Swapping the power supply from a IIfx into a II for testing

[Damian Ward]

I have a Macintosh II that appears to be a problem with the power supply. I really want to get the Macintosh II working again and have seen a power supply for sale.

But I don't want to spend the money if it won't solve the problem.

I also own a Macintosh IIfx that works great. I thought I could swap the power supplies to see if it is defiantly a power supply problem, but I don't want to damage

the IIfx's power supply or the Macintosh II any further. I know that the power supplies are slightly different, but the output looks the same.

Does anybody know if this is safe or will it work?

Thanks in advance.

 

[jruschme]

Have you checked the batteries... both of them.

The II/IIx/IIfx use two of the standard 1/2 AA batteries. Startup problems on those models are usually the result of having one of the two go dead. The bad news is that the earliest of the II and IIx use soldered batteries.

John

 

[Damian Ward]

Thanks for the reply. I have already replaced both batteries on the Macintosh II. Unfortunatly it made no differance.

 

[JRL]

Check the motherboard for cap leakage. Also, make sure you soldered the PRAM batteries correctly; there's always the offchance that you may not have by accident.

 

[beachycove]

I have a Macintosh II that appears to be a problem with the power supply. I really want to get the Macintosh II working again and have seen a power supply for sale. But I don't want to spend the money if it won't solve the problem.

I also own a Macintosh IIfx that works great. I thought I could swap the power supplies to see if it is defiantly a power supply problem, but I don't want to damage

the IIfx's power supply or the Macintosh II any further. I know that the power supplies are slightly different, but the output looks the same.

Does anybody know if this is safe or will it work?

Thanks in advance.

My understanding is that the II -> IIfx upgrade did not include a new ps. Check in the service manual for II upgrades (which I am sure you know how to find) just to be sure that you have seen it with your own eyes, but I'm pretty sure that's how it was.

 

[bbraun]

The II, IIx, and IIfx should all be able to take the same power supply. One test to see if the power supply is working or if it is a problem with the batteries or caps or whatnot is to power on the supply "manually". It needs to be plugged into the logic board (the PSU requires load or it won't turn on), and connect between +3V and +6.5V to pin 15 of the PSU connector (this should be the pin furthest away from the PSU). If your PRAM batteries are fine, connect the + side of the battery to the pin 15 for just long enough for the PSU to power up.

I recommend using a volt meter to make sure you're only sending the single battery (+3.6V) to pin 15. I've sent the full +7.2V to the pin before with no ill effects, but it's best to be sure.

Be careful though, the pin right next to 15 is -12V. Bad things happen if that is shorted to something when the PSU turns on (or tries to).

The PSU in the mac II takes between +3V and +6.5V on pin 15 to turn on. Once it turns on, it provides +5V to its self to stay on so you don't need to continue providing power after it spins up.

If the PSU doesn't turn on, the PSU is probably bad.

If the PSU does turn on but the machine doesn't boot, caps are a likely suspect.

If the PSU does turn on and the machine boots fine, it could be batteries, caps, or something else in the switch circuit (the power switch is somewhat complicated, as you can press the power button on the ADB keyboard or the power button, then that eventually sends power from the pram batteries to pin 15 of the PSU).

 

[papa_november]

Also keep in mind that II-series power supplies die very gradually. If you keep plugging and unplugging the power cord and mashing on both the keyboard and back panel power buttons, you might get it to turn on. If that works, then you know the power supply is hosed.

Check the batteries first though.