[AMRadio] Using the BC-610 on 160

Donald Chester k4kyv at charter.net
Fri Nov 29 23:16:24 EST 2013

I first heard of the  requirement to jumper something in the transmitter
recently on some internet forum. I have some kind of signal corps technical
manual,  but not the regular operator's manual, but I'll check that out and
see if I can find any mention of it there.

I helped Roger, N4IBF (now SK) get  his on the air years ago, and we didn't
do any kind of  modification to cover 160. I think he had a stock tuning
unit that covered 1.5-2.0 mc/s and used that; otherwise he might have
modified a 2-3 mc/s unit, or whatever the tuning range is of the coil that
begins at 2.0 mc/s, to  pull it down a little lower in frequency. He ran
his with an external VFO, a Collins 10B exciter. It worked on 160 and 80,
and he may have run it a few times on 40m.

I gave him a 1.5-2.0 mc/s plate tank coil. With that coil, the 50 pf vacuum
padder is not needed. The plate will hit resonance on 160 with the  main
capacitor about 80% meshed. He said it was a lot easier to change bands
using that coil rather than having to fiddle with the 50 pf padder. One
problem with that  padder is that it is too easy to let it accidentally
roll off the table when not in use and break when it hits the floor.

One modification is needed to  make the transmitter operate properly with an
external VFO fed through the xtal socket. The RF choke in the cathode
circuit of the oscillator tube must be shorted. Otherwise, the stage will
tend to self-oscillate if the VFO output is merely fed into the crystal
socket and the tuning unit switched to XTAL. Of course, that kills the
capability of operating with the xtal. Maybe some kind of switch could be
added to short out the rf choke. I seem to recall that the internal VFO in
the 610 was almost stable and hum-free to be useful on 160, but it is still
better to use an external exciter if possible if a VFO is desired.

I'd like to see a description of what exactly which resistor in the circuit
needs to be jumpered, and what purpose it serves. I can't imagine anything
that would be different about 160 as long as all the tuned circuits
resonance, a 160m signal is fed into the exciter or crystal is inserted, and
a proper plate tank coil is used. The only other problem I could see would
be if an rf choke somewhere in the circuit didn't have enough inductance,
but I don't see how jumpering a  resistor would alleviate that problem.

They even made a special coil that covers 1.0-1.5 mc/s. Someone told me that
coil was used to put the rig on the AM broadcast band. Right after the end
of WWII, the occupying forces were said to have used BC-610s already on
location as makeshift AM broadcast transmitters to provide entertainment to
the troops and possibly to communicate with the civilian populations. I had
a couple of those coils, but tore them apart and used the coil stock to make
the 160m grid coil in my HF-300  rig. I have been using that coil ever since
I built the rig over 40 years ago, so I don't feel that the coils were
wasted. I still have an unmodified/ unused one of those in my junkbox,
although like many coils of that type, the plastic strips that hold the
turns together have disintegrated with age.

Don k4kyv

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