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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