By the way, my schematic misses out the positive rail which the emitter
of Q9 and the R49/R47 junction connect to!
For fault finding purposes, that part of my schematic is annotated with
three typical voltages:
100mV p/p audio at U1D pin 12
1v p/p audio at U3D pin 12
0.8v DC across C34
73,
Steve G3TXQ
On 23/04/2011 14:30, Steve Hunt wrote:
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
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