In 2009 we decommissioned the old PBX system after replacing it with a Cisco VOIP phone system. Most of our campuses just got rid of the equipment lock, stock and barrel, but the whole room full remained at my campus. Over the years, I retrieved a few of the boards, each one about 16 inches square with 24 circuits handling 24 phone lines. The boards connected to the central office had 24 circuits that looked like an analog telephone to the C.O. These used a hybrid transformer with four 600 ohm windings (more about them here). The boards connected to the telephones furnished power and put out a digital signal to the phones. These used a ferrite toroid core transformer to isolate the line from the circuitry. These high permeability cores made excellent Joule Thief coils and were good for many other things.
Removing one from the board was very difficult because the eight pins absorbed most of the soldering iron heat, so I yanked them off with a vice grip pliers,. Each of the 24 transformers had a half inch O.D. ferrite toroid core encapsulated in epoxy. Needless to say, this made them very difficult to get out of the case. I did a few with a hammer and screwdriver, but I broke about 1 in 4 cores, With scores of the cores gathering up, I had to find a better way to remove them from the case.
In late 2010 I took a few outside and used a propane torch to burn them to charred remains. I figured that since the ferrite core, just like the Hobbits’ One Ring, was born in the fire, the heat from the torch couldn’t hurt it. After removing them from the charred remains and removing all the wiring and washing them, the cores came out unscathed and ready to serve as very good JT cores.
But the torching took tens of minutes and the nasty smelling smoke and fumes were blowing in my face. I did about one board of 24 cores this way, but I was looking for a better way. While I was at the hardware store looking at some clear polyurethane coating, I checked out some paint stripper on the shelf. It said that it would strip epoxy, but that was paint, I thought, and it might not work on a cube of solid epoxy. But I decided to give it a try.
I used a pint (473 ml) glass jar with a steel lid, and dropped a few of the transformers into it and covered them with a glob of the stripper. The stuff is viscous, so that it will stick to a vertical surface while it’s dissolving the paint; I had to shake up the cubes to make sure they were coated all over with the goop. I left them for awhile, maybe a half a day. They didn’t change much, so I put them aside for a few days.
A few days later I checked and the cases were cracking open. On the next weekend, I opened up the jar and removed one with a long nosed pliers – if you get the the goop on your skin there will soon be a burning sensation. I used a chopstick (disposable and strong enough to poke the pieces) to pry off the chunks while I held the rest with the long nose. I did it on a sheet of newspaper. I got the cubes down to just the core, wires, pins and a few small chunks of epoxy leftover. I let them dry for awhile so the stuff wouldn’t burn the skin. I used a pair of small diagonal cutters to cut off the tape, wires and shielding*. I washed the cores to get rid of any residue, dried them and tested a few on the LC Meter IIb. I also had some FT50-75 cores I bought, and using a single turn measurement, both cores measured a bit over 3 microhenrys. They are nearly identical in size and shape, so I feel that it’s safe to say that they are equivalent to the FT50-75 cores.
A few of the cores were coated with white epoxy, which became rubbery. When I took off the wires, I damaged this rubbery coating, so I removed it and ended up with the uncoated core, just like the rest of the cores. The rest that were rubbery but undamaged when I removed the wires ended up drying out and the coating turned hard after a few days.
I used the KS-3 Premium Stripper UPC# 30192 11185, which is the pint (473 ml) size can, and cost about 8 dollars (U.S.). I’ll have to buy more of it soon, and the next can I get will be the larger size.
Another experiment
I did another experiment with a PC board. I wanted to get the green conformal coating off of a scrap PCB, so I stuck the whole board into the jar with stripper for a few days. When I removed the board the coating was soft and the copper traces were loose and came off easily, but the board itself was flexible and a bit rubbery. I left it outdoors for awhile and it hardened up, but it’s not flat anymore. The through holes are still coated with copper but there are no copper traces on the surface.
I thought of one other way to remove the cores: when the cooking is done and there are still some briquets still burning in the barbecue, toss a few of the cubes in and let them burn. After a few tens of minutes the heat should have burned all of the outside off. I don’t have a BBQ, so I can’t try it, but someone else might try it and give some feedback. Whatever you do, make sure you burn them after the cooking is finished, because the burning plastic smell is probably not healthy.
*The shielding? Yes, each core has maybe a dozen or more turns wound on the bare core, Then there is a layer of tape, a strip of copper shield, more tape and then more windings of a few turns each. Apparently this shield is to prevent interference from the phone line crossing over to the receiver or transmitter circuits. Along with the signal, the transformer has to pass through 48 to 56 volts DC to the phone for power. That would damage the other circuits if it crossed over into the circuits, so there’s another reason for the shielding.
Update May 12 – More than a week ago I put 15 more of these in the glass jar and poured some more paint stripper on them. They’ve been sitting there, in a big crumpled up blob in the jar, so I finally got out the latex gloves, pliers and chopstick and removed the softened epoxy case. I ended up with fifteen cores, which sat outside to dry for several hours. Then when the stripper had mostly evaporated, I took the pliers, side cutters and chopstick and removed the wires and shielding. Voila! Fifteen more bare cores for making Joule Thiefs!