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Old 29 October 2018, 14:30   #1
sean_skroht
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A500 Power Supply -12v Problem

I recently picked up an A500 240v power supply, part no. 312503-05 that delivers 4.3A on the +5v line. Both +5v and +12v lines measure fine but unfortunately the -12v is reading over -17v. Can anyone suggest where I should start looking to remedy that? Thanks heaps.
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Old 29 October 2018, 14:43   #2
project23
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EDIT: I suggested something a bit silly at first...

Okay more helpful question - are you measuring these voltages while under load (ie when the Amiga is running), or straight off the pins?

John

Last edited by project23; 29 October 2018 at 15:08.
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Old 29 October 2018, 23:50   #3
sean_skroht
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Quote:
Originally Posted by project23 View Post
EDIT: I suggested something a bit silly at first...

Okay more helpful question - are you measuring these voltages while under load (ie when the Amiga is running), or straight off the pins?

John

Thanks for your reply.

Just off the pins, I'm a bit hesitant to plug it into one of my Amiga's with the -12v being the way it is unless someone thinks it should be ok. The +5v and +12v are 100 percent fine, that I'm sure of. A couple of resistors don't measure the values they're supposed to so I might look at those but I am unsure of the semiconductors and how to measure them. Hence if someone has any suggestions as to what the cause might be and where to start looking.

Thanks again.
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Old 29 October 2018, 23:53   #4
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Well, it's not ideal, but you're probably safe enough to connect it at that, at least for a short while. Connect your meter probes so that you only need to turn on the machine for a short while to see the -12V reading. It's very possible that it's simply a result of there being no load on the PSU.
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Old 30 October 2018, 11:44   #5
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I normally see the -12V rail measuring between -15V to -16V when not loaded, and while -17V is a little lower I don't think it should be a problem, at least briefly while you measure the voltage under load.
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Old 30 October 2018, 12:55   #6
sean_skroht
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OK, cool guys. Will give it a look at in the next day or so. Thanks very much for your advice.
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Old 30 October 2018, 13:08   #7
project23
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Overall its an 'odd' problem to arise with a PSU if you ask me - thats why I was making silly suggestions...

Like... why (under load) would a PSU give -17v but +12 perfectly fine?
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Old 30 October 2018, 13:18   #8
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A couple of reasons: there could be a built-in load that's sufficient to regulate the +12V rail, but not on the -12V rail which carries much less power, or there could be a requirement for a certain amount of current flow in the main output to provide regulation in other outputs, or the PSU could be a mixture of design types, with the positive rails using a stable type of PSU technology, but the -12V rail using a different technology to generate the negative voltage that results in different behaviour.
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Old 08 November 2018, 08:37   #9
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OK got to the bottom of it.

Over -17v on the -12v rail was way too much and I wasn't prepared to hook it up to an Amiga motherboard to measure under load and risk damage. It should normally measure around -11.90v.

It turns out the culprit was the 7912 voltage regulator at position IC3 near where the DC cable that goes to the computer is soldered. Replaced it, and now -12v rail measures as it should. Also replaced all the capacitors afterwards while I was at it.
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Old 20 November 2018, 14:40   #10
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Open the source and check the five electrolytic capacitors in the low zone.
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