[ARC5] BC-230 / BC-430 - mod for coil set C-381

[AKLDGUY .]

On Friday 17 October, a BC-230 transmitter came into my possession,
complete with C-381 coil set.

I've rapidly brought myself up to speed on the circuit and physical layout
and have developed a modification for the coil set C-381 that shifts it
from 2500-3200 Kc/s to cover the 80m band.

This modification has been tried first on the oscillator coil, since it
would
be far easier to track changes in the resonant frequency by listening for
an oscillator than trying to measure the shift in frequency of the PA tank.
The idea was that if the oscillator could be shifted successfully, the same
mod could be performed on the PA tank.

In order to test the modification, but not having original tubes or the time
to build and test a 6AQ5 plugin, I decided to connect a FET (MPF-102)
to the oscillator tube socket and run it at 15VDC. The FET worked well
with the two unmodified coil sets (C-381 and C-385) in my possession.
These nominally cover:
C-381:  2500-3200 KHz
C-385:  6200-7700 KHz

The frequency at the bottom end of the dial was 2522 KHz (C-381) and
6217 KHz (C-385). Both are very close to the specified frequencies that
should be obtained when a tube is fitted for the oscillator.

The frequencies at the top end of the dial were 3482 and 9030 KHz
respectively. The C-381 is a little higher than spec but the C-385 is a lot
higher. I put this down to the fact that the FET capacitance is far less
than
that of VT tube internal capacitances, creating a worsening discrepancy
as the dial setting goes higher.

Encouraged by the success with the FET, I went ahead and modified the
C-381 oscillator coil.

The modification does not involve removal of coil turns or capacitor plates.
It consists of soldering a small silver mica capacitor (approx 250 pF) in
series with the hot end of the oscillator coil (inside the coil can).

The mod worked well, but because I'm using a FET I cannot yet publish
a definite value for the capacitor. When a definite value is derived (by
experiment) it should hold for everyone, so that everyone will simply need
to fit a silver mica capacitor of that value.

What needs to be set in motion is a web page that explains the mod,
describes how to do it, and shows the high quality 2 MB photos that I've
taken at each step.

I am currently investigating the setting up of a page that would host that
kind of data so that others can see what I did and try the mod with tubes
inserted.

73 de Neil ZL1ANM