Palm Pilot IIIxe
The Case of the Missing Inductor
Obtober 23, 2024
It's no secret, I'm a big fan of the Palm Pilot III and it's variations. A large part of this is probably nostalgia, but I just feel like the device is so cool, with it's Graffiti stylus input and cool green backlit glow. Also the fact that developers on the platform were able to do some pretty amazing things within the limits of the hardware.
I'm always happy when I get a chance to get my hands on a Palm Pilot, so when I found an auction listing that included two Palm Pilots and a sync cradle I couldn't pass it up. It was actually the sync cradle that I was looking for but the 'extra' Palm Pilots sealed the deal for me. Unfortunately once they arrived, I powered up one of them looking to bask in it's warming green backlit glory, only to be met with disappointment. There was no backlight to be seen.
When faced with electronics that are not doing what you expect them to do, what option do you really have other than to take it apart? The Palm Pilot comes apart easily, just a few screws and of course the obligatory brittle decades old plastic clips that need to be released from their death grip. A screwdriver and a carefully wielded plastic spudger allowed me to get to the enticing internals.
What I came to see was disturbing, no one should ever see what I saw. To this day I will never forget it.
Corrosion *gasp*
More corrosion *gasp*
Even MORE corrosion *double gasp*
But the corrosion was not even the worst of it. Inside was something so menacing, so horrific, that I caution you to look away. Some terrible act of vengeance had been exacted upon the inductor at L4. The majority of the inductor had been torn off, leaving just a plastic stump. Who had done this? Why? Where had the rest of the inductor even gone? How was I going to fix it?
There is no doubt that replacing the inductor was going to be a tricky job since I did not have any kind of schematics to work from to know it's value. But what I did have was another Palm Pilot, which hopefully had an intact inductor that I could measure. I set out to open the other Palm Pilot that I had at my disposal, and discovered that thankfully the inductor missing from the first one was alive and well in the second unit.
The first thing to notice about the inductor is, it's beautiful. I mean how could it not be, it was in fact there, and in one piece. The second thing to notice is that there are no markings. None. Except for half of the top marked white. So nothing to go on in terms of what the value of the inductor was.
No problem you say, just measure it. Yes that was exactly the plan however after rummaging around through all of my tools I had exactly zero tools which would properly measure the inductance on this little guy. Some gave me a resistance, some gave me no reading at all, but nothing, even my trusty component tester would come back with a useable inductance value.
Of course the sensible thing to do would be to try and remove the inductor and measure it out of circuit to see if that improved chances with getting a proper reading. I took all the precautions, masked off components I didn't want to heat, even took the liberty of adding some low melt solder to reduce the time need to remove the inductor and yet I somehow still managed to damage the delicate windings when removing it.
Suddenly I was left with two Palm Pilots that had no backlight inductors and no closer to knowing the values
Ok, so back to the drawing board on this one. I took a roll of the dice and did some reverse image searches of the top of the inductor in hopes maybe I would get lucky with a lead on what it was but this lead nowhere. So then I thought, maybe I can look at the datasheet for whatever IC is driving the backlight and it might point me in a direction.
It turns out that the chip responsible for the backlight is the SIPEX SP4422A and a copy of the datasheet can be found here.
The datasheet contained a sample test circuit for the SIPEX, and within it, there was an inductor which was labelled as 5 mH. Could this be the mysterious value I was looking for? A little further into the document and there was actually a listing of approved inductors. Parsing the list, there was only one that matched with the footprint on the Palm Pilot PCB, a Hitachi part.
Two part numbers were revealed, MD735L902B for a 9mH part, and MD735L502A for a 5mH. I searched DigiKey and other parts sources for the MD735L502A & MD735L902B or for a part compatible with the value and footprint, but initially came up empty. Just when I was about to give up hope I came across a private listing by someone in the US selling a partial roll of the inductors. The ones they had also had white markings and were the 9mH part MD735L902B. I reached out to them and explained I only need a couple for my project and they were very helpful, ultimately sending me what I needed.
A couple of weeks later a small package arrived at my door. Behold the glory of the Hitachi MD735L902A 9mH inductor!
I pulled one from the package and soldered it carefully into place on the first Palm Pilot. Turning it on, I confidently held down the button to activate the backlight and I was awash with the fluorescent green glow of success!
I took some time to clean up all the corrosion on the board which luckily did not reveal any other damage and then I reassembled the Palm Pilot. The takeaway from this one is that even when you may feel like you're at a dead end and you don't have schematics, take a look at some related datasheets because manufacturers do like to use sample circuits from them.