Hardware Insights Forum

Ok, if you did read my intro post you are expecting this one.

I like to restore old computers and i really need help to determine if/how i can fix the power supply for this one. I dont know if the computer is good as i have no equivalent psu to test it . But it is very clean and has all parts.

When i first got the computer it simply would not power on. I dismantled it and tested if the PSU had some voltage when in standby, nothing.

I then checked fuse, fuse is good. I then made a visual inspection and quickly found a burned 1ohm resistor near the ac input of the pcb ( see pictures ) .

I decided to put a new one and try powering the Psu to see what would happen( not connected to computer but still maybe not the smartest move ). Made a loud pop and the resistor burned again. Fuse is still good though, its a 3.3A in 3A socket, cannot tell if its the original or not, but i believe it is as the computer did not appear to have been opened before i got it.

After some more testing i also found that the two biggest transistors have all their pins shorted/grounded( low voltage rectifier ?).

Other transistors (i only tested the big ones with heatsinks) are not shorted in this way but the transistor looking part ( labeled TOP200YAI , apparently a PWM switch ) where i can see a burn mark(see picture) has short between center and rightmost pin ( when looking it front). Pins would be "source" and "drain" according to datasheet, so this switch appears open atm. Cant say if this is normal. I did not check any diodes yet as there are so manu of em and would not know where to begin.

I took some pictures of the PCB and indicated the things i noticed yet. Problem is i am clueless as how to interpret the data i have at this point. If you guys can help me go further and maybe get to fix this psu that would be great. I really love old hardware and do not like it when its broken .


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edited to add a couple details.

About time you posted this
Interesting that you have an old Mac. at work (I work at a tech recyling/computer repair+sales shop), we got in a functional Macintosh Classic...one of those with the tiny little CRT built in. I'd take it home but I'm moving soon and I already have too much stuff.
Anyway, on topic (shut up shovenose nobody cares about your life story lol), c_hegge will probably be able to help you out more than I will, he is our power supply expert.

shovenose wrote:About time you posted this
Interesting that you have an old Mac. at work (I work at a tech recyling/computer repair+sales shop), we got in a functional Macintosh Classic...one of those with the tiny little CRT built in. I'd take it home but I'm moving soon and I already have too much stuff.
Anyway, on topic (shut up shovenose nobody cares about your life story lol), c_hegge will probably be able to help you out more than I will, he is our power supply expert.

Yeah, i know i took my time writing this XD. I have two other macs in my 15+ computer collections, but they are G3 and do not use the same psu. I am thinking about going to a used computer part shop nearby to see if they have one for this performa, would give me a mean to test the computer and a base of comparison to make sure my eventually repaired psu matches a certified one. Id hate to kill this rig.

The two parts on the big heat sink are the output rectifiers, not transistors. They will need replacing, as well as the PWM switch. If you can read the part numbers on them, then I could probably advise you on a suitable replacement part.

Ok so i unsoldered those two rectifiers ( MBR3045PT) And that turned out to be a pretty difficult thing to do considering the way they made it impossible to properly reach the screws with tools...

Turns out they are not shorted outside the circuit. The pads are !!

The rectifiers test as a pair of diodes pointing toward the center pin, diode tester reads 0.16-.0.17 v both of em.

Ill investigate the circuit further tomorrow but now im tired, Got two wisdom teeth extracted this morning, took a toll on me.

In the meantime any other suggestions are welcome

I'd say that the shorted transistor on the primary (with the burn mark under it) is the thing we need to look at first. This unit looks to have a two-transistor 5vsb circuit, rather than the more reliable PWM chip 5vsb. The blown transistor is the main 5vsb switcher. They usually fail when the 'killer cap' dries out. That cap could be any one near the transistor, but is probably one of the two I circled. Try replacing the all of the small caps and the transistor. If it blows again, then it probably has a zener diode on the output side which will need replacing. It's put there because when the 5vsb voltage goes too high (which can be caused by the 'killer cap' failing), the zenner will short internally and blow up the switching transistor, but the motherboard will survive.

I removed the pwm switch ( where the burn mark ) , and also thosr two caps, both 47 uf and they test fine with simple capacitance meter once out( Ive read that this test can be unreliable though) . Anyway, i still have the issue that 5 out of 6 pads where the rectifiers were installed test grounded. I tried following the traces and unless i made a mistake somewhere, At least one of the leg for each rectifiers is connected directly to the ground. And in one case it seems at least another leg is connected to ground via an inductor coil. Could all those grounding be normal ?

Fundamentally, I really have no idea how those switching power supplies work so im kinda trying to learn as it goes. Thats it for tonight.
Thanks for the help so far! [hr]
Btw i read some more and im gonna build one of those 100w bulb in series tester for when im ready to power up the thing.

I have a small problem, the PWM switch labeled TOP200YAI is discontinued and im having a hard time finding it at a reasonable price.
Cannot seem to find information regarding equivalence either.

Where abouts are you located? That way, I can find a supplier and see if what they have.

I should really make the "location" field a requirement...