8100 PSU failure Delta SM-220DB

[mitchW]

Power Mac 8100 was bought as non working. I plugged it in and after 4 seconds, something blew in the PSU. When I opened the PSU, I found out that fuse was blown and the PSU was very dusty. I cleaned it an then replaced the fuse and plugged it again, now there was another bang, but the fuse survived (it is the same rating as the original). Then I tried it again, but now I got some smoke coming out of the area pictured below. There are two a bit charred resistors that measure 84 ohms across both (wired in parallel), however the center marking is burned off, so I don't know what resistance were they should be.
I believe that something must be shorted down the circuit.

Suggestions?

 

[mitchW]

The diode D952 (next to resistors) is shorted. Anyone knows what diode should I use as a replacement?

 

[techknight]

thats probably not the only thing that shorted. you need to check all the transistors, FETs, etc... even controller ICs. 

I dont have a 7100 supply, but you need to find another person who does so you can figure out what the original resistors were. I bet the two transistors near the diode have gone leaky/shorted. 

 

[mitchW]

I disconnected the shorted diode and now PSU stopped making any smoke or blowing fuses, now it just makes very quiet buzzing. The charred resistors are probably undamaged, since they both have the same value of 180 ohms when disconnected. Yes, I am also quite sure that something else must have shorted.

For the diode, I have identified it as a 1N4146, but I am not too sure, since I am not too familiar with these color coded diodes. Can someone confirm if this is the correct one?

For now, I am going to replace this diode and check the nearby components for apparent shorts. If I will be unable to repair it, then I will give it to my friend who runs a electronics repair shop.

EDIT: I must have got a quite an early production unit, since the serial number is 5191.

 

[techknight]

I doubd its a 4146. Maybe a 4148.

 

[mitchW]

Yeah, it is 4148, the last bar is grey, not blue. But now after replacing this diode, the transistor (BD537) gets hot almost instantly.

 

[techknight]

Like I said. You need to go through all the components.

Those transistors are likely leaky/shorted.