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
On 23/04/2011 13:52, Barry N1EU wrote:
I've been trying to figure out the Corsair II AGC circuit on the IF/AF
Board (manual p3-17). I was hoping that someone out there could
either confirm or correct my analysis:
1. D6-D9 form the AGC Detector feeds AGC amp U2B-5 and charges time
constant capacitor C34 through R45
2. Q8/Q9 form the hang circuit and discharges C34 through R54 when Q9
is biased off.
3. The AGC switch (via connector 92, "AGC OFF/AGC TC") functions to:
a. turn off AGC by grounding U2B-5
b. set slow AGC by enabling the hang circuit by grounding C33
c. set fast AGC by disabling the hang circuit by leaving C33 unconnected
Tnx/73,
Barry N1EU
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|