[R-390] 2 diode agc ssb mod c/l meter linearity

[Larry H]

Hi again, Jacques.  Sorry I did not have more time to properly respond to
your thoughtful post earlier.

In addition to what I said, the main factor in picking out a diode was it's
forward voltage.  I found a modern diode stth102 at Digikey that met
the requirements, *except* it's forward voltage is .68 Vf instead of .6
Vf.  I did not obtain one for testing as I had a .6 forward V that I used.
So, the stth102 may be ok, but I don't know.  It's a 200 V - 1 A, which
meets the minimum voltage and the amperage is much more than necessary.

I can say that I tried the 1N4004 and its Vf was not quite low enough.
It's Vf is 1.1 V.

Unfortunately, I looked a lot online and most of what I found in new diodes
that have a low Vf were around 70 V and lower.  Since the diode must be 200
V, there are no correct new ones available.

The ones I mentioned in my announcement are available to purchase, but are
not cheap.  The 1N1695 is available from Electronic Surplus and ePay.

However, there is an exception - if you have the 'moment of silence' (MOS)
fix on your agc speed switch, then a lower voltage and current diode can be
used.    A *big caution* goes with doing this because if you install the
lower voltage diode and swap the IF deck to a unit that does not have the
'MOS' fix on it, it won't last very long -1 switch will do it.  In the case
where you have the MOS fix on, you can use a 1N63, which is readily
available.

Happy listening.

Regards, Larry

On Fri, Aug 27, 2021 at 4:29 PM Larry H <larry41gm at gmail.com> wrote:

> Hi Jacques, The 8 M ohm res is not critical and can be 7.5 to 8.5 M ohms.
> The diode was chosen for it's low forward voltage, .6v or less.  And it
> needs to stand up to the surge when the agc speed is  switched.  Most
> standard rectifier diodes have a forward voltage of 1.2 V, such as the
> 1n4004.  As for a transistor, it could work if it meets the 3 criteria.
>
> Sorry for the rushed answers, I have a Honey Do chore to get  to  asap.
>
> Regards, Larry
>
> On Fri, Aug 27, 2021 at 7:28 AM Jacques Fortin <jacques.f at videotron.ca>
> wrote:
>
>> Hello Larry,
>>
>> Thank you again for providing improvement ideas for our beloved receivers.
>> I have two questions, however.
>> I understand the purpose of the Mod. 3: there is a similar arrangement
>> used
>> in some other receivers, the Racal RA17C12, for example.
>> Question 1: a 8Meg. ohms resistor is not a part of "standard" value, but
>> 8.2Meg. ohms is.
>> I then presume that there will be no difference in the Mod. 3
>> implementation
>> if a 8.2Meg. ohms part is used. Please confirm.
>> Question 2: did you recommend using those 200 PIV, 400ma rectifiers for a
>> special reason, like a super-low reverse leakage ??
>> They are all Silicon rectifiers of old manufacture, so they will be hard
>> to
>> find.
>> Could a "good old" 1N4004 be used instead ???
>> Thinking about another trick, does the B-C junction of a high voltage
>> silicon transistor can be used (like a 2N3440, or similar) ??
>>
>> 73, Jacques, VE2JFE
>>
>> -----Message d'origine-----
>> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
>> part de Larry H
>> Envoyé : 26 août 2021 23:43
>> À : R-390 Forum <r-390 at mailman.qth.net>
>> Objet : [R-390] 2 diode agc ssb mod c/l meter linearity
>>
>> Hi all,  The c/l meter linearity may be skewed if you have the Lankford 2
>> diode agc mod on for ssb reception.  The reason is that the original
>> 'suppressor grid' clamping at the junction of R544, R546 and R547 is not
>> effective enough to overcome the increased voltage from internal noise, an
>> RF signal or the antenna noise.
>>
>> The clamping circuit provided by this fix (Mod 3), improves the situation
>> considerably, but does not completely solve it for a high noise antenna.
>> This problem has been there all along, but with the original design, it
>> was
>> not very noticable. With Mod 2 and Mod 3 on, it looks like the c/l meter
>> noise tolerance operation is a little better than with none of these AGC
>> mods on. Mod 3 also improves the c/l meter linearity close to what it was
>> without these mods on. How does this affect the c/l meter linearity, you
>> say? Well, with mod 3 on, the meter 0 set point is at a better place. This
>> is what improves the linearity. The diode needs to be 200 volts at 400 ma
>> with a .6 volt forward drop or less because of the high voltage pulses
>> created when switching the AGC speed.  There were a number of diodes made
>> that meet the requirements in the 60's and 70's, but very few available
>> now.
>> Ones I found are: 1N1693, 1N1694, 1N1695, or 1N2092.
>>
>> However, if you have the 'moment of silence' fix on for the AGC speed,
>> then
>> a lower voltage one (such as a 1N63) can be used.
>>
>> I have attached the updated schematic for this fix.  If you don't receive
>> it
>> from the list server and would like a copy, please let me know.
>>
>> Regards, Larry
>>
>>