Dead 128k +3

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amateus
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Dead 128k +3

Post by amateus » Wed Mar 21, 2018 3:46 pm

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

amateus
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Joined: Wed Mar 21, 2018 2:28 pm

Re: Dead 128k +3

Post by amateus » Sun Apr 08, 2018 9:37 pm

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

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1024MAK
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Re: Dead 128k +3

Post by 1024MAK » Mon Apr 09, 2018 11:28 am

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

amateus
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Re: Dead 128k +3

Post by amateus » Mon Apr 09, 2018 4:28 pm

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

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1024MAK
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Re: Dead 128k +3

Post by 1024MAK » Tue Apr 10, 2018 8:19 am

I would replace TR2 first.

Mark

amateus
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Re: Dead 128k +3

Post by amateus » Tue Apr 10, 2018 10:31 am

Yeah seems to be the weirdest.

will do. thanks

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Re: Dead 128k +3

Post by amateus » Thu May 03, 2018 12:34 pm

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

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Re: Dead 128k +3

Post by RWAP » Fri May 04, 2018 8:58 am

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

amateus
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Re: Dead 128k +3

Post by amateus » Mon May 07, 2018 12:23 pm

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 :)

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