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Macintosh II PSU/Power On Headaches

KGLlewellyn

Well-known member
Hey there,

I've been having a bit of a nightmare trying to get my Macintosh II Rev. A to power up. My board is one of the older models which doesn't have the SMD electrolytic capacitors on, it's only got the axial caps which don't typically leak (info on this board here: https://recapamac.com.au/macintosh-ii-a/). I've been looking through all of the schematics for the power on circuitry, however it seems that they all reference the Rev. B board. Does anyone have any ideas how the batteries hook up in the original Mac II board so I can find out where to test continuity and voltages from the batteries? Even when the PSU wasn't giving me issues, installing known-good batteries would not resolve the issue, the Mac always had to be jump-started.

I used to be able to jump-start this Mac, however that stopped working around 3 years ago. I've since recapped the power supply to no avail. Has anyone tried doing any ATX conversion of these supplies or aware of any other components in it that are known to fail? My PSU is made by Sony, if that helps.

Thanks!

Kai
 

joshc

Well-known member
If it works from jump starting, this indicates the problem is in the start up circuit.

You can find diagrams and an explanation of it how it works here: https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/

My Mac II had the same problem (rev B board) and there was a broken trace between R3 and R18 in the Power On circuit. Ended up with a big bodge wire underneath the board which did the trick.

If you want any more pointers/help following Doug's page, just let me know.

As for the power supply, have you read the voltages from it, or is it not switching on at all?
 
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joshc

Well-known member
There should be a lot of ATX power supplies that would fit in the Mac II power supply enclosure, and hooking it up is no different to doing an ATX conversion for other Macs.
 

KGLlewellyn

Well-known member
Yeah, I definitely concluded that there is an issue with the start-up circuit. The Rev. A boards are significantly different from the Rev. B...to the point that I physically cannot find the referenced components, as in I can find R3 (It's about 8 cm below the edge connector on the underside) but no R18. Couldn't find the referenced Diodes (the board doesn't seem to have any?), there aren't any components that are prefixed with 'UB' and so forth.

The PSU is not switching on at all, I understand with how the PSU's work with these boards is that there is no trickle voltages that come out of it, so unless the PSU is actually on, nothing would be picked up on the multimeter.

The ATX should be simple enough, there's 12v rail, then several 5v rails and a number of ground wires...I think the bigger complexity is that the ATX is expecting the power on trickle voltage to drop for it to switch on, while the Mac II board instead will send the 3v into the supply which ofc it isn't expecting. So one would need to find a way of building a custom circuit which could simulate the power on voltage drop for the ATX supply.
 

cheesestraws

Well-known member
I just had a look at the II power system and it gave me a headache. Good start ;-)

I think you could probably still use the same kinds of strategies as with a PSU with trickle, though, especially as an ATX PSU would provide that trickle. So you could either use an inverter powered off the 5V trickle from the PSU (though you'd have to be careful about logic levels) or just a transistor that takes the 3V on its base and pulls the ATX power on line low.
 

KGLlewellyn

Well-known member
I just had a look at the II power system and it gave me a headache. Good start ;-)

I think you could probably still use the same kinds of strategies as with a PSU with trickle, though, especially as an ATX PSU would provide that trickle. So you could either use an inverter powered off the 5V trickle from the PSU (though you'd have to be careful about logic levels) or just a transistor that takes the 3V on its base and pulls the ATX power on line low.
You and me both, what a pain. xD So I basically modded a 300W Apevia TFX-AP300W PSU for the Mac II's wiring, was pretty simple to do in the end. I matched the Pinout of the II's requirements to what was provided from the ATX connector, 1x 12v rail (yellow), 5x 5v rails (red), 6x ground rails (black), and 1 -12v (blue). I then installed a rocker switch which shorts the green ATX PS_ON wire to ground when switched on. With this the Mac II operates like a compact Mac with a Power Switch bodged into one of the expansion slots in the back. It'll even say "It's now safe to switch off your Macintosh" when shutting down.

It's a temporary solution until I can figure out what's going on with the power on circuity on the board itself. Once I get that fixed I can think about building a custom circuit to get the Mac II to bring the voltage from PS_ON low using its existing power on circuitry.
 
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