[Milsurplus] GRC-109, RS-1 transmitter - keying

[Kenneth G. Gordon]

On 15 Feb 2006 at 11:54, B. Smith wrote:

> Has anyone modified the Transmit Receive (TR) circuit for the 
> T-784(GRC-109 or RT-3 (RS-1) to improve receiver sensitivity when
> using the internal automatic antenna switch between transmit and
> receive?

I have.

> is keyed.   The  problem is that there are  extreme loses in the
> receive side of the TR  circuit  due to the  receiver coupling
> capacitor C14 which is only 18 pf but is in the output circuit of the
> transmitter.

No. That isn't the problem: the problem is called "suck-out" and 
is caused by the transmitter's final tank circuit being tuned to 
the same frequency as the receiver's input circuitry. One of the 
list members on the Glowbugs list calculated the losses due to 
the final tank circuit being connected effectively in parallel or 
series (I have forgotten which) with the receiver's input 
circuitry, and it amounts to -30DB LOSS!!! in the case of the 
GRC-109.

Suck-out was a common problem for hams who used TR 
switches which were connected directly to the final amp's tank 
circuitry.

> The receiver sensitivity loss is paraphrased in TM-11-5820-474-14,
> Chapter two, 14e, under "Preliminary Connections, Separate
> Transmitting and Receiving Antennas."

Yes, but it doesn't tell you why: it is "suck-out", period. The 
receiver coupling cap is plenty big enough to do the job.

> In Vietnam at base camps the sets were used with separate antennas.

Yes, and that method was suggested in the manual too. The 
internal TR switch was only to be used in an emergency.

> I have been using an external antenna change over switch for the  set
> to prevent the receive loses but I am preparing to modify the set
> using either pin diodes (ugh) or a small relay.  :-)

There are several ways you can do it: what I did was non-
destructive. I mounted one of those small 5 VDC Radio Shack 
relays internally on a bit of double-sided tape, and used a single 
diode to rectify the filament voltage as a 1/2 wave rectifier. I 
oriented the diode in such a way that if you used 6 VDC on the 
filaments, the diode would pass that voltage on to the relay so 
the circuit works the same way whether you are using AC or 
DC.

I use the relay to both key the rig and change the antenna so 
that QSK is preserved. It easily follows my bug at 20 WPM or 
more.

It works very well.

Or you could build an external TR box that does all that. 
Electric Radio mag recently had an article on just that subject.

BTW, I REALLY like the AN/GRC-109. It even runs off my PE-
162C generator, although I have not yet gotten my DY-88 to 
work with it. My GN-58 does a superb job with it, especially 
since I mounted the GN-58 to an old exercise bicycle. I can even 
send while pedaling. :-)

Ken W7EKB