[ARC5] Command Set Receiver/Transmitter Frequency Spotting

[Mike Morrow]

>On the receiver control box, the volume knob is too close to the
>cranks. In certain positions, if you are not careful, you will bump
>the crank on either side.

>We all know that the pilots and radiomen were not really supposed to
>fiddle with the radios in flight. 

A question about the ARA/ATA, the SCR-274-N, and (lesser) the AN/ARC-5, is how the receiver frequency was supposed to be set.  The associated transmitter would have been set with some care and precision using the LM or BC-221.  The controls would be locked after completion of transmitter tuning.

But...for these MF/HF command sets, there is NO provision for spotting the receiver to the associated transmitter.  The receiver frequency must also be set using some external reference (LM, BC-221, O-4/ARC-5), then it must be hoped that that the receiver frequency adjustment doesn't get disturbed or suffer changes due to environmental factors.  Many/most AN/ARC-5 MF/HF communications R-25, 26, 27 installations used lock-tuned stabilized receivers with less opportunity for disturbance (but not in all cases).  The ARA and SCR-274-N receivers always stood in grave danger of unintended disturbance.

The SCR-287 and AN/ARC-8 have a spotting provision using a MONITOR-NORMAL switch.  In MONITOR this switch (1) Disconnects the transmitter sidetone audio output from the audio bus so that it doesn't override the audio out of the BC-348, and (2) Maintains HV to all stages of the BC-348 that the transmit-receive relay would normally interrupt on key-down.  The antenna terminal of the BC-348 is grounded during during key-down regardless of the switch being in MONITOR or NORMAL.  In MONITOR, the actual RF signal from the transmitter can be heard in the BC-348 output and the receiver easily adjusted to the transmitter.  For the AN/ARC-8, this allowed the BC-348 to be very quickly (manually) tuned to the precise new frequency that has just been selected on the T-47A/ART-13 auto-tune transmitter.

In the ARA/ATA, SCR-274-N, and AN/ARC-5, during key-down, relays in the receiver rack energize to completely disconnect ALL receiver outputs from the audio bus, and to connect the transmit sidetone audio from the modulator to the audio bus.  There is no MONITOR-NORMAL switch to keep these relays from energizing on key-down so that the actual receiver output could be heard.  A MONITOR-NORMAL switch for these command sets would simply need to open the connection to pin 5 in the cable between the receiver rack and the modulator.  This would make it simple to adjust a receiver to the associated transmitter.

I suspect that the HF command set system of receiver frequency alignment during flight relied upon a detuned receiver being broad enough to hear a signal from another nearby aircraft, during which the pilot could make necessary adjustment at his receiver control box.

I have read a number of WWII USAAF pilot testimonials in praise of the SCR-522 that say its pushbutton channel selection was far superior to the "coffee-grinder" controls of the SCR-274-N.  No wonder!

Mike / KK5F