Help in resurrecting my ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

I mean, with the computer disconnected from the power. Use a multimeter on the low resistance range (e.g. 200 ohms range) or the continuity range (bleeper or buzzer), to test between the transistor TR2 collector and R5. Then between the transistor TR2 collector and pin 6 of the Z80 CPU. If you test on the component legs / pins, if you get resistances of below 2 ohms (not including the resistance of the meter leads), then the tracks and solder joints are likely good.

Mark
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

Martin, do you have a 555 timer chip? Either just a chip, or a module (PCB) with one on?

Mark
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Re: Help in resurrecting my ZX81

Post by mrtinb »

Multimeter set for 200Ω.
  • Between TR2 collector and R5 leg 1 - no reaction
  • Between TR2 collector and R5 leg 2 - no reaction
  • Between TR2 collector and CPU pin 6 - no reaction
Strange thing: This was working when I bought it 10 years ago.

I have a small SMD SOIC 555 from a "Try soldering SMD"-kit.

Does anyone have a scan of a component-less issue 3 board? That would greatly help debugging.

Otherwise I'll have to wait for my desoldering gun, and remove the ICs.

Edit: I just found that measuring on the pins get no read out. I works by measuring on the solder joints. :oops: Maybe the old legs are oxidiced. I'll measure from the start. :(
Martin
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Re: Help in resurrecting my ZX81

Post by mrtinb »

Measuring correct I get:
  • Between TR2 collector and R5 leg 1 - 330Ω
  • Between TR2 collector and R5 leg 2 - 0,8Ω
  • Between TR2 collector and CPU pin 6 - no reaction
Martin
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1024MAK
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Re: Help in resurrecting my ZX81

Post by 1024MAK »

mrtinb wrote: Tue Aug 07, 2018 11:01 pm Measuring correct I get:
  • Between TR2 collector and CPU pin 6 - no reaction
That looks like a problem...

Mark
ZX81 Variations
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Re: Help in resurrecting my ZX81

Post by mrtinb »

I’ll wait for my desolder gun, and remove the Z80 so I can see the traces and measure further.

Again if anyone has a scan of a component-less issue 3 pcb, that would help as well.

Mark, thanks for helping me this far.
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Re: Help in resurrecting my ZX81

Post by zx-heinz »

Hello Martin,
you should not unsolder the socket if there is no serious reason to do so! Look to the scheme again: The amplifier of the clock is Tr3 (not Tr2) for Iss3 board. So, you should check the join between Tr3(collector) and CPU (pin 6). Tr2 is mixing the FBAS signal and has nothing to do with the clock.
Last edited by zx-heinz on Wed Aug 08, 2018 6:54 pm, edited 1 time in total.
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Re: Help in resurrecting my ZX81

Post by mrtinb »

Arghh!!! So they've changed component names from issue 1 to issue 3.

How dare they! :o :shock:

That's not very helpful, when refering to components in the forum. :cry:

I'm not desoldering the socket, but the IC as there is no sockets on this pcb.
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Re: Help in resurrecting my ZX81

Post by zx-heinz »

Sorry, Martin,
what I said was nonsense! I have mixed it up with the ZX Spectrum scheme! You are right: The collector of Tr2 should show the clock signal, i.e. it should pulsing (or 2-3V on your multimeter in DC). The same at CPU(pin6).
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Re: Help in resurrecting my ZX81

Post by zx-heinz »

mrtinb wrote: Wed Aug 08, 2018 6:21 am Again if anyone has a scan of a component-less issue 3 pcb, that would help as well.
Hi Martin, as a compensation for the trouble I have made I send you the wanted scan.
Heinz
Zeddy-board2.jpg
Zeddy-board.jpg
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