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Succes: ATX PSU in Mac IIsi

gijsmans

Member
I have replaced a dead IIsi PSU with a used Fortron Source FSP300-60GHS 300W PSU which I got from our IT guys at the office. It runs great, very cool. It is a tight fit, I had to alter the original PSU metal case a bit. Of course the 300W is overkill but the Mac will only use what it needs. Overall it is a very nice and solid upgrade. The IIsi is a surprisingly nice vintage Mac as it is able to run system 6.0.8 and has HD20 compatibility with the Floppy Emulator. It put in a SCSI2SD 3x 2GB: System 6.0.8, System 7.1 and one disk for our family 80's-90's documents archive.

One thing could be improved: soft power by keyboard is not working. Anyone having a suggestion how to fix this?

C4CF3B65-3E51-4882-8624-56C06CAC8B12.jpg
 

joshc

Well-known member
One thing could be improved: soft power by keyboard is not working. Anyone having a suggestion how to fix this?
Did you hook anything up for that or not? ATX PSUs don't support soft power, you need to add your own little circuit for this.

Something like this

1631291403674.png

Also see here: https://bylenga.ddns.net/index.php?page=Centris_ATX.php

1631291469060.png

(from that page) - basically same as the above but drawn a little differently.

Also see here, refers to the same method https://68kmla.org/bb/index.php?threads/stealth-macintosh-iicx-iivx-style-sfx-psu-rebuild.38439/
 

cheesestraws

Well-known member
ATX PSUs don't support soft power, you need to add your own little circuit for this.

Slight clarification: ATX PSUs do support soft power in the ATX way, they don't support soft power in the Mac way. In ATX, the soft power pin is grounded to turn the PSU on (and I expect you, @gijsmans, have tied the ATX soft power line to ground to get it to turn on permanently?). In the Mac, the soft power line is taken high to turn the PSU on. So you need one of the circuits posted to essentially invert that line.
 

bdurbrow

Well-known member
On the first one, the 74ls04 (or any other inverter - I’m using a single-gate package inverter) should be powered off of the trickle (standby) 5v line, not the main 5v lines. Also, as it’s a totem-pole output device, the second 1k resistor is unnecessary.

And on both circuits, you can cut the power consumption by increasing the 1k resistors to 10k or higher (however, I probably wouldn’t go above 50k - it’ll start getting really sluggish when it’s switching).
 

bdurbrow

Well-known member
Ugh... me too. o_O

However - any small signal NPN transistor should work (2N2222, 2N3904, BC848, 2N5551, etc).
 

Daniël

Well-known member
Slight clarification: ATX PSUs do support soft power in the ATX way, they don't support soft power in the Mac way. In ATX, the soft power pin is grounded to turn the PSU on (and I expect you, @gijsmans, have tied the ATX soft power line to ground to get it to turn on permanently?). In the Mac, the soft power line is taken high to turn the PSU on. So you need one of the circuits posted to essentially invert that line.
What is slightly interesting is that when a Mac of this era that utilizes soft power detects the PSU not shutting itself off on power down, it will show the "It is now safe to turn off your Macintosh" screen instead.
 
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