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

[Fuqua, William]

  I think the diodes may also limit the click heard in the headphones while keying.

Yes there have to be low capacitance since the coupling capacitor from the antenna is so small.

I'll let you know later this week how it turns out.

73

Bill wa4lav


________________________________
From: Dennis Monticelli <dennis.monticelli at gmail.com>
Sent: Tuesday, February 19, 2019 2:25 AM
To: Fuqua, William
Cc: ARC5 at mailman.qth.net
Subject: Re: [ARC5] AN/GRC-109 antenna clamping diodes CR1, CR2

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<mailto: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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