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View Full Version : Commodore Amiga 500 PSU Repair



derekb
October 31st, 2014, 11:23 AM
Hi
Can anyone help with some info on repairing an old Amiga 500 PSU. I dont want to replace it with an atx psu etc, rather try to sort out the old unit in practice for some other older kit i have.

21290

i have no output with a ticking noise, if left on for a min, two of the resistors feeding the transormer get realy hot !

i have had a few mis haps allready, capacitors need replacing so their on their way, tried removing the 2 x 47ohm resistors and powering up hmmm.. blue a thyristor so that on order too along with a new power transistor just in case. most of the board looks ok and most of the resistors and diodes seem good.

any one had experiance or has advice. ??

barythrin
November 3rd, 2014, 08:46 AM
I'm curious if anyone has a diagnosis of this as well. Have you replaced the caps yet? (I know you said you ordered them). Have you measured the voltages? Is it clicking when disconnected from everything?

derekb
November 3rd, 2014, 11:10 AM
Thanks for viewing

No the caps are still comming in.
I have now removed the main transformer, im trying to get to a point that allows the power to be connected whilst beeing stable (no smoke) so i can get my osi on it. I have drawn out the schematics in the hope i could get my head around it, which is not getting me very far !

It seems to be using the main transistor as a chopper with feedback via a opto isolator.

Im new to this but determined. Most users throw them away for newer units. This is more a discovery of talent (assuming i have any :) so will persivear untill its done. i would rather find the fault than just replace all the components though.

Derek

Pet Rescue
November 8th, 2014, 01:38 AM
Thanks for viewing

No the caps are still comming in.
I have now removed the main transformer, im trying to get to a point that allows the power to be connected whilst beeing stable (no smoke) so i can get my osi on it. I have drawn out the schematics in the hope i could get my head around it, which is not getting me very far !

It seems to be using the main transistor as a chopper with feedback via a opto isolator.

Im new to this but determined. Most users throw them away for newer units. This is more a discovery of talent (assuming i have any :) so will persivear untill its done. i would rather find the fault than just replace all the components though.

Derek

Derek that's the best way to learn and exactly how I am. If you try and fail you still win because you gain a bit more knowledge and experience each time. If you try and succeed then it's a win win situation.

On the few power supplies I have worked on both switching and linear the problems have been capacitors gone short, diodes short or open circuit and voltage regs gone bad.

I always like to test suspect components out of the board and one of the best tools are your eyes. Look for damage on the boards or components like burning etc or bulging caps etc.

I haven't worked on an Amiga supply yet, they have always behaved well.


Oh another word of advice when you get frustrated just put it to one side and come back to it later, even if it's a month or more later. Sometimes you get a fresh idea and it's the right one.

KevinO
November 12th, 2014, 09:42 AM
Had a chance to get back to this yet? Interested in hearing what you've found. I've never worked on an A500 supply, but I have done a couple of 128 power supplies, and they were interesting. Maybe not remotely the same, but the same case makes me think they might be. :)

derekb
November 15th, 2014, 09:38 AM
Sorry for the delay, im New and replys hang around awhile befor they go live

Yes i have now replaced all the caps, found a bad Diode and replaced a few transistors. Im having trouble working out how the main transistor gets switched on. i have drawn up the circuit but just cant see it as the diode on the base is taking power from the emmiter ??.

I removed the transformer as i dont want to load the circuit yet.

I had the idea that using some low voltage input may help work out wats up. i have plumbed in an 18v ac supply. the bridge diodes fire up ok and i get 25v dc. But i still cant get the transistors to switch.

21499

Any idears how i can test parts of this circuite before i plug in the 230v ?

Derek

KevinO
November 18th, 2014, 09:43 AM
No, a low voltage isn't going to do it. You're not getting the circuits up to operating voltages. I'd say plug it in. Stand back if you're worried about something blowing, but you started out with it just clicking, so unless you've replaced a component with the wrong value or bridged a solder joint, I can't see it going any worse.

If you had a bad diode, that's probably the culprit. Was it open or shorted?

derekb
November 19th, 2014, 03:58 AM
Open circuit on the output side

I have been chatting with the chaps at my collage (all be it im 46) and started putting some stuff into Multisim.

I will put the transformer back in and give it a go.


Will Get Back ASAP

derekb
November 28th, 2014, 08:09 AM
My apologies if this is a double post!

Put it together and nothing for about 2 seconds and then took out a resistor.

As I said, I have been badgering my tutors but as always they are too busy when it comes to real world stuff.


Not sure what to try next ?

Derek

geoffm3
December 1st, 2014, 06:55 AM
My apologies if this is a double post!

Put it together and nothing for about 2 seconds and then took out a resistor.

As I said, I have been badgering my tutors but as always they are too busy when it comes to real world stuff.


Not sure what to try next ?

Derek

Sounds like either a short or the thing isn't switching causing too much current draw through those resistors. Your schematic is incomplete so it's hard to follow what you have there without it in front of our faces.

I am thinking that part of the problem is that since you are running the circuit open loop the input switcher doesn't have the feedback via the opto-isolator to know if it's driving things like it should. You've also removed the input winding from the picture by pulling it out so you've removed that current path to ground as well.

KevinO
December 1st, 2014, 04:03 PM
My apologies if this is a double post!

Put it together and nothing for about 2 seconds and then took out a resistor.

As I said, I have been badgering my tutors but as always they are too busy when it comes to real world stuff.


Not sure what to try next ?

Derek

I'm responding to you and geoffm3, who asked an important question....when you said you put it together, did you put it all the way together, or just put the transformer back in? I'm by no means an expert on switching power supplies, because I believe they run on magical pixie dust, but I do know that everything has to be there....

derekb
December 3rd, 2014, 11:04 AM
Yes its was all back together when the resistor popped.

The comment about the switcher not running sounds like you could be on the right track, the drawing i did does not show the other side from the transformer. I will add some more bits onto the drawing.
Worked out to day that with no transformer the large transistor would never get turned on, so no output to the smaller transistors..

I will keep on pluging at this.. If my replies are a little slow my appologies, us newbies to the forum have to go through a reply filter befor they apear in the thread.

Thanks for looking and replying though.

Derek