Repairing Mac G4 Power supply unit.

A friend of mine who runs a printing company still needs to use his Mac G4s

for his business since the printing industry uses Mac widely. Problem is

one of his Mac G4 has no power problem. He has been trying to fix it but he

had no luck. I suggested him,“Why don't you let me to repair it for free?”

                                       Broken power unit

As I investigate the inside, following parts were  broken.

The power line fuse 8A – Open

 

 

One of the FETs for PFC, S-D-G all electrically short circuited.

The block electrolytic capacitor – The capacitance was less than half.

 

 

In order to replace FETs, you have to remove the heat sink and unscrew the

nut that holds FETs.

The broken FET is W20NM50 of ST-Micro. I tried to obtain 2 of them but they

were not available here. Instead of W20NM50, I tried to use 2SK790 of Toshiba. I bought 2 of 2SK790 at the Bonton Co. Ltd. at Ameyoko in Osu Nagoya.

The 2SK790 was almost the same size with W20NM50 and the pin assignment

was just the same. Other specifications were a bit different except the

maximum drain voltage. In the past, I had been experiencing value analysis

and cost down activity in the company replacing originally designed parts to 

some newer parts which were inexpensive with the same performance. So I tried

something similar for my repair. As I check the power loss, the original circuit

for W20NM50 is not OK for “2SK790 since it uses a resister 100 ohm for the

gate. This is too big for 2SK790 and I decided to use 22 ohm in parallel with

100 ohm observing the wave form to be the best. The result was satisfactory

both for the loss and the EMI. You even can touch the heat sink without using

fans at 150W load. With fans on, a bigger power can be expected to dissipate,

I can assume easily by the result.

Below photo is the aging test after fixed with dummy loads. I always use bulbs

for automobiles for loads. The reason is very simple since I have them many

in my junk box. Some has lower beam filament open but upper side is OK.

Another reason is those bulbs have a big inrush current and this is convenient

for the stress test.

 

So far 4 hours have passed after the aging test has started and there has been

no problem at all.  The next step is to install this to the Mac and to confirm

everything is OK.