[ARC5] NORMAL/MONITOR Switch in USAAF Liaison Sets

[Mike Morrow]

SPOT/NET "MONITOR" SWITCH FUNCTIONS

For:
SCR-287-A [BC-348-* and BC-375-*]
AN/ARC-8 [AN/ARR-11 (BC-348-*) and T-47*/ART-13]

- Typically found in USAAF (but not USN) liaison sets.
- It's a simple DPST toggle switch usually located on the radio operator table near receiver.
- MONITOR SWITCH positions are labeled NORMAL and MONITOR.

The keying relay is part of the transmitter.  When the transmitter is UNKEYED, the MONITOR switch has no effect in either position.  The unkeyed relay:
(1)  Switches antenna from transmitter RF output to receiver RF input.
(2)  Ungrounds receiver RF input.
(3)  Connects receiver power plug pin 2 to pin 6 to unmute receiver by applying HV to the screens of one RF and two IF stages.
(4)  Disconnects transmitter AF sidetone from system AF bus.


When the MONITOR switch is in NORMAL position and the transmitter is KEYED, the keyed relay:
(1)  Switches antenna from receiver RF input to transmitter RF output.
(2)  Grounds receiver RF input.
(3)  Disconnects receiver power plug pin 2 from pin 6, which removes HV from screens of one RF and two IF stages for receiver muting.
(4)  Connects transmitter AF sidetone to system AF bus on top of muted receiver AF output.

When the MONITOR switch is in MONITOR position, actions (3) and (4) above are overridden:
(1)  One pole of the DPST MONITOR switch closes to connect receiver power plug pin 2 to pin 6.  This UNmutes the receiver while the transmitter is keyed.
(2)  One pole of the DPST MONITOR switch opens to disconnect transmitter AF sidetone from the system AF bus.  This allows only receiver AF output to be heard on the system AF bus without interference or masking from the keyed transmitter sidetone AF output.

The MONITOR switch is most definitely an official feature of these USAAF liaison sets, although early SCR-287-A installations seem not to have it.

The MONITOR switch is not shown on the manual for the SCR-287-A because there is no such manual.  The manuals for the BC-348-* and BC-375-* are all there is.  The associated aircraft's wiring diagrams document the SCR-287-A system interconnections between receiver and transmitter, including the MONITOR switch.  Some aircraft manuals that document their SCR-287-A insrallation show the MONITOR switch.

The MONITOR switch in the AN/ARC-8 liaison set is shown in USAAF manuals for the AN/ART-13, -13A, -13B.  These are the principle source documents for the AN/ARC-8.

The USN typically did not use a similar switch or receiver muting in equivalent liaison installations, other than disconnecting and grounding the receiver RF input when the transmitter is keyed.

Mike / KK5F