Amiga 500 PSU 312503-03 wrong -12+12 output voltage

[Mr KS]

Hello, I'm new to the forum, as well as a big fan of retrocomputers. I ask for your help as I am trying to fix the transformer for amiga 500 312503-03 25090 (made in W. Germany) output +5v (4.5A) +12v(1.0A) -12v(0.1A). This one for instance:



With an output voltage (no load) of +14.2 and -14.1 (instead of +- 12). The 5v is correct. I did a complete recap but the voltages I output are always identical. I can't figure out what they are (or is) the linear voltage regulator. On various sites I understand that LM304T-12 is identified but honestly I can't find any 3-pin component (apart from a bu508af that has other functions..). Someone could tell me which one it is, so that I try to replace it? Thank you very much!

[giantclam]

There's a bit of info here -> http://electev.blogspot.com/2020/03/...su-repair.html

[derSammler]

Originally Posted by Mr KS

With an output voltage (no load) of +14.2 and -14.1 (instead of +- 12).

Most basic mistake: never check voltages with no load. Add some load to the rails you are testing. Also, tolerances on the 12v rails are pretty high, +/- 10-15% in most cases.

I maintained exactly the same PSU not long ago. What happened to the heaksink on yours?

[Mr KS]

Originally Posted by derSammler

Most basic mistake: never check voltages with no load. Add some load to the rails you are testing. Also, tolerances on the 12v rails are pretty high, +/- 10-15% in most cases.

I maintained exactly the same PSU not long ago. What happened to the heaksink on yours?

..there's not any heatsink!

So are the -12+12 tolerances correct? Because 14v seems like a lot to me. I compared the output on 2 other PSU (amiga 600 model) and they are around +12-12, with minimal tolerances. The Amiga 500 works correctly...I'll try to see how they change with the Amiga 500 connected. Thanks for now.

[r.cade]

Exactly- plug it into the Amiga and then measure there with it turned on.

The voltage is spiked a little to get through the 6' cable and connectors, and then there is the drop from the actual load.

[Orionis5]

Do not risk actual Amiga for measuring load. Try something else for particular voltage, at least for +5v and +12v. 12 is easy with connecting simple fans.