[TenTec] Corsair II AGC circuit

[Don Rasmussen]

The AGC is the "magic" of Corsair II. Having experimented in the circuit I can say the parts selection seems to represent a precarious balance. There must have been some trial and error in the development. It really makes me wonder who figured it out and how much time they had to spend on it. It is weird but in a genuis kind of a way, I think. The previous audio AGC Ten Tec transceivers like OMNI C suffered from some loud gunshot upon occasion, would put me under the desk when it went off. Corsair II does not suffer this. My ideal AGC is *none* all the way up to S9 then just enough attenuation after that to keep out of overload. Corsair II fits the bill. ;-)


Re: [TenTec] Corsair II AGC circuit
from [Steve Hunt] [Permanent Link][Original] 

To:  Discussion of Ten-Tec Equipment <tentec at contesting.com> 
Subject:  Re: [TenTec] Corsair II AGC circuit 
From:  Steve Hunt <steve at karinya.net> 
Reply-to:  Discussion of Ten-Tec Equipment <tentec at contesting.com> 
Date:  Sat, 23 Apr 2011 14:30:55 +0100 
List-post:  <tentec at contesting.com">mailto:tentec at contesting.com> 

Barry,

That sounds right.

U2A is included as an audio phase inverter - that allows *full-wave* 
rectification of the audio using the D6/D7 + D8/D9 diode pairs. 
Full-wave rectification provides a faster attack time - the Achilles' 
Heel of most audio-derived AGC systems.

The inclusion of R46 across C34 provides a leakage path which allows the 
AGC to follow a very slowly-varying signal; if you don't have it, the 
AGC voltage pretty much "freezes" at its highest value until the hang 
circuitry discharges it.

The inclusion of R54 establishes a sensible discharge time for C34 once 
the hang circuitry activates - it sounds very unpleasant if it's not 
included.

U2B provides a very high input impedance so as not to shunt the main 
capacitor C34; Q6 then buffers its output to provide low impedance drive 
to the IF stages and the S-meter.

In the SLOW position C33 is effectively in parallel with C32; in the 
FAST position C32 alone is in operation and establishes the hang period 
in combination with R49 which is the charge path. Positive-going audio 
half-cyles which exceed the Vbe threshold of Q8 cause it to conduct and 
discharge C32/C33; absence of audio allows C32/C33 to charge via R49. 
Once the voltage on C32/C33 has risen high enough, Q9 is cut off and C34 
discharges via R54 and D10.

But you'd probably already worked all that out :)

73,
Steve G3TXQ