[ARC5] A discussion on AGC.

[Mike Morrow]

> Receiver muting may be a topic closely associated with the AGC line.  The
> AN/ARC-5 receiver may have developed overwhelming AGC voltage that effectively
> silenced it while transmitting,..

No.

> It's not clear from the SCR-274-N manual how, or even whether, muting was
> achieved.
> ...I'm curious to know more about muting and how it was achieved in the
> ARA, SCR-274-N,.and AN/ARC-5 models.

The muting process in these receivers is very simple and positive indeed.  Whenever the transmitter is keyed, there is a relay in the receiver rack for each receiver (K1, K2, K3 in a three-receiver rack) that energizes.  That relay completely isolates the AF output of the receiver and connects modulator AF sidetone output to the downstream AF routing circuits.  The operator never hears the actual output of the transmitter through a receiver.

SIDE COMMENT:  This means that there was no way to key the transmitter and then tune the associated receiver to match.  The operator (pilots) could only tune the receiver's dial to the associated transmitter's known setting, then trust that its very broad selectivity and close proximity to the command transmitters of other aircraft would allow hearing the other aircraft until the receiver tuning could be fine adjusted to the output from other aircraft, such as during pre-flight radio checks.

Pilots disliked this characteristic of the "coffee-grinder" command sets, and how easily the tuning could be upset if something hit the crank.  The AN/ARC-5 system tried to correct these failings by making frequency-stabilzed receivers for 1.5 to 9.1 MHz that could be technician-tuned on the ground and locked, without any remote pilot tuning posssible.  The pilot had control only of the AF level from a receiver in these lock-tuned configurations.  Typically, the remote tuning control box C-26 was supplied only with the R-23* beacon band receiver.

It's little surprise that pilots greatly preferred the switch or push-button frequency selection of VHF command sets.  Pilot tuning of an HF command set was a concept that **should** have been obsolete by the late 1930s.

Mike / KK5F