Sinclair ZX80 / ZX81 / Z88 Forums

Hi all,

I have a 128k +3 that is for all purposes... dead.

I have changed fuses from the PSU, (both of them - one was damaged), and now I have right signals from PSU pins, but I still can't get a signal from the computer. I think the problem might be with the PCB power connector, because I tested the connections on the PCB and the values were virtually non, But I would like some opinions on that, and also, if possible, some advice on first tests someone should do on a 128k +3.

Other question is if anyone knows where to find PCB power connectors for the +3. I tried searching but came up with no results.

Thanks in advance,
António

Hi All,

So an update on this situation. I've replaced the PCB power connector and now have +12V and -12V, but 5V is missing (multimeter only shows a 0.17v on several points).

Some detail:

PSU output -12.40, +12.40 on 12v pins which are ok.
PSU outputs +7.20 (on both 5v pins, which I think it's a little high?)
IC12 shows -12 and +12 on pins 1 and 14, so should be ok
IC13 shows 0.17 on pin 14, which is wrong but it's consistent across the board on several +5v
I check for continuity from 5v PCB socket to other points and everything seems to be fine and they have continuity, so hopefully no broken tracks

So I take a closer look on the pcb power socket and did some measures and found something odd.
If I measure the voltage behind the power socket, I get the 0.17v, but the PSU is ouputing the mentioned +7.20, which is weird? (+12 and -12 are correct).

I also found that there is continuity between +5v pins and ground on the pcb. I turned the pcb abnd check the pins and confirmed:

ºG ºG
º-12 º12 º0.17 (should be 5v?)
º12 º0.17 (should be 5v?)
ºG

So directly on the power socket 5v pins I have the wrong voltage. Also, I have continuity between the 0.17 pins and the G pins which I don't think it's normal?

I've connected the PSU to another power socket without solding it (I bought a few), and took some measures and the voltage is consistent with the psu pins (eg 7.20v)

Hopefully the explanation is not as confusing as I am confused about this.

Any idea? I'm a little lost here

Thanks for any help,
António

When measuring the +5V output from the power supply unit (PSU), was this with the computer connected? Or with the computer not connected? I suspect the answer is that the computer was not connected.

This draws me to the conclusion that the circuitry that generates the +5V rail in the PSU is faulty.

There is some useful information on the +3 PSU here.

Likely causes of a faulty +5V rail are:

• Transistor T2 (2N2222)
• Transistor T1 (H1061)
• Fuse F1 (3.15A)
• Sometimes a poor solder joint on one of the transformer winding connections on the PCB.

Mark

Hi,

Thanks for the info Mark,

So when the PSU is on but not connected to the computer, I get ~7v on the 5v pins. When the psu is on and connected to the computer, I get 0.17v on 5v pcb power socket connection.

----

I followed the +5v circuit and the values are somewhat strange, as expected.

T2 collects ~12v and emits ~7.45v and T1 collects 9.98v and emits 7v. I don't think those values are correct, according to the schema. But I'm really a noob in this, so I can be wrong

I've measure TR1 (on PCB):
- negative on base to emitter and collector returned 1
- positive on base to emitter: 600
- positive on base to collector: 598

and TR2 (on PCB):
- negative or positive on base to emitter: 1
- negative or positive on base to collector: 035 (continuity bip)

So I *think*, because I know there was a short before that burned the F1, that T1 or T2 or both are faulty.

I guess I can change both, but I would really like to know what you think about it, as this is my first time messing with a psu.

Cheers,
António

I would replace TR2 first.

Mark

Yeah seems to be the weirdest.

will do. thanks

Hi guys,

Didn't have much time to mess around this until last night.

So I've replaced both TR1 and TR2 on the PSU and now voltages are ok, and the +3 is working.

Drive does not read any disks, which was expected. I hear the motor working, but nothing happens.

Probably the belt, but have to check.

One (dumb) question that is bugging me. The drive led is always on. Is this normal? i thought the led only worked when the drive was working.

Thanks once again for all your help and patience

Cheers,
António

No - the drive LED only comes on during access - check the cable connection to the 3" drive.

So I changed the drive (didn't see anything weird with the cables) for another I knew it worked was working and everything was ok. I was able to load a few games and played them.

I assume that the drive is faulty. I'll have to try to see what's happening.

Thanks for the help guys