Help me repair an Amiga power supply?

[Jeff]

I recently picked up an A1060 sidecar with a dead power supply board. I have a fair electronics background with good component replacement skills, but I have never worked on troubleshooting a dead SMPS beyond replacing old caps.

I would like to keep the power supply board original if possible. So far I have replaced the rectifiers and electrolytic caps, and either tested or replaced most of the rest of the discreet compontents including the switching transistor.

The "shotgun" approach has always worked for me in the past, but I just can't get this one going. I don't have a schematic for it either. I can provide good photos of both sides of the board, or send it to somebody and offer to pay you for your time if you can take a look at it for me.

I can also sub in a board from a modern power supply and be done with it, but I was trying not to.

Thanks and Happy Holidays,
Jeff

[Hodgkinson]

I fixed a SMPS in a 24-port network hub some time ago. The large beefy 100K or so resistor from the rectified mains side at ~300v DC had gone open-circuit [cooked, charred, and falling to pieces], and in the process it had caused the controller IC (A small 8 pin DIP IC on a sub-board) to go short circuit across what appeared to be it's supply pins.
Replacing the resistor with a even larger (Higher power) resistor and swapping the controller IC did the trick.

PS. Phihong power supplies in the US are extremely helpful with regards to fixing their power supplies - BOM, circuit diagram, etc, can't praise them enough.

PPS. The other common failure is the power devices going short circuit.
EDIT: Oh, you've already swapped them :-)

Just thought I'd point out some of my experiences,
Hodgkinson.

[CLS2086]

Hi,
check if the transformer is not cut/open...
and post some good photos

[Plaz]

@Hodgkinson

Dang, I'm ringing you up the next time I'm working on something. I'll probably save a day or two. :-)

Plaz

[Hodgkinson]

Dang, I'm ringing you up the next time I'm working on something. I'll probably save a day or two.

:-) :-D I'd be glad to help

[Jeff]

Here is a link to the pictures.
Power Supply Front Side
Power Supply Back Side

The line voltage goes in at the spades by the fuse. The output should be +12vdc, +5vdc, and -12vdc.

I do have 300vdc out of the main rectifier and across the filter caps with no AC leakage. I can follow it to the main switching transistor, but it seems to stop there. I don't understand the switching circuitry beyond that.

One of the low voltage rectifiers was open (they have both been replaced). I have also replaced both of the small IC's along with all of the electrolytics. One is an LM393N and the other is an LM339N.

I can email higher res versions of these pictures if needed. I  am pretty much stuck and I hate to give up on it at this point. There is low resistance across the windings on both sides of the main transformer, but I'm not sure what it should be.

There are also a couple of components that I can't identify and don't know how to test (Like the blue Schaffer box at the bottom).

Anyway here are the pictures, and thank you in advance for any ideas. BTW I do have an older Tektronics scope too if that will help.

Thank You,
Jeff

[Ohforf]

The blue schaffer Box looks like a small Transformer.
Schaffer is a German Transformer / Power Supply Manufacturer.
Which other Parts are not identified yet ?

And dont forget : switching Power Supplys need Load to work.

[Hodgkinson]

Hi there,
I’m afraid my knowledge stops there, too. The only reason I managed to fix my SMPS was because I could see visual damage.

I've had a word with a electronics engineer, and the best advice that we can give you is to try loading the PSU outputs, and to go around and check the resistance of all of the higher valued resistors (>=100K) by unsoldering one leg of each resistor and checking each one in turn - The higher valued resistors have a tendency to commonly fail by going open circuit.

Regards,
Hodgkinson

[Jeff]

Yes, I have done all of my tests with a proper load on the output. I forgot to mention that.

I will begin the search for an open resistor. That's how I was able to test the small NPN and PNP transistors, by lifting a leg.

Thanks again,
Jeff

[metalman]

Jeff wrote:
The "shotgun" approach has always worked for me in the past, but I just can't get this one going. I don't have a schematic for it either. I can provide good photos of both sides of the board, or send it to somebody and offer to pay you for your time if you can take a look at it for me.

Thanks and Happy Holidays,
Jeff

The Most probable failure items:
fuses
electrolytic caps
trim pots
diodes

look for swelled electrolytic caps,
visual look or smell of burned diodes or resistors
use a meter to check diodes

check pot resistance readings
turn the trim pots, then back to original position.
check pot resistance again.

[Jeff]

It ended up being a bad winding in the transformer. I won't be repairing that:-(

On the lighter side, I have fitted the board from a old 250 watt AT power supply in it's place and it works great. I guess I should have done that to start with. My A1060 is finally finished and working!

Thanks for all the advice.
Jeff

[Hodgkinson]

Glad to hear that it’s working :-)
I guess the winding was short/open circuit? You're lucky that you were able to spot it - It only takes a single turn of a winding to short out in an EHT transformer and that'll stop everything dead - And an ohms meter won’t pick it up.

:-)

Hodgkinson.