Here's my success story of a 75A-4 set screw.
I acquired the receiver in the early 80's, and never had any occasion to
remove the front panel until one of the dial cables broke. I disassembled
the tuning knob and vernier mechanism without any problem, but once the
panel was removed, the plastic disc kilocycle dial had to come off the shaft
so that I could gain access to the pulley to attach the new dial cord. Then
I discovered that someone had rounded out the spline-head (Bristo) set
screw. I had the proper Bristo wrench, but the hole was so rounded out
that it wouldn't catch. It would just rotate in the hole. I suspect
someone had attempted to use an allen wrench to turn it and broke off the
splines inside the hole.
The receiver sat for months while I tried everything I could imagine with no
success. I was about to the point of drilling through the rivets and
removing the plastic disc from the bushing to get better access so I could
drill out the screw. But one day, I probed the hole in the screw-head using
some long drill bits, and discovered that a #49 drill would latch firmly
into the hole. But the problem was that this would tighten the screw, not
loosen it. I made some inquiries over the internet and found a mail order
company that sold left-hand (anti-clockwise) drill bits. I ordered several
#49 left-hand drills. They were too short for the job, so I took a piece of
1/4" metal shaft material and drilled a hole in the end of the shaft, a size
just barely large enough to admit the shank end of the drill . I filled the
hole with JB Weld epoxy and shoved the drill bit shank inside, slowly
forcing the epoxy to ooze out the hole around the shank as I inserted it,
until the shank bottomed out in the hold I had drilled. Once the epoxy
hardened, I tested the tool, and I believe the drill bit would have broken
before the expoxy bond would have failed and allowed the drill shank to
rotate inside the hole.
Using this tool, I was able to get the drill into the hole in the set-screw
head. It caught firmly, but the set screw would not budge. Collins had
used some kind of green bonding agent similar to Loc-Tite to secure the
screw. I suspect that to be the reason the screw was rounded out in the
first place. I decided that I needed to heat the screw to soften the
Loc-Tite, but it was located just behind the plastic dial disc, and how was
I going to heat the screw without damaging the plastic dial?
I found a slim, miniature low-wattage soldering iron. The barrel of the
iron was no more than 5/16" in diameter, and the tip was threaded to screw
into the end of the barrel. The threads were the same as a #8-32 machine
screw. I found about a 2 1/2" long 8-32 brass screw in my junk box. I
ground off the screw head, and ground the end down to a pointed tip just
large enough that it could be inserted part way into the hole in the screw
head. That would heat up the set screw, but now the problem was to avoid
damaging the plastic dial. I made a heat shield using a piece of aluminium
foil sandwiched next to a thin piece of insulating material - either wood or
bakelite (don't remember). That allowed the thin, hot screw and barrel of
the soldering iron to remain for a considerable length of time in close
proximity to the plastic disc without melting it.
I tried heating the screw with this arrangement. I didn't damage the dial,
but the screw still wouldn't turn. I came to the conclusion that the
problem was insufficient heat transferring between the end of the makeshift
brass soldering iron tip and the steel screw. Then it occurred to me that
the perfect medium for improving the heat transfer would be heat-sink
compound. Alas, my tube of sink compound had completely dried up and was
hard as a rock. But I found in my junkbox the remains of an old solid state
stereo amplifier. I took the power transistors off and sure enough, there
was enough soft heat sink compound left behind that I could scrape it up
into a blob, then I applied the blob to the end of the soldering iron tip
and inserted the tip into the hole. I plugged in the soldering iron and let
it cook for several minutes. When I took it out I immediately inserted the
tool that I had made from the #49 left-hand screw. Applying just a little
torque, the screw broke loose, and I was able it remove it, threads intact.
After I re-strung the dial cord, it was time to re-mount the kilocycle dial.
That set screw has extra fine threads, and my junk box seemed to contain
many small screws the proper diameter and length, but they were all coarse
threaded. But when I dumped the whole box of screws into a sorting tray, I
managed to fine ONE lone screw the proper length with the correct thread
pitch. It was a slotted screw instead of Allen or Bristo, but that didn't
matter to me. Using a jeweller's screwdriver, I tightened the new set screw
just enough to make sure it was a snug fit, but I made a point not to over
tighten it, and I didn't use any Loc-Tite.
That was several years ago, and I have used the receiver almost daily ever
since.
Don k4kyv
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