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

[Mike Morrow]

>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.  

>Plug P-151 on the modulator is unused.

That's a problem then.  PL-151 to the BC-456-B should connect to a
PL-151 going to the receiver rack, right side, to carry sidetone
audio and sidetone relay control.

>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? 

That's not standard either.  A cable with a PL-156 on each end connects
BC-442-A antenna relay to the FT-226-A transmitter rack to provide nothing
more or less than a three-wire circuit to control the antenna relay.

>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.

It takes three wires to the BC-442-A:  GROUND, 28 VDC, and KEY (grounded
on KEY DOWN or PTT pressed).

>The BC-451 tone,cw, voice selector switch is stuck in the voice position.

That's not normal.

>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.

Not a good sign!

>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 doesn't power up until the mode switch on the control box is
placed in in CW or MCW position if you are using a proper conttrol box.

>The receiver has local controls to toggle it between tone, cw, and voice,
>and to adjust the gain.

There is no difference between TONE and VOICE on the receivers.  It should
have a CW-OFF-MCW switch which controls power, and whether or not the BFO is ON.

Only the transmitter needs a selection of TONE, VOICE, or CW.

It appears *not* to be a proper FT-260-A local control box.

>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? 

The DM-33-A on the BC-456-B is on continuously only in CW or MCW mode.  In VOICE,
the DM-33-A is on only when the PTT or KEY is depressed.

>I'm assuming that the transmiter on-off switch on the BC-451 starts and stops
>the modulator dynamotor.

No, see above.

>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?

In CW and MCW mode, the dynamotor runs continuously and a K52 keying relay in
the BC-456-B energizes when the KEY is closed or the PTT is depressed.  K52
applies all transmitter B+.

The BC-442-A swaps antenna to the transmitter and grounds the receiver antenna
terminal on KEY down or PTT depressed.

It sounds like you have a system that has been hammed-up some, so that it
is not standard in some respects.  You need the manual, available for free
download at http://bama.edebris.com/manuals/military/scr274/

Mike / KK5F