ZX81 rebuild, take 2

Discussions about Sinclair ZX80 and ZX81 Hardware
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

I desoldered the chips on my ZX81 recently.
Apart from pins 23 and 24 on the cpu, it went ok.
The two cpu pins lost most of the pads around the hole on the top side, but as they're not connected and there's a bit left, it's ok.
Component side
Component side
Solder masked
Solder masked
Lardo Boffin
Posts: 2155
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild, take 2

Post by Lardo Boffin »

Nice job!
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
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

Hopefully it'll work when I've soldered in the sockets and cleaned up the chips, a black screen was all that happened previously but the rom socket wasn't soldered in properly, one end was sticking up out of the board and the pins didn't show on the underside.
Some of the solder joints looked dry, as well.

It's got 24-pin Mostek rom and ram chips, also I'll solder in a socket that a vLA81 will fit in.
The original Augat sockets are much more difficult to desolder in comparison with modern single-wipe sockets.

This is the first ZX81 I've had, although it was the first computer that I saw, a relative had one with a ram pack.
The typing woes with a ram pack fitted can be avoided if a 'plinth' that keeps the ram pack from touching down is used, I recall from the age of wonder!
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

I've noticed that my ZX81 has odd looking 'mod' in the ear socket, a capacitor and resistor.
This pcb outputs nothing via the modulator, I'd expect something but there's nothing whatsoever.

I'll try and fix this and see if anything changes.
Moggy
Posts: 3222
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 rebuild, take 2

Post by Moggy »

When you say "in" the ear socket do you mean literally inside the socket or on the board itself? I only ask because I don't see anything on the board in your first picture that shouldn't be there.
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

Moggy wrote: Tue Jun 21, 2022 1:23 pm I don't see anything on the board in your first picture that shouldn't be there.
I mentioned the 1K resistor and the 47nf capacitor soldered between the power socket and the ear socket because Lardo's board from Trevor has the same arrangement and likewise no output from the modulator, whereas these components are alternatively R27 and C11 on other ZX81s that I've seen.
Moggy
Posts: 3222
Joined: Wed Jun 18, 2008 2:00 pm

Re: ZX81 rebuild, take 2

Post by Moggy »

R27 and C11 are usually found connected on the later issue 3 boards not earlier issue 1's, no doubt done to try and improve the load/save characteristics of the earlier board, perhaps these later boards are the ones you have seen as your issue 1 looks in order regarding the capacitor and resistor you mention?

Hopefully Mark (1024MAK) will see this and confirm or not as the case may be.
Attachments
6A98DA52-D254-4D23-AAB4-FF2CF0D49313.jpeg
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

This board has the components in the alternative position but it's an issue 1 and although it doesn't boot up, there is a black screen from the UHF modulator.
Issue 1 with C11 & R27
Issue 1 with C11 & R27
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

I think the difference between the two motherboards is because the ULA is different, the resistor and capacitor is a standard mod for the 2C184 ULA.

I don't know why there's no output from the board with the 184 ULA, it doesn't seem to have any blown components.
The board with the 158 ULA may work but as I haven't got a black and white UHF TV it might be because my Apple Performa with TV card can't display the output properly.
sP1d3r
Posts: 118
Joined: Mon May 16, 2022 9:20 am

Re: ZX81 rebuild, take 2

Post by sP1d3r »

Moggy wrote: Tue Jun 21, 2022 3:01 pm no doubt done to try and improve the load/save characteristics of the earlier board
Do you think that it's a good idea move the resistor and capacitor to the ear socket on the my earlier issue one motherboard?
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