PowerMac G4 Sawtooth Repair?

[mitchW]

Hey,
In the attic I found a forgotten Mac that I stored there almost 20 years ago - back in 2008 or so.
As the title says it is a PM G4 Sawtooth (AGP graphics). I would like to get it going.
The case is in really nice condition, with no yellowing going on, but it has issues:

1. Power supply won't stay on, it just stays for a second or so and it makes a strange whining noise. It know was same back when I stored it.
2. Mainboard is presumably dead (which is why I put it in storage in the first place)
3. Graphics card is missing, along with the all the drives
4. Front cover/panel that covers the optical drive/ZIP drive is also missing

So now the question is, should I attempt to restore it or just part it out.
Power supply would be probably the easiest to repair, it could just have some dried up caps, so after a full recap it might just work fine. Back then I didn't even own a soldering iron, so this is why I didn't tackle it.
Motherboard on the contrary, would be probably the hardest to repair as there are no schematics from what I know and if the "northbridge" or some other special chip is dead, then there is no way of repairing it. Even if just something quite simple like the flash ROM got corrupted, it would be quite a challenge to repair I guess.
Anyone got schematics for this mainboard?

Graphics card could be bit hard to obtain, but I read that some of PC AGP cards could be modified to work in these, and I have a bunch of those.
For the missing front panel, I didn't see any that are on sale currently on eBay, at least from EU, but I know those do pop up from time to time.

So should I put this up as a next project, or just try to get another PM G4?
They are quite rare in this part of Europe though.

Oh, and I almost forgot. I own a second one, the exact same model, that is fully working. Which would make things much easier I guess

 

[indibil]

hello. It all depends on the value you give to that specific G4. If it has value for you, look for another cheap one and make one of the two.

If it has no special value, look for another one in good functional and aesthetic condition, and use that one.

 

[mitchW]

I managed to repair the power supply, it had a shorted tiny diode on the primary side.
The Mac now turns on, but shuts off after 2 seconds. No sounds or any other stuff is present. I am testing it just with CPU installed, and nothing else.
PSU is tested OK, as I ran it for an hour with load.

If I remember correctly, this was the same problem it had when I put it in the attic.
So, adventure continues

 

[mitchW]

And at last, I managed to repair the logic board.

Long story short, it was the SI4467DY P-Channel MOSFET that shorted from gate to drain (in layman's terms: from control input to the power input).

I used the PM G4 Digital Audio schematics as a reference (sadly they are not the best quality), and checked where the power is missing or something is not right. I also put on the other working G4 AGP on the bench and compared the voltages.
The red LED that should be always on was turning on dimly, compared to a working board. It was only getting about 1.5V instead of 3.3V. I then traced the power source to the Q900 and back to the U33 LM339 control chip. I thought that the Q900 was the culprit as it had 3.3V on one side, but just 1.5V on the other. But it was the Q22 (SI4467DY) that failed. When computer was turned on, 3.3V appeared on the gate, and when it was off it should have been 5V, but was just 1.2V.

After replacing that MOSFET, the board turned on, and chimed. SUCCESS!
For the donor, I found one on a defective 2008 MacBook logic board. Has even better specs
My 10 years of board level repairs paid off this time.

Now if I want to make a nice machine out of it, I will need quite a lot of stuff which I initially forgot, but realized when I had another one on the bench.
It needs:
- GPU
- Front drive cover
- ODD drive tray/bracket
- HDD bracket
- HDD IDE cable (short one)
- Power supply bracket (small one)
- Main fan (has a broken propeller at the base which looks like it happened due to an old age - might glue it back together)
- Most of the screws (Allen type for the PSU, normal ones for the rest of the components)

So essentially it would be the best just to get a parts unit from the beginning and make one good out of 2.
But where's the fun in that?

If anyone has those parts for sale, let me know.

 

[mitchW]

For much faster navigation and search I cleaned up (straightened) the scanned schematics that were provided on this forum, and did the OCR, so you can CMD+F (or CTR+F) the document.

As I said, the Sawtooth mainboard is quite similar to the Digital Audio one, but of course there would be some differences
Hope this helps somebody in the future.

 

[Byrd]

So essentially it would be the best just to get a parts unit from the beginning and make one good out of 2.
But where's the fun in that?

Since you basically need another Mac, check locally they are hard to ship/sell and often offered as pickup for < $50. Then you'll be able to pick and choose the best parts, plastics (which often crack and handles go missing too), and make up your dream G4. By the time you look for these parts separately, shipped it'll cost you $200+

 

[applesos]

I have the ATI Rage GPU that came stock with my Sawtooth. I switched it out with a nicer Radeon supplied by a fellow MLAer. As far as drives I've been using solid state IDE drives from OWC with great success. If you'd like my GPU I'm happy to mail it.