[ARC5] Netting Switch

[Mike Morrow]

> 1.  Do the Command Set transmitters let the oscillator run
> all the time?

No.  **All** high voltage supply to the transmitter is disconnected
when the unit is not keyed, by K-52 in the modulator.

> If not, why not? It would seem to be a superior approach for
> frequency stability and crisp keying.

It would prevent the associated receiver that is tuned to the
same frequency as the transmitter from receiving anything
*but* the transmitter oscillator.  Not too useful.

The A.R.C. Type 12 transmitter oscillators do not run all the
time either.  The whistle-though signal is generated from the
transmitter only when the appropriate control is depressed
while tuning the receiver.

> 2.  If you have a receiver and transmitter fired up and
> sitting essentially side by side as they commonly were,
> even without the Zero Beat mods that David describes,
> can't you receive the transmitter oscillator to some
> degree with the receiver?

In the "command set" system, the transmitter has NO HV anywhere
except when it is keyed.  Thus...there are no signals of any
sort being generated by the selected transmitter when the
system is in receive mode.

When the transmitter is keyed, now there is a lot of signal
being generated.  The associated receiver is *in fact*
receiving that signal, even though the receiver antenna
terminal is grounded during transmit by the antenna relay K-55.
But, nothing will be heard in the headphones from the receiver
because there is a sidetone switching relay K-1 (-2, -3) in
the receiver rack that energizes when the transmitter is
keyed.  This relay completely disconnects the audio bus from
the output of the receiver and connects it to the sidetone
AF output from the modulator.  That sidetone output will be
a 1000 Hz tone generated by the modulator if in CW or MCW
mode, or the VOICE AF from the modulator transformer if in
VOICE mode.  The only time the operator hears the AF output
from the receiver on the audio bus is when the transmitter
is *not* keyed.  When the transmitter is keyed, the operator
hears *only* sidetone that is generated by the modulator,
and nothing generated by the receiver.

A simple netting switch can be added to an otherwise stock
command set system by installing a SPST switch that opens
the keying line from pin 5 of modulator connector J-52 to
the receiver rack.  Then, when the transmitter is keyed,
the sidetone relay(s) in the rack won't operate.  The
AF output of the receiver will remain on the AF bus, and
the modulator sidetone AF will remain disconnected from
the AF bus.  An actual received signal will be heard when
the receiver tuning is adjusted near the transmitter 
frequency.  It'll still be a strong siganl, even though
receiver antenna terminal(s) are grounded by K-55.
When the SPST switch is closed, the configuration returns
to normal such that only modulator sidetone is heard in
the headphones when the transmitter is keyed.  I don't
know why that simple arrangement wasn't incorporated into
the command set design, because the as-designed system does
NOT allow the receiver to be netted to the transmitter.
If, in flight, the receiver tuning is upset (easy with
the pilot's grinder-tuning cranks that have no mechanical
locks), it is very difficult to get the receiver back on
the frequency of the associated transmitter unless someone
in the flight is transmitting while the adjustment is
being recovered.

The USAAF liaison systems SCR-287-A and AN/ARC-8 have
a MONITOR-NORMAL switch at the operating desk near the
BC-348-* expressly to allow the BC-348-* to be netted
with the BC-375-* or AN/ART-13*.  If it is in NORMAL,
when the transmitter is keyed, the receiver HV to
several stages is interrupted to mute the BC-348-*,
and headphone audio is sidetone AF generated by the
transmitter.  If it is in MONITOR, when the transmitter
is keyed, all receiver HV remains on (the BC-348-*is
NOT muted), the transmitter sidetone AF is disconnected,
and headphone audio is AF generated by the receiver.
Netting is performed with the switch in MONITOR and
keying the transmitter.  The receiver is tuned to 
the signal generated by the transmitter.  Then the
switch is taken from MONITOR to NORMAL.  In both NORMAL
and MONITOR the receiver antenna terminal is grounded
when the transmitter is keyed.  This reduces the strength
of the signal from the transmitter to the operating receiver
during the netting operation.

The USN practice was not as sophisticated for their
systems.  When the transmitter is keyed, the receiver
is never muted electronically.  Only the receiver
antenna terminal is grounded.  That allows the
transmitter signal to always be heard in the receiver,
though it may be competing with transmitter AF sidetone.

Mike / KK5F