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Power Mac 7200: Power Supply

Whilst searching for a CRT on eBay for the Quadra (that's another story), I came across this item. It was listed as being PC Compatible and, after my last efforts of trying to get a DOS Card to work, I decided to tackle this one.

The Power Mac 7200 was marketed as a PC Compatible because it came with a PCI card containing all the required hardware to run a PC. This card includes a 100 MHz Pentium processor that can have a maximum of 64 MB of RAM dedicated to the DOS/Windows operating system.

Powering it on

The case contents rattled a little, so I opened up (there are two push-buttons on the front under the lip) and inspected it. A few plastic lugs had snapped loose, but there was nothing conductive. With the case open I jammed in a power cable and hit the power button.

Nothing. Not even a zap, whirrr or beep. Just silence. I hate that...

Basic principles told me to disconnect all unnecessary components and try again. No love... Pulled the CMOS Battery, held the CUDA button (but not for 15s), left power off 10minutes... no go.

Testing the power supply (Delta DPS-150GB A) (Apple 614-0039)

From here I thought I'd tackle the power supply first. I didn't want to believe it was the logic board. I disconnected the cables and removed it from the case.

There are 4 screws holding the lid of the power supply on. After removing these, you'll then have to slide the lid along the main trunk of power cables. Pushing the cables into the supply will make this easier. The fan has a 2-wire cable which easily pops off the main power supply circuit board.

Once open, I first inspected all components for damage... all looked good. Testing the fuse showed that it worked fine... conducting power. I then grabbed the multimeter and tested all of the pins. I found 5v on pin 4, but no voltage anywhere else.

Here's the pinout for the main 22-pin connector, looking at the plug as if it was facing away from you. Pin 1 is top-left, pin 22 bottom-right.

+5v	+5v	+5v	+5v	GND	GND	GND	GND	GND	N/A	-12v
TRKL	+5v	+5v	+5v	+5v	GND	GND	GND	GND	/PFW	+12v

And then the secondary 10-pin connector. Pin 1 is top-left, pin 10 bottom-right.

GND	GND	GND	GND	-S
+12v	+12v	+12v	+12v	+S

These power supplies are triggered by the power switch on the motherboard. Plugging them into the wall will get them in to 'stand-by' mode and power should be visible on certain pins. To test this, plug the power supply into the wall and check the following:

• (22-pin connector) +5v is visible on pin 12 (Yellow)

One important point is that, in my case, the 5v above showed up on the correct pin. The power supply still would not power up. It turns out that the above test is not fool-proof; it'll simply tell you if the initial 'trickle' power is available.

To get this thing to power on whilst disconnected from the motherboard, follow these steps (Thanks goes to LT.Maddog420 on the Tech Guy forums):

1. On the 10-pin secondary cable, connect pin 5 (White) to any of pins 1-4 (Black)
2. Still on the 10-pin secondary cable, connect pin 10 (Brown) to any of pins 6-9 (Orange)
3. Confirm the power supply is plugged in to the wall and that the pins connected above are not exposed or touching metallic surfaces.
4. Now, on the main 22-pin power plug, connect pin 12 (Yellow) to pin 21 (Purple)

The above should jump-start the power supply. The fan should at least fire up. If you get nothing then you've got a dead power supply. This would be the first item to replace in a Macintosh that doesn't power on. Of course, when the supply died it could've also fried other parts... but replace the power supply first as it's cheaper. I tried to jump-start mine and it didn't work at all. I decided to pull it completely apart for thorough inspection.

Cracked Circuit Board

Once out of the casing, the issue became quite obvious. The base circuit board for the power supply had a crack in it. The crack formed a semi-circle around the area where the circuit board is held by notches in the metal case. It seems that, somehow, the board has received lateral pressure and cracked! It doesn't quite make sense as the board is thoroughly enclosed. I can only imagine that weather/heat has caused this. I have a hunch that this machine had been out in the elements for a while.

There also seemed to be new flux and solder at the base of a 3-pin component. I really wouldn't be surprised if this had been repaired in its past life; this machine is nearly 20 years old.

Repairing the crack was pretty straight-forward. I was really lucky that this was a single-sided circuit board with large tracks. If there had been tracks on the top side, then this would've been a lot more difficult.

I bared the copper on the tracks around the solder joints and messily bridged all of the component pins. I made sure that I followed the tracks and didn't bridge any circuits. My butane-powered soldering iron decided to let its tip melt in the process and so the first fix shown below looks really crappy. I bought a proper soldering station (I needed it to work on the DOS Card for the Quadra 950) and re-did the soldering. Hence the two pictures of the finished job below.

And then... the friggen thing spun up! Using the jump-start instructions above, I bridged the two wires and then joined yellow to purple. It felt like I was hot-wiring a car and I, honestly, had very little faith in my hack working. I had a celebratory beer as the fan kicked off and I then crossed my fingers that the logic board wasn't also damaged.

A final note: be careful when pulling/pushing the bunch of wires in and out of the power supply case. You can easily peel the insulation off the individual wires and the last thing you would want is a short!