[Milsurplus] Re TA -12 tx

[Mike Morrow]

pete williams wrote:

>The TA-12 (Bendix) was obviously used with a  receiver (RA-10? ?) ..
> How was the companion rx muted when going to transmit ? ----- there
>does not appear to be any PTT facilities with the tx or the external
>modulator/dyno. Pins 12. 13 and 16 are mic inputs  to the  dyno /mod
>but no tx/rx facilities obvious.

Mike Hanz wrote:

>It's a bit circuitous, Pete.  The wire from microphone tip in the MS-18B 
>jack box goes to pin 16 in the modulator/power supply through an MS-14B 
>junction box.  Microphone sleeve runs through the same junction box and 
>ends up at pin 2.  That's how it gets the PTT signal.  The set isn't as 
>self contained as some other designs because you really need some kind 
>of junction box to tie everything together.  See 
>http://members.cox.net/aafradio/flightdeck/bendix1.htm for the piece 
>parts.  Unfortunately, you also need an MT-51 control box to select CW, 
>MCW, or phone - you can't do it from the TA-12 front panel.

I've never understood why those Bendix adjunct items are so hard to find.  TA-12B/C transmitters and the associated RA-10D/DB receivers, and MN-26C RDFs are everywhere, many in near new condition.  MT-53B LF coils aren't too hard to find either.  But the MP-28BA modulator/dynamotor units are fairly hard to find.  The MT-51, MR-9, and MN-28C (as opposed to the common but oddball 28Y) control boxes are harder still.  Same thing with the RTA-1B 10-channel HF transmitter-receiver.  It's pretty easy to find them in great condition, but not the rack and control box.

Regarding the TA-12/RA-10 receiver muting system, I seem to recall that the only muting of the receiver comes from the antenna relay which grounds the receiver antenna lead on transmit.  Navy airborne sets seem to use that as the only muting method for the receiver, such as in the systems comprised of the ARB/ATB, ARB/ATC, ARB/ATD.  The USAAF systems using the BC-348 did an "honest" job of receiver muting by grounding the receiver antenna lead and removing high voltage from some of the receiver front end tubes.  Of course, that meant that these USAAF systems needed an external MONITOR-NORMAL switch to restore that HV in the receiver to allow netting the receiver to the transmitter.

73,
Mike / KK5F