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.
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.
October 5, 2025 at 8:54 PM
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.
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. :-(
September 14, 2025 at 1:17 AM
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. :-(
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.
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.
September 1, 2025 at 5:18 PM
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.
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.
August 30, 2025 at 10:36 PM
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.
Okay, my IIsi riser board replica is looking good. Everything will be uploaded to GitHub shortly, and I'll figure out a way to offer them for sale for convenience.
As for the failure: it was a bad solder joint on pin 10 of the chip. @48kram.bsky.social had the closest guess and is the winner!
As for the failure: it was a bad solder joint on pin 10 of the chip. @48kram.bsky.social had the closest guess and is the winner!
August 25, 2025 at 10:52 PM
Okay, my IIsi riser board replica is looking good. Everything will be uploaded to GitHub shortly, and I'll figure out a way to offer them for sale for convenience.
As for the failure: it was a bad solder joint on pin 10 of the chip. @48kram.bsky.social had the closest guess and is the winner!
As for the failure: it was a bad solder joint on pin 10 of the chip. @48kram.bsky.social had the closest guess and is the winner!
Built up my first IIsi power daughterboard replica. It looks nice, but it doesn’t work! The power supply immediately shuts off like it’s going into protection. Dang. I did end up with random tiny solder balls here and there from the paste. Could just be a soldering mistake.
August 24, 2025 at 12:27 AM
Built up my first IIsi power daughterboard replica. It looks nice, but it doesn’t work! The power supply immediately shuts off like it’s going into protection. Dang. I did end up with random tiny solder balls here and there from the paste. Could just be a soldering mistake.
Remember my research into the “lost” Performa 550 recovery partition? This hard drive arrived today, and it also has the partition! Even better, all the original Performa software is still on it too. This should hopefully solve the mystery of how the partition was originally created.
August 22, 2025 at 10:54 PM
Remember my research into the “lost” Performa 550 recovery partition? This hard drive arrived today, and it also has the partition! Even better, all the original Performa software is still on it too. This should hopefully solve the mystery of how the partition was originally created.
I probably won’t get around to building a replica up and trying it in my IIsi until this weekend, but look what arrived today!
August 18, 2025 at 11:15 PM
I probably won’t get around to building a replica up and trying it in my IIsi until this weekend, but look what arrived today!
My tester has voltmeters for all three power rails now. It would sure be nice to arrange it into a clean enclosure instead of a jumble of wires though!
August 12, 2025 at 9:48 PM
My tester has voltmeters for all three power rails now. It would sure be nice to arrange it into a clean enclosure instead of a jumble of wires though!
Getting closer to trying out my replica design…just waiting on PCBs to arrive.
August 11, 2025 at 10:59 PM
Getting closer to trying out my replica design…just waiting on PCBs to arrive.
Someone else did a similar test and found their 5V rail doesn't stay quite as high as mine when it shuts off. More like 1V. I even made a power supply tester and confirmed with no capacitance added to the 5V rail at all, the power supply behaves totally fine and shuts on and off as it's supposed to.
August 3, 2025 at 6:46 PM
Someone else did a similar test and found their 5V rail doesn't stay quite as high as mine when it shuts off. More like 1V. I even made a power supply tester and confirmed with no capacitance added to the 5V rail at all, the power supply behaves totally fine and shuts on and off as it's supposed to.
The 5V rail (pink) is actually keeping a fair amount of voltage -- it's still at 1.2V after 250 ms. After the Egret stops driving /PFW low, it creeps slowly back up toward the (now dead) 5V rail due to the 3.3k pullup through a diode on the logic board. It's reaching the transistor turnon threshold.
August 3, 2025 at 6:41 PM
The 5V rail (pink) is actually keeping a fair amount of voltage -- it's still at 1.2V after 250 ms. After the Egret stops driving /PFW low, it creeps slowly back up toward the (now dead) 5V rail due to the 3.3k pullup through a diode on the logic board. It's reaching the transistor turnon threshold.
This is maybe the weirdest PCB I've ever laid out. The pad sizes are inconsistent. I had to manually edit each one to match the original. The traces are weird too. I had to make a copper fill for each trace. It was almost like it was hand-drawn originally or something. It's basically done though!
July 31, 2025 at 5:07 AM
This is maybe the weirdest PCB I've ever laid out. The pad sizes are inconsistent. I had to manually edit each one to match the original. The traces are weird too. I had to make a copper fill for each trace. It was almost like it was hand-drawn originally or something. It's basically done though!
I'm making some pretty good progress on a Mac IIsi power supply riser board recreation design. With how much effort I've put into understanding the schematics, why not? Plus, having it all in KiCad will make it easier to understand for people going forward.
July 29, 2025 at 5:33 AM
I'm making some pretty good progress on a Mac IIsi power supply riser board recreation design. With how much effort I've put into understanding the schematics, why not? Plus, having it all in KiCad will make it easier to understand for people going forward.
Last picture for tonight: scoped the base of the DTC124. It's not turning on until *after* ON/OFF goes low.
If I haven't missed a short on the riser board, isn't this potentially a bad M51977FP? Brand new DTC124, it's not being asked to turn on, and 5V standby is good. ON/OFF should not drop to 0.
If I haven't missed a short on the riser board, isn't this potentially a bad M51977FP? Brand new DTC124, it's not being asked to turn on, and 5V standby is good. ON/OFF should not drop to 0.
July 20, 2025 at 6:43 AM
Last picture for tonight: scoped the base of the DTC124. It's not turning on until *after* ON/OFF goes low.
If I haven't missed a short on the riser board, isn't this potentially a bad M51977FP? Brand new DTC124, it's not being asked to turn on, and 5V standby is good. ON/OFF should not drop to 0.
If I haven't missed a short on the riser board, isn't this potentially a bad M51977FP? Brand new DTC124, it's not being asked to turn on, and 5V standby is good. ON/OFF should not drop to 0.
Thank you for all your ideas today! I will definitely look at the Zeners. While I had my scope out, I added the main +5V rail as well as the +5V standby rail to my scope trace. Confirmed: the 5V rail does shut off, but then something drives ON/OFF low. Also confirmed: 5V standby stays at 5V.
July 20, 2025 at 5:28 AM
Thank you for all your ideas today! I will definitely look at the Zeners. While I had my scope out, I added the main +5V rail as well as the +5V standby rail to my scope trace. Confirmed: the 5V rail does shut off, but then something drives ON/OFF low. Also confirmed: 5V standby stays at 5V.
I set up to probe /PFW and the ON/OFF pin of the PWM chip. Here is what happens when the IIsi does a Special -> Shut Down. It drives /PFW low, which definitely causes the ON/OFF pin to go high (makes sense so far). But...after it releases it, ON/OFF goes low again. Seems wrong.
July 20, 2025 at 12:06 AM
I set up to probe /PFW and the ON/OFF pin of the PWM chip. Here is what happens when the IIsi does a Special -> Shut Down. It drives /PFW low, which definitely causes the ON/OFF pin to go high (makes sense so far). But...after it releases it, ON/OFF goes low again. Seems wrong.
I also think I've solved one mystery about the Bomarc IIsi schematic that confused me. They identified the chip marked "44" as a DTA124, but I think that's wrong. It's not like the others. Thanks to marking code searches, I think I may have identified it as a M51944BML. It looks just like mine.
July 13, 2025 at 10:26 PM
I also think I've solved one mystery about the Bomarc IIsi schematic that confused me. They identified the chip marked "44" as a DTA124, but I think that's wrong. It's not like the others. Thanks to marking code searches, I think I may have identified it as a M51944BML. It looks just like mine.
I think I've narrowed down the circuitry involved in IIsi ON/OFF control. Pink is GND, bluish purple is the ON/OFF control starting from /PFW on pin 3. I'm tempted to replace the BAS16 and DTC124 since that's what the shutdown signal from the logic board would go through, and they're cheap.
July 13, 2025 at 5:22 PM
I think I've narrowed down the circuitry involved in IIsi ON/OFF control. Pink is GND, bluish purple is the ON/OFF control starting from /PFW on pin 3. I'm tempted to replace the BAS16 and DTC124 since that's what the shutdown signal from the logic board would go through, and they're cheap.
So I did this yesterday in an effort to fix my IIsi. It works fine, but there's something wrong with the soft power circuit because when I tell it to shut down, I hear the relay click but then it turns right back on.
Removing every component and cleaning everything didn't fix it! I'm at a loss now.
Removing every component and cleaning everything didn't fix it! I'm at a loss now.
July 12, 2025 at 3:41 PM
So I did this yesterday in an effort to fix my IIsi. It works fine, but there's something wrong with the soft power circuit because when I tell it to shut down, I hear the relay click but then it turns right back on.
Removing every component and cleaning everything didn't fix it! I'm at a loss now.
Removing every component and cleaning everything didn't fix it! I'm at a loss now.