Doug Brown
@downtowndougbrown.com
Classic Mac geek, Linux geek, blogger, embedded firmware developer, Blazers fan.
https://www.downtowndougbrown.com/
https://www.downtowndougbrown.com/
Yeah, I think they do. My 550 has a hole for it. So maybe it’s not too risky after all!
November 9, 2025 at 4:46 PM
Yeah, I think they do. My 550 has a hole for it. So maybe it’s not too risky after all!
That’s fascinating! My family’s caddy-loading Performa 550 came with the exact same CD (I still have the CD) and I happen to have a caddy-loading 660av here too! It would be interesting to see if I can replicate the problem here — maybe not in my 550 though, haha. That would be a pain to take apart.
November 9, 2025 at 4:27 AM
That’s fascinating! My family’s caddy-loading Performa 550 came with the exact same CD (I still have the CD) and I happen to have a caddy-loading 660av here too! It would be interesting to see if I can replicate the problem here — maybe not in my 550 though, haha. That would be a pain to take apart.
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!
Then I tried my original known good riser board instead, and it blew up again. Damn. Should have recorded it!
October 6, 2025 at 1:15 AM
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!
Then I tried my original known good riser board instead, and it blew up again. Damn. Should have recorded it!
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.
October 5, 2025 at 10:51 PM
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.
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
October 5, 2025 at 8:32 PM
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
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!
October 5, 2025 at 8:27 PM
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!
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.
October 5, 2025 at 7:22 PM
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.
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
October 5, 2025 at 7:08 PM
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
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.
A faulty shorted D103 originally caused my PSU to go into protection, btw.
October 4, 2025 at 11:32 PM
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.
A faulty shorted D103 originally caused my PSU to go into protection, btw.
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…
October 4, 2025 at 10:29 PM
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…
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.
October 4, 2025 at 2:53 PM
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.
Yep, the MOSFET and Zener are shorted again. Fun fun fun!
September 14, 2025 at 2:54 AM
Yep, the MOSFET and Zener are shorted again. Fun fun fun!
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.
September 14, 2025 at 2:53 AM
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.
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.
September 14, 2025 at 1:21 AM
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.
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…
September 7, 2025 at 7:15 PM
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…
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?
September 7, 2025 at 6:42 PM
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?
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.
September 6, 2025 at 11:45 PM
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.
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.
September 6, 2025 at 9:11 PM
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.
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.
August 30, 2025 at 10:46 PM
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.
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
August 28, 2025 at 2:30 AM
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...