[ARC5] AN/GRC-109 antenna clamping diodes CR1, CR2

[Tim]

Hi Bill - If either or both of those diodes are shorted, not much signal
will get from the TX circuits into the receiver of course.
But are you sure you are not seeing the "system" issue with the GRC-109
"single antenna" effect?  The RX signal is coupled from the TX into the RX
via C14, an 18 pf SM cap.  It is pretty small for the application,
especially at the lower freqs.  Putting a larger cap in there gets more
signal to the receiver but that cap is also across the Pi network variable
loading capacitor in the TX circuit upon Key Down - thus restricting the
impedance matching range of the TX.  A well known problem.

I think C14's value was picked as a compromise to enable the system to work
with a single, random Z  antenna (simplifies things operationally) and also
parts-wise to eliminate a mechanical T/R switch or antenna relay.  Also in
use, getting a message out from a team was probably considered to be more
important than getting a message into them.  Also, the "base" transmitter
could be high powered...  My speculation.

My solution, also mentioned in the manual is to use a separate antenna for
the receiver directly.  Works great.  K4CHE also did a nice job to
incorporate a small T/R relay inside the transmitter to get around the
problem in Ham usage...

Prof Lee:  It would be great if you could have one of your students create
a Spice model of that T/R circuit in the GRC-109 to numerically nail down
the effects of that implementation!  Would also quantify the effects of the
TX Pi network tuning on the amount of antenna signal getting shunted from
the RX at resonance!

A fun radio set for sure!
Tim
N6CC

On Mon, Feb 18, 2019 at 10:04 PM Fuqua, William <wlfuqu00 at uky.edu> wrote:

>   I was testing the GRC-109 and found that the TR switch in the
> transmitter was not passing any signal.
>
> I suspect CR1 and CR2 HD2172 diodes are bad. What would be a good
> substitution for these? I can't find any data on them.
>
> The receiver works well down to less than a uV on CW with the signal
> applied directly to the antenna terminals.
>
> 73
>
> Bill wa4lav
>
>
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