[Milsurplus] Re: HF/VHF Again

[Mike Morrow]

Jay wrote:

>I think with the "monitor"switch you could tune up on frequency and 
>then the final tuning was done according to charts as these controls 
>were switches not infinately variable.Or am I missing something ? My 
>AN/ART-13A has been idle for so long...

The MONITOR-NORMAL switch has nothing to do with allowing transmitter
tuning at low power levels or disabling PA output.  It is there
ONLY to allow netting the receiver to the transmitter, once the
transmitter has been adjusted to frequency and tuned in the normal
manner.

The NONITOR-NORMAL switch is used in all but early installations
of the USAAF SCR-287 (BC-375, BC-348), and in all USAAF AN/ARC-8
(T-47A/ART-13, BC-348) installations.  The switch is a DPDT,
installed in close proximity to the BC-348.  

In NORMAL, 
(1) Transmitter sidetone output is connected to the interphone
    system, and
(2) BC-348 keyed HV is disconnected when the transmitter is
    keyed, to mute the receiver.

In MONITOR, 
(1) Transmitter sidetone output is DISconnected from the interphone
    system, and
(2) BC-348 keyed HV is NOT disconnected when the transmitter is
    keyed, so the receiver is not muted.

In MONITOR, the transmitter still puts out FULL power on key down,
and the transmitter keying relay still grounds the receiver antenna
terminal, but the UNmuted receiver may now be tuned to the output of
the transmitter.  The diconnection of the transmitter sidetone
audio output prevents that from covering the audio coming from
the receiver.

Thus, the transmitter must be tuned to frequency IN THE REGULAR
MANNER, only then can the MONITOR function be used to net the
receiver to the transmitter.

I don't know of any USN-style systems that used a similar switch.  Most
USN "liason" systems didn't disable the receiver on key down, except for
grounding the antenna terminal.  Even the USN AN/ARC-25 (T-47/ART-13,
R-105/ARR-15) didn't have this switch, but that system is unique because
both the transmitter and the receiver can each be precisely tuned to
frequency independently of each other.

>BTW,as far as I know the "monitor" switch was "custom" for each 
>aircraft.It was not a component per se but something the aircrew or 
>service guys installed.

This switch was standard, installed at the factory, shown in the wiring
diagram equipment.  Oddly, the common RADIO volume of the B-24D
aircraft manual contains a wiring diagram of the SCR-287 without the
switch, but all associated pictures shown in the book clearly show the
switch in place and wired.

If an early installation lacked such a switch, it would be child's
play to install.  I've always though that the ATA/ARA, SCR-274-N, and
AN/ARC-5 (with non-locked tuned receivers) should have had such a
switch, but I know of no instance that one was added.

Mike / KK5F