Greetings Friends of the Filament,
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. 73 Mike
KF6KXG

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.
----------------
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

Don,
Thank you for the reply. That is what I have heard from others too. But how
come my new SX110 (in 1961) and my current SX99 (same circuit) doesn't "block
out"? Mike KF6KXG

Mike posted:
Thank you for the reply. That is what I have heard from others too. But
how
come my new SX110 (in 1961) and my current SX99 (same circuit) doesn't "block
out"?
--------------
Very good question. Could it be that some version of the receiver used a dual
gang switch that disabled the AGC when the bfo was turned on? If I can find a
SX99 schematic I'll check for differences.

Don

Don and others following this string,
The dual gang switch idea would explain it. I have a W7FG copy of the SX99
manual for the Mark 1B and 1C and the AM / CW switch is single gang. I am not
sure what versions my SX99 is. I am going to spend some time this wknd and
compare the schematics and pts lists a little closer. Maybe I can find a
difference. I am also going to finish recapping it while I have it apart.
Has anyone else out there with a SX110 noticed this behavior? Mike KF6KXG

In article ,
(Dbowey) writes:

Very good question. Could it be that some version of the receiver used a dual
gang switch that disabled the AGC when the bfo was turned on? If I can find
an SX99 schematic I'll check for differences.

Almost every rx of that era required you to turn off the AGC when using the
BFO, except for some high-end jobs that picked off the AGC ahead of the BFO
injection.

The Halli engineers who designed the SX-99 would have been doing everyone a
favor by adding the extra switch gang to cut off the AGC when using BFO.

This auto-mode switching was carried to further extremems later on by all
makers. --Mike K.



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