Macintosh SE Power Supply Replacement?

[SteveHere]

Hi All,

I've recently tried something in a Mac SE that I've been working on, and I wanted to run it by the community here and see if anyone has tried this before, and how it's worked out (or not). After recapping the Power Supply on a Mac SE, I was not able to bring it back to working order, perhaps there were bad semiconductors on the board or something else, but I decided I'd rather have a new supply if possible.

The stock power supply has the following ratings:
V1 +5V at 6A
V2 +12V at 1.25A
V3 +12V at 2.1A
V4 -12V at 0.5A

I found this MeanWell Power Supply with the following ratings:
V1 +5V at 8A
V2 +12V at 3.35A
V3 -12V at 0.5A
And, it fits inside the SE Power Supply enclosure with a bit of room too spare.

Because the MeanWell supply has only a single +12V output, this option presumes tying the two +12V supplies together. The current capacity of the MeanWell should be enough to do supply both loads but perhaps there was some other reason that Apple wanted to isolate the Sweep circuits on the SE Power Supply from the Drive 12V power supply? I did look at the schematics a bit, but I wish I had a better understanding of that, was that a requirement to provide isolation, or simply a design convenience?

I have done this, and it's working for now, but if there's a compelling reason to add a second 12V supply I will do that as well, although it will probably not fit inside the original enclosure as the Meanwell RT-85B does. Thanks!

 

[joethezombie]

The belief is that the two were "isolated" to reduce screen noise with drive activity. Do you see any visual impact on the CRT with floppy or HDD (if using 12V mechanical drive) activity? If not, I wouldn't be particularly concerned. In my experience, I've used a mini-ITX power supply (FSP220-60LE) inside the case, and also a Seasonic SSP-250SUB with no issues or visual anomalies. The mini-ITX advertised as having isolated 12V rails, but the Seasonic did not.

 

[SteveHere]

@joethezombie thanks for that. While I was working on the restoration I had the original analog board in the machine. With this board I did see screen geometry changes with drive activity but this board was in rough shape and only a couple hours after a complete recap it failed again. With a replacement SE Analog board I do not see any changes to screen geometry or flickering. I was even able to add a mechanical drive back into the machine and I still see very small voltage fluctuations on the 12 volt rail during drive access (around 0.2V), and no visible problems w/ Screen Geometry or flickering. I'll call that a win. Thanks again!

 

[eric]

If you havent seen @JDW has a video going over some aspects of replacing a SE PSU -

 

[SteveHere]

Thanks @eric, I had not and that was very helpful. The 68KMLA forum thread referred to on the video is linked here. In hindsight, it does seem that the Seasonic unit would have been a better way to go. I will keep this in mind in future should I add an accelerator or additional loads.