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

[Dennis Monticelli]

Bill,

When the transmit key opens on the GRC-109's transmitter (the T-784), the
cathode of the PA tube is open circuited which suddenly interrupts the
current through a 1mH RFC.  There is a path from that RFC through a 0.01uF
cap straight into those back-to-back protection diodes.  I suspect
repetitive current surges may occasionally do them in.  Or a single
lightning strike.  The back-to-back configuration provides inherent reverse
voltage protection, which leaves robust surge capability as most important
survival parameter.

I recommend a common cheap 1N4148 Si switching diode.  Low capacitance and
400mA of repetitive peak current.  Not that it matters, but it can also
handle 100V reverse peak repetitive.  Yes, I know....it's not original.

If you're worried about the 0.7V vs 0.3V drop being important then keep in
mind that the CIA version of this TX, the RT-3 (which I have), uses the
same TX circuit minus the two protection diodes.  Hmmm.

Dennis AE6C

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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