[ARC5] "Curing Chirp in Command Transmitters"

[AKLDGUY .]

We need to be very careful in taking the advice about chirp reduction
from someone who probably modified the original VFO heater wiring.

Look at the original circuit diagram of the Command transmitter.

The heater line (which was originally DC) is connected to a terminal on
each selector relay (K53, K54), one of the 1625 heater pins, and one
of the 1629 heater pins. None of that wiring is decoupled for RF.

The other side of the 1629 heater goes to a terminal on winding B of
VFO coil T53. This terminal is decoupled with a 0.006 uF capacitor.

The other end of winding B goes to heater (pin 2) of the VFO tube,
type 1626. The other heater terminal (pin 7) is connected directly to
cathode (8) and to a tap on winding A of the VFO coil, from where this
branch of the heater currents flows to ground.

The justification for connecting pin 7 to the cathode is given in the
manual:
"...*so that variations of cathode-to-heater capacitance within the tube*
*will **not affect the frequency of operation.*"

It can be seen that pin 7 of the 1626 is well above ground RF-wise,
since it is connected to the tap-connected cathode. It appears that the
reason for feeding pin 2 through winding B is that the latter is connected
in opposite sense to that of the cathode tap-ground winding, so any RF
appearing at pin 2 is firstly countered by the equal-and-opposite induced
voltage in its winding, with anything left being mopped up by the 0.006 uF.

Now let's assume that the writer of the article did the usual mod and
disconnected VFO tube pin 2 from its winding and connected it directly
to the 1629 heater pin, thus dispensing with winding B and the 0.006 uF
altogether.

When the VFO is now keyed, either by K53 or by switching elsewhere,
the RF voltage at pins 7 and 8 travels through the heater to pin 2, but
instead of being countered by (opposite sense) winding B and 0.006
decoupler, it is free to get onto the heater line via the 1629 heater.
That may be an explanation for the writer's measurement of 6-10 volts
of RF.

If I were modifying a Command transmitter today to run on AC heaters,
I'd leave the VFO circuitry strictly original. That retains the frequency
stability measure referred to in the manual, and I believe there cannot
be any AC induced into the VFO coil due to the counterbalancing effect
of the two windings, if they are in fact counter-connected.

73 de Neil ZL1ANM

On Sun, Oct 5, 2014 at 12:38 PM, Dave Merrill <r390a.urr at gmail.com> wrote:

> One of my 'finds' at Shelby last month was a 1947 edition of "Hints and
> Kinks for the Radio Amateur."  In the 20 page "Converting War Surplus"
> section, I found the following:
>
> <begin quotation>
>
> Curing Chirp in Command Transmitters
> Alfred Scott Cline, W6LGU
>
> My BC-459-A chirped and from what I've heard on the air, most everybody
> else's does too.  I tried various methods of keying, and extremes of
> voltage stabilization, but the chirp persisted.
>
> Checking with a good v.t.v.m. showed 12.6 volts on the filaments with the
> key up, but from 18 to 22 volts when the key was closed!  The added voltage
> was r.f.
>
> To remedy this situation, shielded filament wire was substituted in the
> rig, with by-passes at each end of the wire.  Old microphone cable (with
> high r.f. losses) seemed best.  A heavy copper strip was run across the
> chassis and the "cold" ends of the 1625 filaments and the cathodes were
> connected to it to get a good ground.  This change resulted in chirpless
> keying for me and has done the same for all the others to whom I have
> passes this hint.
>
> <end of quotation>
>
> Okay, I know Command Set keying chirp has been beat to death but perhaps
> this is still worth discussing.
>
> W6LGU does not mention how he is keying the BC-459-A which might be
> helpful.
>
> Even without this information, my suspicion is the unwanted r.f. is coming
> from unshielded filament leads OUTSIDE the transmitter.
>
> Thoughts?
>
> 73,
> Dave
> N9ZC
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