[Milsurplus] scr-287 integration question

[Mike Morrow]

Jack wrote:

>The '375 mutes the '348 by opening a set of
>normally closed contacts, this takes screen voltage
>off the the IF and one of the RF amps in the receiver.
>
>Mine does this, but the receiver still operates well enough
>to hear the transmitter. Even with the antenna leads
>disconnected from the receiver. It is not grid rectification
>or audio coming in the power socket, as it is tunable.
>
>Just wondering if this is a design feature to provide side tone,
>or do I have some other issue?

Hi Jack,

The BC-375 sidetone on MCW and CW is generated by the VT-25 speech amp set up as an audio oscillator, and fed to the interphone system through pin 33 of SO-44.   In VOICE, the same path provides voice audio to the interphone system.  Switch 1179 adjusts the sidetone output.

The sidetone output of the BC-375 is simply connected in parallel with the audio output of the BC-348.  I haven't heard one of these combos in operation, but I'd always assumed that this sidetone coming out of the BC-375 on transmit would cover whatever audio was coming out of the BC-348, if switch 1179 was set properly.

There should also be in the BC-348/BC-375 interconnection circuits a "NORMAL-MONITOR" DPST switch, located near the receiver.  In NORMAL, one pole of this switch closes to connect the BC-375 sidetone at pin 33 of SO-44 to the interphone input.  Another pole of the switch is connected to pins 21 and 22 of SO-44, and is open.  In MONITOR, one pole of the switch opens to disconnect the BC-375 sidetone output completely, and the other pole of the switch closes to connect pin 21 to pin 22 so that the switched screen voltage that is normally removed from the BC-348 during transmit remains connected.   Thus MONITOR allows netting the transmitter and receiver together by maintaining the BC-348 in full operation (but with its antenna terminal grounded by the BC-375 relay) and with the BC-375 sidetone audio disconnected, so that the BC-348 can monitor the signal from the BC-375.

Oddly, the 1943 Consolidated B-24D Radio Manual wiring diagram for the SCR-287 installation does NOT show this NORMAL-MONITOR switch, though it is clearly visible in photos at the BC-348 desk in the same manual, and described in the text.  

The exact same switch was used in the AN/ARC-8 successor to the SCR-287.

Do you use the NORMAL-MONITOR switch in your set up, is the BC-348 connected through the BC-375 such that its antenna is grounded on transmit, and is the BC-375 sidetone output connected in parallel with the BC-348 output?  

It would be interesting to know if most of the SCR-287 sets being operated on the ham bands today reproduce this useful switch.  The SCR-274-N set does not have such a switch, so it must have been a real challenge to assure that the receivers were kept tuned to the transmitters.  Most of the WWII USN combos seem to have ignored receiver muting on transmit except for grounding the receiver antenna input.

73,
Mike / KK5F