8

I have tried searching for "amiga 1200 psu schematic" and many variations, but haven't been able to find an A1200 power supply schematic. I was able to find an A500 PSU KiCAD schematic and re-capping instructions for the A1200 PSU (though, this photo doesn't look identical to mine so maybe that's a PSU from a different region) but all other results seemed to turn up nothing that I could recognise as a schematic for the board.

Amiga 1200 PSU circuit board

The PSU has a UK plug.

Side note: I suspect a shorted cap but I haven't checked all of the caps yet. I believe a schematic would be useful to know what tracks should and should not have continuity. The line filter—at least, it looks like it could be the line filter—is damaged and I need to replace it, as well as the damaged 30 ohm resistor (above the word 'caution'). This damage was due to using an incorrect fuse (I have no excuse, the warning was right there). It seems the board is shorting, but I'm unable to determine why (it was shorting before the line filter and resistor were visibly damaged... I damaged them, oops).

Edit: Turns out 3 of the 4 diodes on the full bridge rectifier had shorted. I guess that there was a power surge causing a large reverse voltage. There was a thunderstorm recently. I'm surprised this is the only thing in the house that failed. It's a 1N4007, so the reverse voltage must have been above 1 kV.

The common reasons for a diode failure are excessive forward current and a large reverse voltage. Usually, large reverse voltage leads to a shorted diode while overcurrent makes it fail open.

I bought a new used PSU (which I suspect may have the same fate eventually), and I'm aware there are modern PSU alternatives for the A1200... but until it arrives, I thought I'd at least attempt to fix this board and get some (hopefully non-lethal) experience with mains voltage boards. I cut the wires to make it easier to work with. I'll probably end up using crimped connectors when (..if) I reconnect the wires.

Edit: There are also some burn marks on one of the jumper wires below the heatsink in the middle of the board. I believe a schematic would help me investigate that.

Edit: I actually probably won't use it once I've fixed it, since I'd indeed be worried about taking the computer with it as you mentioned.

Improve this question
10
  • 3
    Just a thought, wouldn't it be easier, safer and cheaper to scrap that thing and get a new PSU? Old commodore power supplies have a tendency to die, and take the computer with them.
    – Omar and Lorraine
    Jan 26 at 13:32
  • 2
    @Героямслава Yes. As I mentioned in my question, I already bought a new one. I'm attempting to fix this one for fun/experience/learning/etc. I actually probably won't use it once I've fixed it, since I'd indeed be worried about taking the computer with it as you mentioned.
    – Nick Bolton
    Jan 26 at 13:33
  • 2
    @Героямслава Older Commodore power supplies like for the C64 can be dangerous to the computer when they fail but that doesn't really apply to the later ones for the C128 and Amiga models. When they fail they don't fail in a way that provides dangerous voltages to the computer like the older style ones do. In general the Amiga power supplies are both safe and relatively easy to service as long as the circuit board itself isn't cracked or damaged.
    – mnem
    Jan 26 at 13:42
  • 1
    To my knowledge Commodore used multiple manufacturers for the 1200 PSU and as a result there is a few different designs out there.
    – Edders
    Jan 27 at 12:16
  • 1
    @Justme Turns out it was a shorted diode, not a shorted cap.
    – Nick Bolton
    Jan 29 at 11:07

1 Answer 1

Reset to default
13

As one did not seem to exist, I created an A1200 PSU schematic and have published my work in progress. This schematic is based on the A500 PSU schematic by Greg McCarthy.

These screenshots need updating; for the latest version, check the GitHub repository.

Amiga A1200 PSU schematic

Amiga A1200 PSU PCB

Amiga A1200 PSU PCB Front 3D

Amiga A1200 PSU PCB Back 3D

In case anyone is wondering, I fixed my A1200 PSU board. This would have been significantly harder without a schematic. The FET and the PWM IC had failed short, and a bunch of resistors had fried (between the PWM IC and the FET). The current sense resistors also fried. Fairly extensive repair, about 10 parts replaced in total. Oh, and of course, the caps were leaking, but were still working (I replaced them anyway).

Improve this answer
1
  • 4
    I love when I see someone make KiCad look easy. I get as far as two LEDs and a resistor before giving up. :-)
    – Geo...
    Feb 11 at 15:02

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .