[ARC5] BC-454-B, tough problem

[Ian Wilson]

I've been resurrecting a BC-454; everything worked FB except for the local
oscillator stage (triode part of the 12K8 triode-hexode).

The triode plate voltage appeared to be absent. This part of the circuitry
is a little obscure, since there is a 200k plate series resistor that is
contained
in the RF oscillator subunit of the plug-in coil thingy. Also, the plate
supply
for this stage is routed through the capacitor cover.

OK, I now know more about the LO stage than I ever wanted to. Removing
the capacitor cover (which involves approximately every screw plus removal
of 2 of the IFTs and most of the tubes) and disconnecting the bare wire
going to
the variable cap showed 8 ohms to ground. A strange number. The almost-short
turned out to be one of (there are two!) the air-variable trimmer
capacitors that
are in parallel with the oscillator tuning part of the 3-gang capacitor.

I can't see the moving parts of this trimmer, but turning it a few times
must
have broken up the whisker (guessing that's what it was). I left the trimmer
off its previous position. All working now.

All the coil cans contain more than just inductors. Worth studying during
these
dark hours while awaiting Armageddon, perhaps.

73, ian K3IMW
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