[R-390] AGC Line Again

[Jacques Fortin]

Hi Ross,
Does the AGC "speed" change anything ?
In "fast" mode, it still behaves bad ??
If so, it is not related to C551 because it is out of the circuit in the
fast position (and this should not prevent to hear a BC local station
properly anyways).

If my theory is right, it can well be the internal leakage of one or many of
the Mechanical Filer(s).
With the module hot and the P112 disconnected from the J512, you should be
able to measure some leakage value (resistance) between J512-6 and the GND.
A perfect IF module will measure infinite between those points.
I agree with Dave that it only take few megohms of leakage to screw up the
AGC from there.

Dave, I do not have a perfect solution, as the perfect one is to replace /
repair (rebuilt) the offending Mech. Filter once it is found.
But I had the idea to bring the output side of the four Mech. Filters OUT OF
THE AGC LINE.
I did not invent anything: just looked at how the AGC distribution is done
in a 51S1.

1_	Locate R507 (22K ohms) and remove it completely.
	One side is soldered to a post just below the BW shaft (the AGC
"distribution post"), the other end to S503-8
2_ 	There is a piece of insulated wire between S503-7 and V502-1: remove
it completely.
3_	Connect a 220K ohms, 10% or better resistor between the AGC
"distribution post" and V502-1.
	Insulate the resistor leads if needed: this is the AGC line here !
4_	Connect a 470 to 510pF capacitor of proven low leakage between
V510-1 and S503-7.
	I used a 510pF, 5%, 630V polypropylene there.
5_	Open the Mech. Filters top cover, and find C512 within (usually on
the top of the 2kHz filter) and jump it with a piece of wire.

With the C512 shorted, all the output sides of the Mech. Filters are now
referenced to GND.
The BW switch continues to select which of the four filters will provide a
signal to the 2nd IF amp (V502) thru the 470/510pF.
The AGC now controls the grid of V502 thru the 220K ohms resistor.
If any of the Mech Filters have or develop internal leakage, it may still
affect the BW shape for that filter only, but no longer create havoc with
the AGC line.

Keep me aware of your progress.

73, Jacques, VE2JFE