX8-CCB.
rmbrowngr
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Joined: Thu Dec 09, 2021 2:58 pm

Re: Zx81 composite mod with backporch-Advice

Post by rmbrowngr »

1024MAK wrote: Sun Feb 05, 2023 12:47 pm Oh, and it looks the all the chips are in sockets. It’s very unusual for factory made boards to have the RAM in sockets.
Yes, all the IC's are socketed.
ZX81 (purchased November 1982), my first computer.
ZXpand+, ZX8-CCB.
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