[AMRadio] BC-610 on 160

Donald Chester k4kyv at charter.net
Tue Nov 5 01:17:59 EST 2013

From: rbethman <rbethman at comcast.net>

>>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.>>


I found my "repair manual" for the BC-610, which includes the schematic. R9
is referred to as the "plate choke shunt" and is listed as 700 ohms. The RF
choke is 2.5 mHy, 8.5 ohms DC.

The final amplifier is the standard single-ended triode final with plate
neutralisation, using a balanced plate tank circuit and single-ended grid
circuit. The 150 pf/section split stator plate tuning capacitor resonates
the tank coil, which is link coupled to the output load. The tuning
capacitor floats above ground, with the full modulated DC applied to the
frame and rotor plates, with an rf by-pass capacitor to ground to keep the
stator plates at rf ground potential. The HV modulated DC is fed to the
plate of the 250TH through the rf  choke and the tank coil, the standard
series feed. The purpose of the rf choke is, since the mid-tap on the tank
coil is only approximate and will rarely end up at the exact rf cold spot on
the balanced coil, the choke is needed to isolate the DC feed point on the
coil from rf ground, allowing the precise cold spot to form on the coil
wherever it may and float free, rf-wise. The tank coil balance is achieved
by rf-grounding the frame/rotor of the split stator capacitor, independently
of the mid-point of the coil, allowing the opposite end of the plate tank
coil to produce the 180 degree out-of-phase rf voltage needed for the
neutralisation circuit. The  reason for applying the modulated plate voltage
to the frame of the capacitor is to maintain the same DC voltage on both the
rotor and stator plates, so that only the modulated rf voltage appears
across the capacitor plates without any additional DC potential. This allows
much closer spacing of the plates in the tuning capacitor without arc-over.

This is the same exact single-ended triode final circuit used in many
transmitters of the era of the BC-610, and can be seen abundantly in the
pre-WWII ARRL and Radio Handbooks. The BC-610 manual I have doesn't mention
what purpose is served by R9. It would seem to me to be detrimental to  the
performance of the RF choke.  Maybe the 2.5 mHy choke is self-resonant at
some odd-ball frequency and the resistor is used to damp out a tendency to
cause self-oscillation. I would simply remove the resistor and discard it.
If the rf choke  gives any problem on other bands without the resistor, I
would  try a different rf choke. I notice in the 1941 and 1943 ARRL
handbooks they show similar tank circuits, using a 1 mHy rf choke. The choke
must be rated to carry the full DC plate current pulled by the tube, about
300 MA. The problem may have occurred on some frequency outside the amateur
bands and not even be a problem at all if the transmitter is used only for
amateur service. If it acts flaky on one of the ham bands, I  would suggest
trying something else for a choke until one is found that allowed the
transmitter to work well on 160 and the rest of the amateur bands. There
should be no need for a resistor in parallel with that rf choke. I  hadn't
noticed it in the BC-610 before, and this is the first time I have ever seen
this in any transmitter. 

The problem may be that the rf choke or other component(s) in the grid
circuit may resonate at some odd frequency that is the same as a resonant
frequency of the RF plate choke, and the two chokes together form a TGTP
oscillator, causing the 250TH to go into self-oscillation outside of the
operating frequency. Adjusting the inductance of the rf  plate choke by
trial and error should do the trick. I would be curious if any of the
technical support manuals for the BC-610 mention anything about the reason
this resistor was added in parallel with the rf choke in the first place.

>>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.>>

I think few  hams who used  the BC-610 have ever used the antenna tuner
designed to go with the rig. The transmitter has a standard link-coupled
output tank circuit, and there are plenty of well-known effective antenna
tuners that hams have used over the years to work with  link-coupled
transmitters. The BC-939 would be useless for the vast majority of amateur
antenna systems, and I wouldn't worry about any problems with the thing on
160, unless one planned to use it to feed a short wire as the Signal Corps
did. Even if it worked perfectly with such an antenna, the results would be
disappointing, compared to a standard antenna, for operation on the amateur
bands. The best amateur use for the "BC-610 antenna tuner" is to part it out
for the turns counters, RF switch and rotary inductors, although they may
have by now become a pricy but useless collector's item.

>>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.>>

They tend to be unstable, hard to set to frequency, and produce an
irritating 60~ a.c. FM hum on the carrier. I  would either use a crystal or
external VFO with the transmitter.

>>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.>>

Does that manual mention why the shunt resistor is there? If the design of
the transmitter works OK down to 2 mc/s, just barely nudging it a little
lower in frequency, to 1.8 mc/s shouldn't have any significant effect. I
wouldn't worry about the BC-939, and would try the transmitter on all bands
without the 700-ohm resistor. If this causes any trouble with the rf choke
on 160 or other bands, I would try replacing or modifying the rf choke.

Don k4kyv

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