[AMRadio] AMRadio Digest, Vol 118, Issue 10 (BC-610 on 160)

rbethman rbethman at comcast.net
Mon Nov 4 08:51:53 EST 2013


You got it through this time.

There is a Choke, CH4 and resistor R9 in parallel in the PA output to 
the center pin of the PA Coil jackbar.

Apparently CH4 becomes an issue when that circuit resonates in the 160 
band.  The solution came out in the MC-509 kit and instructions.

R9 is to be unsoldered on one end and lifted clear of the circuit.

The jumper I referred to is actually in the BC-939 Antenna Coupler.  The 
BC-939 is labeled "Antenna Tuner" both on its tag and in the manual.  It 
is a misnomer.  The input to the BC-939 is 50 ohm, and its output is 
approximately 300 ohm.  The BC-939 is intended to feed a "short" whip or 
a "long wire".

The Signal Corps meaning of "Long Wire" is not what Amateurs consider a 
long wire.  In 160 use it is around 90 feet.

The Tuning Units *is* the VFO when placed in the M.O. position in lieu 
of a crystal.  It "may" be stable, but is very touchy to tune.  It is 
the one place that a reduction drive would have been a great addition.

You and I are probably referring to the same choke.  However, the Signal 
Corps in its testing must have discovered this issue prior to releasing 
the MC-509 kit.

My reference is TM 11-826, pp 43 through 48.

It also includes the modification of the BC-939 for operation below 2MC.

Bob - N0DGN

On 11/4/2013 2:29 AM, Donald Chester wrote:
> Let me try this once more. Like once before, the AMRadio server bounced my
> message saying it looked like SPAM.
> 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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