[AMRadio] BC375 repair


Donald Chester k4kyv at charter.net
Tue Nov 26 13:42:29 EST 2013


>>Yeah that stuff is Glyptol (sp?) a WW II version of Locktite. You may be
able to flood the shaft / knob bushing with something like Flux Remover from
a spray can or MEK. I don't think Lacquer Thinner will do any good, just be
careful not to get any solvent on the plastic knob.....
?
Bill AD5OL>>

I had success removing the kilocycle disc dial on my 75-A4, which uses
similar stuff, by making a special attachment to a small soldering iron. I
used a brass screw with threads the same as those on a tip that would fit
the iron (6-32 IIRC), but much longer than a standard tip. You may find a
stock tip long enough. Just make sure it is small enough to pass through the
set screw hole in the knob without contacting the plastic. File the tip to a
point so that it will fit into the hexagonal hole meant for the Allen driver
(or Bristo if that's what it uses). Then apply a fairly large dab of heat
sink compound to the tip, let the iron come to temperature, and carefully
insert the tip into the hole in the head of set screw and hold it in place
for a few minutes, long enough for enough heat from the iron to transfer to
the screw,  via the heat sink compound, and soften the Glyptol or whatever
it is. Carefully remove the hot iron and immediately insert the proper
driver to rotate the screw, and it should easily break loose. You  might use
a small tool to dab some sink compound onto the screw head before inserting
the iron, in addition to the dab on the tip, to assure the best heat
transfer.

In the case of the 75A-4, I fabricated a heat shield, using a piece of thin
cardboard sandwiched between two pieces of aluminium foil to keep the hot
tip from damaging the plastic disc on the kilocycle dial. The Bristo screw
head was hopelessly rounded out because someone had attempted to turn it
using an Allen driver. I tried several rotary drills until I found the size
that would fit inside the hole and firmly grab the screw, but I did not
attempt to turn it. I noted the drill  size and  then ordered from
McMaster-Carr a left-hand drill (one designed to be rotated in reverse of
the normal clockwise direction when  drilling a hole). When it arrived, I
heated up the set screw as described above, with the  heat  shield in place,
and then used the reverse drill chucked into a small handle to grab the
screw inside the hole and back it out. The reverse  drill bites into the
screw when turned to loosen the set screw. A conventional drill bites into
the hole only when rotated to tighten  the screw, so one of those won't
work. The special drill wasn't terribly expensive, so I ordered two just so
I would have a spare in case I broke one. The shipping cost more than did
the drills. The "handle" I used was a short brass rod, with a hole drilled
in one end, slightly larger than the shank of the #49 drill, and glued in
place with JB Weld.

But hopefully, you won't have that problem, and a regular Allen driver will
work. I don't think they used  Bristos in those units.

Don k4kyv



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