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

[Tom Lee]

If you don't have a 1N34A handy, give the 1N914 a shot (its capacitance 
is usually very close to 1pF). The 1N34A's capacitance is typically only 
a little bit lower in practice.

--Tom

-- 
Prof. Thomas H. Lee
Allen Bldg., CIS-205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/19/2019 3:52 PM, Fuqua, William wrote:
>
>   One diode shorted.
>
> I will get back to it later this week
>
> 73
>
> Bill wa4lav
>
>
>
> ------------------------------------------------------------------------
> *From:* arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on 
> behalf of Fuqua, William <wlfuqu00 at uky.edu>
> *Sent:* Tuesday, February 19, 2019 6:36 AM
> *To:* Dennis Monticelli
> *Cc:* ARC5 at mailman.qth.net
> *Subject:* Re: [ARC5] AN/GRC-109 antenna clamping diodes CR1, CR2
>
>   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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