Farfisa tuning coil slug repair

[glebert]

Just a post to share a repair I had to do on the neverendingfarfisacompactproject (TM). I had all the oscillators tuned to pitch, and was playing and found the A# was off (what was I doing playing a black key?). I went to tweak the tuning coil (basically an adjustable transformer that has a screw or slug in the center to adjust..something). My heart sank when a chunk of the tuning slug broke off. I had read the coils were fragile, I didn't think people meant the slugs. The slots in them are such that most flat blade screwdrivers are too small or too big. I found using a too small one can cause real problems. Attempts to grab onto the exposed bit of the slug just broke off more. Luckily, the coil is open on both ends and the slug is slotted on both ends. I was able to go in through the back of the coil and (carefully) drive the slug out (turning righty-tighty because its from the back) . The slug appeared to be made of pot metal, and I don't know how much the material properties like magnetism would affect the coil performance, so I wanted to use the original slug if possible. I did find out that the thread on the slug is 1/4" 28tpi which isn't the most common but not impossible to find. I did buy a couple of types of screws at the hardware store of that pitch, along with some nut. I decided to try to give the original slug a more durable turning mechanism before I tried a different type of screw. I took an angle grinder to the slug (which I held by putting two jam nuts on it and held with vise grip pliers) to smooth down the break. I also cut the shaft off a sheet metal screw that I had lying around. I used metal epoxy (like jb weld) to attach them and let them dry for 24 hours. The ridge on the sheet metal screw head was really convenient because I could stand the slug vertically on that while the epoxy cured. I put it back in the coil and it works great. To get it correctly tuned to pitch I ended up having to replace a capacitor on the oscillator, which is another lesson: if you have to turn the coil a lot, that probably isn't the problem.

I think I'm going to make a tuning tool by grinding a plastic putty knife or something to fit the slug slots better, and if it isn't metal it won't thow off the pitch when turning either.

Anyway, pictures for posterity.

[Jon Snell]

The slug is made of ferrite. An iron core will change the characteristics and inductance unfortunately but it may work as it is a fixed frequency.

[Mick Bailey]

I have quite a few trim tools for slugs from my radio days. They're a plastic shafts with a non-magnetic stainless blade, though some are all-plastic. The reason being that a regular steel screwdriver or similar changes the inductance of the coil when it's being used, making frequency adjustment difficult because when the screwdriver is removed the frequency changes.

There are two main types of slugs I encounter - some are ferrite or often powdered iron bonded in resin, where screwing it into the coil increases inductance, and some are brass where the inductance is reduced when screwing into the coil.