Mike posted:
I have a problem on my SX110 that I have been trying to trace down for a
while. When the BFO is turned on, the set goes quiet unless I turn off the AVC.
I know the directions say to do this, but...in the 60's when I had a new SX110,
it didn't behave this way. Also since it is the same basic circuit as the S 40,
etc. and they don't behave this way, I am trying to trace out the problem.
Pin 2 (plate) to the 6SC7BFO tube reads 217v instead of 235v. R28 that
feeds it is about 10% too high. But my problem is too much injection
overwhelming the AVC circuit. Changing R28 would increase the
injection...wouldn't it? When I touch Pin 3 (grid) with a digital meter, the
signals return as if it is bleeding off some of the injection.
Is my problem with the BFO circuit or the AVC? Could someone else who has a
SX110 let me know if theirs behaves this way? All help is appreciated.
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I don't have an SX110, but I did peruse the schematic for one. The problem is
in how the receiver is designed and used. The BFO signal is injected to the
control grid of the last IF amp. The amplified BFO signal voltage from this
stage drives the diode detector, which provides the AGC voltage back to the
earlier stages.

When you turn on the BFO, which is a fairly high-level signal, it causes the
AGC to greatly reduce the gain of the RF amp and the two IF amps. This will
cause the receiver to go quiet. It is normal, given the design.

Don