A1200 PSU no-load voltages?

Hi,

Trying to get my A1200 back up and running after many years. It ran, but the audio is crackly/glitchy.

Couldn't see any obvious capacitor leakage, and I don't have SMD soldering experience, so I took the easy option, and bought a complete replacement motherboard on eBay. Got it installed, but have the same/similar audio problem.

Sounds like a common problem, so I could have two boards with faults - but reading up on it a bit more, I've also seen references to PSU problems (specifically the -12V ) causing bad audio.

So I've taken a multimeter to my PSU, with no load. I'm getting:

+5v to gnd: 5.43v
+12v to gnd: 12.37v
-12v to gnd: No measurable voltage. (meter fluctuates a little around zero. Tried the probes both ways around to be sure)

I'm assuming that I should be getting *something* from the -12v? - even with no load? (just the multimeter probes on the connector pins)

With a completely dead -12v output would you expect the Amiga to work as normal, except for the audio? - does it look like I may actually have two working boards and just a dead PSU?!

(Not got a spare Amiga or ATX PSU for testing at the moment...)

[hese]

Yes, you should be getting near -12V even without a load but the best is to plug the PSU to A1200 motherboard and measure the actual voltages from the PSU connector.
IIRC -12V is used only for audio so the Amiga boots even without it.

[prowler]

Forget testing the A1200 PSU on no load. The results will be unpredictable and meaningless.

[roy bates]

Originally Posted by prowler

Forget testing the A1200 PSU on no load. The results will be unpredictable and meaningless.

another sensible comment

always test a supply under load,thats what it was made for.

but try not to use the amiga as the load,try to make a dummy load for it.

[hese]

Originally Posted by prowler

Forget testing the A1200 PSU on no load. The results will be unpredictable and meaningless.

Not unpredictable, I've always got near the same voltages than what is should provide under load.

[demolition]

Originally Posted by hese

Not unpredictable, I've always got near the same voltages than what is should provide under load.

If you got the heavy transformer type PSU it will be stable without load, but the switch mode types may or may not.

Originally Posted by prowler

Forget testing the A1200 PSU on no load. The results will be unpredictable and meaningless.

Hmmm...

This morning, I opened up the A1200, and measured the voltages from the back of the power connector:

+12v: 12.28v
+5v: 5.248v
-12v: -6.125v

So there was something coming out of -12v, but it looked suspect.

Then I unplugged the PSU cable, tried to measure from the pins again - and unexpectedly, I was reading a good -12v output.

Reconnected it, booted up the machine again, tested the audio, and it's working now!

No idea if it'll keep working - but it looks like I do just have a dodgy PSU then - and won't have to get soldering capacitors just yet... Might mod an ATX PSU as a longer term solution.

[FOL]

Off load some A1200 PSU's give odd -12v volatages.
I have seen no reading at all on some.

You have a definite faulty component in the Audio Power / Audio section.
Prowler makes a good point.

If I recall, had something like this in the past. Turned out to be post amp.

[hese]

Originally Posted by demolition

If you got the heavy transformer type PSU it will be stable without load, but the switch mode types may or may not.

All the Amiga PSUs I've measured (both types, dozens of them) have given valid voltages even without load.

Of course measuring a PSU without load is a bit useless because that doesn't tell how the PSU behaves under load.

[FOL]

Originally Posted by hese

All the Amiga PSUs I've measured (both types, dozens of them) have given valid voltages even without load.

Of course measuring a PSU without load is a bit useless because that doesn't tell how the PSU behaves under load.

Considering where I work, I have measured hundreds of PSU's.
Both Prowler and Demolition are correct. You cant measure offload and expect to get accurate readings.

Cost Reduced A1200 PSU's gives no reading for -12v.

Originally Posted by FOL

Off load some A1200 PSU's give odd -12v volatages.
I have seen no reading at all on some.

You have a definite faulty component in the Audio Power / Audio section.
Prowler makes a good point.

If I recall, had something like this in the past. Turned out to be post amp.

My problem in this case is definitely the PSU.

I've now found that wiggling the power cable where it comes out of the PSU causes/solves the problem. There's a bad connection on the -12v somewhere. May just have been over-strained at some point. Haven't opened up the PSU yet to have a look, though.

(Kind of annoying, as I'd rushed out and bought a replacement motherboard - I really didn't expect the PSU to cause that sort of glitch, was convinced it was dead capacitors or worse. Oh well, got a spare board now...)

[roy bates]

it sounds like you might have a break in the cable somewhere,you can test the cable on its own with a multimeter on continuity test.
pretty easy to do if you can get the supply apart,and test it when its off.

this happens alot with mouse and joystick cables because there moved around alot.
they normally snap inside where it bends the most,about 3 inches from the ends of the thing that is in use.

[FOL]

Originally Posted by bluescrn

My problem in this case is definitely the PSU.

I've now found that wiggling the power cable where it comes out of the PSU causes/solves the problem. There's a bad connection on the -12v somewhere. May just have been over-strained at some point. Haven't opened up the PSU yet to have a look, though.

(Kind of annoying, as I'd rushed out and bought a replacement motherboard - I really didn't expect the PSU to cause that sort of glitch, was convinced it was dead capacitors or worse. Oh well, got a spare board now...)

If it is broken, then it should definitely not read -6v off load.

Originally Posted by roy bates

it sounds like you might have a break in the cable somewhere,you can test the cable on its own with a multimeter on continuity test.
pretty easy to do if you can get the supply apart,and test it when its off.

this happens alot with mouse and joystick cables because there moved around alot.
they normally snap inside where it bends the most,about 3 inches from the ends of the thing that is in use.

Yep, that's all it seems to have been - I've cut about 4" off the cable and resoldered it all up, and it seems good now. Can measure a reliable -12v with no load (it's the 'heavy' PSU with a big transformer), and my audio works, even when wiggling the cable.

Managed to do a fairly tidy job, re-using the original strain reliever. One of the plastic 'legs' that the screws go into sheared off the case whilst taking it apart, but it's held back together fairly securely by the remaining 3 screws.

(Next job... get a data transfer solution sorted out, to retrieve some old files from the Amiga's HD, and move it all over to a 4Gb CF drive... )