Low -12V on A1200 PSU..

jbenam · 04 October 2009, 20:56

Hi mateys,
some days ago, I noticed my A1200 PSU (one of those little black ones) displayed a lot of noise on screen. Shaking it a bit fixed it.
Now sometimes when I shake it, the Amiga just resets.

I've just tested the voltages with a multimeter, and the -12V never goes higher than -10V. Is the on-screen noise caused by that?

I don't see anything wrong on the PCB... any general hints in what should I search for?

Thanks

kipper2k · 04 October 2009, 21:20

Where did you actually measure the -12V... at the power supply plug or points on the motherboard ?. If the power at the power supply is low by 2V then that will affect everything else downstream and probably cause resets under high load (ie, HD, Floppy access and CPU at full load.

If you have a spare PSU, try that.

If your PSU is showing good voltage (but probably not if you been shaking it to get it to work) then your Caps would probably be bad on the motherboard

edit... shaking it is not a good idea, you could really cause major damage to the motherboard if something in the PSU is loose and shorts out!

prowler · 04 October 2009, 21:23

I don't think the low -12V rail is causing the video noise, but if your Amiga is resetting sometimes when you shake the PSU, I would look for a dry or unmade joint on the PCB (you will have to remove it to check underneath).

But before you do this, try wiggling the mains lead and Amiga power lead while holding the PSU still to check for (very unlikely) breakage or loose connections in these cables.

Also, it has been known for the connections between the Amiga's power socket and motherboard to work loose with the repeated plugging and unplugging of the PSU lead and the wiggling that this usually entails.

jbenam · 04 October 2009, 22:32

@kipper2k
I've measured them at the power supply plug. Sadly, my spare PSU died some months ago. I should have a A500 powerpack somewhere, but even that one is not in top conditions.
5V and 12V are fine. Yeah, I was thinking about getting new caps.

It does not need to get shaken to work, it works fine normally. And it wasn't real "shaking", more like little pushes on it

@prowler
Still, when giving little pushes on it the noise disappeared... Yeah, it has a lot of dry joints. I'm probably gonna resolder all of them to be sure, thanks

Now I've put some contact cleaner on it and I'm waiting for it to dry, as the PCB was pretty dirty

jbenam · 05 October 2009, 15:57

A lil bump with new measurings after the PCB cleaning:

5V: 4.70-4.78V
12V: 11.27-11.30V
-12V: -10.70V

They aren't too good, huh?

Luckily my miggy 1200 works fine... I guess I'm gonna change all the caps and see how it fares

Shadowfire · 05 October 2009, 22:10

The 5V is marginal. However, it is not the source of the problem of "reset when shaken"... that is a loose connection somewhere.

The +12V and -12V are fine. They are only used for the audio section of the board, and as long as you aren't noticing audio distortion, let it be.

04 October 2009, 20:56	#1
jbenam Italian Amiga Zealot Join Date: Jan 2009 Location: Italy Age: 36 Posts: 1,910	Low -12V on A1200 PSU.. Hi mateys, some days ago, I noticed my A1200 PSU (one of those little black ones) displayed a lot of noise on screen. Shaking it a bit fixed it. Now sometimes when I shake it, the Amiga just resets. I've just tested the voltages with a multimeter, and the -12V never goes higher than -10V. Is the on-screen noise caused by that? I don't see anything wrong on the PCB... any general hints in what should I search for? Thanks

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04 October 2009, 21:20	#2
kipper2k Registered User Join Date: Sep 2006 Location: Thunder Bay, Canada Posts: 4,323	Where did you actually measure the -12V... at the power supply plug or points on the motherboard ?. If the power at the power supply is low by 2V then that will affect everything else downstream and probably cause resets under high load (ie, HD, Floppy access and CPU at full load. If you have a spare PSU, try that. If your PSU is showing good voltage (but probably not if you been shaking it to get it to work) then your Caps would probably be bad on the motherboard edit... shaking it is not a good idea, you could really cause major damage to the motherboard if something in the PSU is loose and shorts out!

04 October 2009, 21:23	#3
prowler Global Moderator Join Date: Aug 2008 Location: Sidcup, England Posts: 10,300	I don't think the low -12V rail is causing the video noise, but if your Amiga is resetting sometimes when you shake the PSU, I would look for a dry or unmade joint on the PCB (you will have to remove it to check underneath). But before you do this, try wiggling the mains lead and Amiga power lead while holding the PSU still to check for (very unlikely) breakage or loose connections in these cables. Also, it has been known for the connections between the Amiga's power socket and motherboard to work loose with the repeated plugging and unplugging of the PSU lead and the wiggling that this usually entails.

04 October 2009, 22:32	#4
jbenam Italian Amiga Zealot Join Date: Jan 2009 Location: Italy Age: 36 Posts: 1,910	@kipper2k I've measured them at the power supply plug. Sadly, my spare PSU died some months ago. I should have a A500 powerpack somewhere, but even that one is not in top conditions. 5V and 12V are fine. Yeah, I was thinking about getting new caps. It does not need to get shaken to work, it works fine normally. And it wasn't real "shaking", more like little pushes on it @prowler Still, when giving little pushes on it the noise disappeared... Yeah, it has a lot of dry joints. I'm probably gonna resolder all of them to be sure, thanks Now I've put some contact cleaner on it and I'm waiting for it to dry, as the PCB was pretty dirty

05 October 2009, 15:57	#5
jbenam Italian Amiga Zealot Join Date: Jan 2009 Location: Italy Age: 36 Posts: 1,910	A lil bump with new measurings after the PCB cleaning: 5V: 4.70-4.78V 12V: 11.27-11.30V -12V: -10.70V They aren't too good, huh? Luckily my miggy 1200 works fine... I guess I'm gonna change all the caps and see how it fares

05 October 2009, 22:10	#6
Shadowfire Registered User Join Date: Aug 2001 Location: Connecticut USA Posts: 617	The 5V is marginal. However, it is not the source of the problem of "reset when shaken"... that is a loose connection somewhere. The +12V and -12V are fine. They are only used for the audio section of the board, and as long as you aren't noticing audio distortion, let it be.

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