Help in resurrecting my ZX81

[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

[1024MAK]

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

Mark

[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. Maybe the old legs are oxidiced. I'll measure from the start.

[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

[1024MAK]

Measuring correct I get:

• Between TR2 collector and CPU pin 6 - no reaction

That looks like a problem...

Mark

[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.

[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.

[mrtinb]

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

How dare they!

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

I'm not desoldering the socket, but the IC as there is no sockets on this pcb.

[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).

[zx-heinz]

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