• Hello MLAers! We've re-enabled auto-approval for accounts. If you are still waiting on account approval, please check this thread for more information.

Sawtooth ATX PSU conversion

J

joshc

68LC040
I am starting to think about converting my Sawtooth to run from an ATX PSU. The original Astec PSU still works OK but its 23 years old and probably getting toward the end of its life now.

The challenge as I see it is how to achieve a neat/OEM look, in terms of: no case modification, retaining the rear mounted fan, etc.

Hardly any modern ATX PSUs have a rear fan, they all have a fan on the top/bottom of the unit. A lot of the ones I'm looking at are modular but I'm not sure how those will fit.

I am not sure that even an SFX PSU will fit inside the original Astec unit, at least not without modification.

My requirements are something 80 plus gold/platinum rated, with good ripple, efficiency at lower loads, regulated rails, all the modern safety/protection features, high quality caps/components, probably 300-500W (the original Astec model in my Sawtooth is 237W).

I did notice the Astec unit is rated for 24A on the 3.3V rail - the highest I've seen in an ATX PSU that would be suitable is 22A.

Most documentation of converting a G4 to run from an ATX cover the MDD model, not Sawtooth, so any thoughts at all are welcome.
 
joshc said:
I did notice the Astec unit is rated for 24A on the 3.3V rail - the highest I've seen in an ATX PSU that would be suitable is 22A.
Click to expand...
I wouldn't worry too much about dropping from 24A to 22A. The usual sort of thing, avoid heavily loading the machine and use good parts and you'll be fine. Its usually the 5V rail I struggle to find PSUs that are able to match, interesting that it is the 3.3V on this one.

Not got any suggestions for a PSU that the guts would easily fit sorry. I've tried getting suggestions for my QS or MDD before but... people just argued with me that it wasn't my PSU that was faulty (it was).
 
Yeah. I'm fine with doing the wiring harness and know the pinout is documented in a few places.

My question is more about mechanical engineering... physically how to do this neatly, and retain the rear fan, and AC inlets. I guess the neat thing to do would be to replace the secondary/monitor AC inlet with a toggle switch for the replacement PSU?

It would also be useful to know how much I could drop 3.3V to - would 20A also suffice? I don't know how much current the Sonnet upgrade in my G4 draws. Even 20A seems like a heck of a lot.
 
joshc said:
It would also be useful to know how much I could drop 3.3V to - would 20A also suffice? I don't know how much current the Sonnet upgrade in my G4 draws. Even 20A seems like a heck of a lot.
Click to expand...
You could wire the 3.3V (don't forget to connect the GNDs) into a bench powersupply set to 3.3V and monitor the current usage while you did a number of heavy load things? See what the peak real load you get is?
 
Or you could put a 0.01Ohm 100W resistor in line with all of the 3.3V wires, then use a multimeter to measure the (small) voltage drop over the resistor. Current would be equal to the voltage drop / 0.01.

You'd have a voltage of 0.1V if the computer was drawing 10A, or 0.2V if it was drawing 20A. Your meter would need to be accurate to 0.01 of a volt to give accurate readings though.

Might be pushing it for home stuff.
 
I can't say for the sawtooh specifically but I recently ATX converted one of my QS's as it had a dead PSU and found that the board out of a cheap ATX PSU (Chosen as I had a pile of them at work that where new but free (pulled from cases when we where building PC's)) fitted perfectly in the case of the original PSU except for some of the mounting holes having to be redrilled in the PCB. I was also able to desolder the original wiring harness and move it over to keep that OEM look. I suspect the sawtooth PSU should be pretty similar

On the +3.3V question. What kind of PSU's have you been looking at? even the chepo 500W ATX psu I used will do 26A on the 3.3V rail
 
Interesting, I've been wondering the same thing. Most of the ATX PSUs I've looked at have a power switch on the back and the AC plug in what appears to be the wrong place. But I don't think that layout is part of the standard at all.

I've seen the pinout and quite a few people who say they've done it, but noone's named a recent PSU that gave a good result.

Another thing I've wondered is that Seasonic do some fanless PSUs, which'd be great, but then I have no airflow at all... even for a stock 400 MHz model with no mechanial drives, that's probably not a good idea, is it?
 
paws said:
Another thing I've wondered is that Seasonic do some fanless PSUs, which'd be great, but then I have no airflow at all... even for a stock 400 MHz model with no mechanial drives, that's probably not a good idea, is it?
Click to expand...
You really want a fan in the PSU, and pointing in the same direction, and with inlets inside the case. The PSU fan is part of the whole case's cooling in macs.

Reusing the original enclosure and fan and mains connectors is the best starting point.
 
I am wondering if anything made by MeanWell would have the right spec and fit inside the original Astec enclosure...

Yeah that rear PSU fan needs to be kept, it is part of the entire system's cooling.
 
Phipli said:
You really want a fan in the PSU, and pointing in the same direction, and with inlets inside the case. The PSU fan is part of the whole case's cooling in macs.
Click to expand...
Yeah, that's what I thought.

Phipli said:
Reusing the original enclosure and fan and mains connectors is the best starting point.
Click to expand...
You mean rehouse the PSU basically? Interesting idea, hadn't thought about that.
 
First thing to do, when I can be bothered/time allows, is to crack open the Astec unit and take some measurements. I want to choose something 'better' than any random ATX PSU, as I plan to keep this system. I know that almost any ATX PSU would suffice for a G4 but it feels like if I'm going to the effort I may as well use the best one I can, without going over the top, that's why I thought something in the 300-500W range should be relatively sensible.
 
I remember there used to be a guy who sold adapter cables to convert ATX power supplies, to work with G4 Powermacs. By the time I found him, though, it was too late. He said he no longer makes the cables. I don't think there ever was a way to adapt the ADC power line to an ATX PSU, so once you go ATX, you lose the ability to use your ADC monitor. That was one of the caveats that the guy making the cables had on his sales page.
 
Oh, and I think you might also have to tape off the power pins on your graphics card, just to make sure the ADC is completely non functional, and not sending power to your monitor.
 
You must log in or register to reply here.

Similar threads

C
ATX PSUs with high amp 5V and 3.3V rails
Replies
1
Views
2K
Phipli
Phipli
bigmessowires
designing the ideal ATX power supply replacement for vintage Macs
2
Replies
33
Views
10K
jmacz
jmacz
jmacz
My Macintosh Quadra 700 Clean Up and Fixes
2
Replies
28
Views
9K
Phipli
Phipli
CircuitBored
A guide for doing a SuperMac S900/C600 ATX power supply conversion
Replies
5
Views
3K
Angelgreat
Angelgreat
M
Project: Modern Macintosh Plus mini conversion + paint job
Replies
1
Views
1K
s_pupp
s_pupp
Back
Top