[ARC5] How do I operate a SCR-274N system that's wired together?

[Tom Bridgers]

Was most fortunate to recently obtain a complete SCR-274N system that has all the plugs wiring for a BC-696, BC-454, BC-442, BC-451A, and BC-456 which are connected together.  The transmitter and receiver are mounted in correct racks.  The BC-451A is pretty rough looking cosmetically and shows signs of bad corrosion in several places.  If anyone has a BC-451A that they would like to sell, I'd be interested.

 

The 12 pin and 18 pin plugs and wiring look standard. These connect the modulator to the BC-696 and BC-451.

 

The 28 vdc source and 3 pin plug that connects to the BC-456 looks standard.

 

Plug P-151 on the modulator is unused.

 

Plug PL-156 on the BC-696 rack has 3 wires that go to a taped up "wire bundle" from which three pairs of wires exit.  Two pair of wires go to the receiver rack.  One pair, I'd think, supplies 28 vdc to the receiver.  Not sure what the other pair does; maybe it supplies the side tone? 

 

The remaining pair of wires go to the BC-442.  I'm assuming this supplies 28 vdc to the ant relay which switches the antenna from receiver to transmitter.

 

The BC-451 tone,cw, voice selector switch is stuck in the voice position.  The selector switch supposedly activates the relays in the BC-696, but I'm not sure under what conditions the relays are actually activated. 

A hole has been drilled in the BC-696 rack and about an 18", two-conductor wire exits from it.  This wire is not connected to anything.  I suspect this wire was probably connected to a 0-200 ma meter.  Need to do more detective work.

 

I'm guessing the side tone circuit that normally goes to the BC-454 from the BC-456 is unused.

 

Until I do more detective work, I'm guessing that once  28 vdc is supplied to the system, the receiver filaments come on and its dynamotor immediately begins to operate.  The receiver has local controls to toggle it between tone, cw, and voice, and to adjust the gain.  Audio output from the receiver is via the rack's headphone jack.

 

I am unclear about the sequence to use to transmit.  Does the dynamtor run all the time, but B+ is not applied until a mic is keyed or a key is used?   I'm assuming that the transmiter on-off switch on the BC-451 starts and stops the modulator dynamotor.  Depending on whether the voice or cw is selected, and assuming either a mic or key is connected, then does keying the mic (i.e., PTT) or taping the key 1) apply B+ to the TX, 2) activate the relay in the BC-442 and switch the antenna from recever to transmitter, and 3) ground the receiver antenna? 

 

I'm hoping that someone on the list has a similar SCR-274N system and can suggest how to proceed to understand what I have and then to suggest how I might safely test the system in receive and transmit modes.

 

Many thanks in advance for any help or thoughts,

-Tom  KE4RHH