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Doug Brown @downtowndougbrown.com · 5d

Ugh...it worked okay with my tester, but then I tried to use it to boot my actual IIsi, and it cycled itself on and off over and over again, and refused to stay on.

Then I tried my original known good riser board instead, and it blew up again. Damn. Should have recorded it!

Doug Brown @downtowndougbrown.com · 5d

Haha, that video is a nice consolation prize at least! I hope you're able to fix it! If it helps at all, the 2SK1402As I got from activeparts on eBay were legit. Looks like they've more than doubled the price since I got mine though.

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Doug Brown @downtowndougbrown.com · 5d

The bad news: I don't get to share video footage of my IIsi PSU frying. The good news: it's fixed! Even though I wasn't feeling optimistic after the first Q101 and D104 replacement failed, I tried again and it worked. The difference: this time, I didn't try to power it up with the dim bulb tester.

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Doug Brown @downtowndougbrown.com · 5d

For what it's worth, replacing Q101 and D104 fixed my PSU. This time I only used the dim bulb tester to make sure it wouldn't blow the fuse initially, and for testing the 5VSB circuit. After that, I just went straight to the wall outlet. I think the dim bulb tester is what fried my first replacement

Doug Brown @downtowndougbrown.com · 5d

Ahh, gotcha! That makes sense. I'm considering trying to come up with some kind of daughterboard tester module that I can use to test all the circuits on it, because I learned the hard way that these things can fry the PSU!

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Doug Brown @downtowndougbrown.com · 5d

Interesting, my megaohm-level readings are quite a bit different, but I'm guessing those don't matter much. My readings do differ quite a bit on the M51977FP though. Are you sure about the high resistance measurement on pin 3 of the IC? It's tied directly to GND on the bare PCB.

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Doug Brown @downtowndougbrown.com · 5d

Ah yeah, the variable resistor being bad could cause a problem! Nice reference info! I'll be interested in hearing if replacing the transistor and diode fixes it for you. I'm slowly trying to gain the courage to try it again. I'm realizing I haven't really tested my replica in stuff like protection

Doug Brown @downtowndougbrown.com · 6d

Dang! I would definitely check D104 as well because it has shorted out both times I’ve seen the K1402A go boom. I am not sure if the K1402A going bad kills the daughterboard. The gate goes through T102, so maybe not.

A faulty shorted D103 originally caused my PSU to go into protection, btw.

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Doug Brown @downtowndougbrown.com · 6d

Oh no! Welcome to the club I guess, lol. I’ve been spending a lot of time studying the circuit. I wonder if yours blew the same transistor and diode that mine did…

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Doug Brown @downtowndougbrown.com · 6d

I decided to take a break after the initial repair attempt seemed to re-blow the replacement MOSFET, but it’s possible the dim bulb tester was the cause of that. Each time it fails, it destroys the IC on one of my known-good riser boards, so I have to be careful. I’m going to look at it again soon.

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Doug Brown @downtowndougbrown.com · 27d

Yep, the MOSFET and Zener are shorted again. Fun fun fun!

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Doug Brown @downtowndougbrown.com · 27d

The main fuse that blows in this failure state is a slow blow fuse that doesn’t seem to protect the components either, though :-( I made my own little dummy load box I’ve been using (2 ohms on 5V, 24 ohms on 12V), but maybe I should be diagnosing this problem with a much lighter load.

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Doug Brown @downtowndougbrown.com · 27d

I did use a 60W dim bulb tester, so a fuse didn’t blow, but I’m back to square one. I’m guessing the diode and/or MOSFET is shorted again. I’ve heard dim bulb testers are bad for SMPS though, so maybe I’m testing this the wrong way.

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Doug Brown @downtowndougbrown.com · 27d

My IIsi power supply is proving to be tricky. I put in a new MOSFET and Zener, and this at least restored it to a state where it doesn’t blow the fuse when I plug it in. Good +5VSB too. Tried to turn it on, and it failed again and something’s shorted again. :-(

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Doug Brown @downtowndougbrown.com · Sep 7

Answering my own question, I think the transformer might actually be okay? I was worried because of low inductance on the damaged side, but it’s literally just a wire going through the core once. Matching the PCB silkscreen. And it’s on a current sense part of the circuit. Whew…I think…

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Doug Brown @downtowndougbrown.com · Sep 7

It also took out the 18V Zener diode on the gate, and I think it may have damaged this transformer too, which is a major bummer. It’s a custom part, no off the shelf replacement. Why does the PCB silkscreen show an inductor on one side and a straight line on the other?

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Doug Brown @downtowndougbrown.com · Sep 6

I’m not sure if it’s _the_ problem, but this is definitely a problem! This is the main transistor that the riser board controls too, so maybe that’s what was damaged by the bad board.

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Doug Brown @downtowndougbrown.com · Sep 6

Welp, my IIsi PSU is dead. Replacement fuses arrived today, and this time the fuse blew as soon as I plugged the power cord in. I guess the one faulty riser board really badly screwed something up. Damn.

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Doug Brown @downtowndougbrown.com · Sep 1

This is not a time machine. Blowing the fuse in my IIsi PSU gave me an excuse to finish putting my IIci/IIsi PSU tester into a nice enclosure. Pretty proud of this! First time I’ve made a case like this that wasn’t 3D printed. Although I did need to design and 3D print some internal brackets.

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Doug Brown @downtowndougbrown.com · Aug 30

Absolutely! I need to come up with a good set of sanity tests for the board before I try powering it. Hopefully the fuse is the only thing that was damaged.

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Doug Brown @downtowndougbrown.com · Aug 30

Got out the toaster oven and built up some IIsi PSU daughterboards today. First two test out perfect. The third…I turn it on and kablooey! Visible spark and F101 blows on my power supply. Dang, this is what I was worried about. Hard to test these without risking my PSU. Hope it’s not shot.

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Doug Brown @downtowndougbrown.com · Aug 28

Here's my blog post with the latest info about the recovery partition, plus a few other tidbits discovered since the first article came out: www.downtowndougbrown.com/2025/08/an-u...

Downtown Doug Brown » An update about the hidden Performa 550 recovery partition

www.downtowndougbrown.com

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Doug Brown @downtowndougbrown.com · Aug 27

If you're getting tired of hearing about my IIsi riser board replica, never fear! It's coming to a close! My design is now live on GitHub. Built with all new components from Digi-Key except the two obsolete ICs. Still deciding how to build/sell them to recoup some costs. github.com/dougg3/iisi-...

GitHub - dougg3/iisi-riser-board: Replica of Macintosh IIsi power supply riser board model AP41S

Replica of Macintosh IIsi power supply riser board model AP41S - dougg3/iisi-riser-board

github.com

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Doug Brown @downtowndougbrown.com · Aug 26

Ahh, got it. Well, if you ever think of someone who might need one, I’d be happy to donate the prize to them instead!

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Doug Brown @downtowndougbrown.com · Aug 25

Haha, absolutely! I soldered it with hot air and didn’t do the greatest job. I bet my toaster oven with controller will do better!

As the winner you are entitled to a free board when I have some built up and ready to go! I’m not sure how to get in contact with you for that, but can you email me?

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