I have seen pictures of several SX-28 or SX-28A receivers that have
what appears to be an additional IF stage modification. This mod is
right in front of the power transformer, and has a miniature tube
(6BA6 ?) and an IF can mounted on a small sub-chassis.

I would appreciate any information regarding this modification. A
pointer to anything published would be ideal.

Thank you.

73,
Jeff
N6MNI

Although I am not familiar with an additional if stage, it seems there would
not be much advantage in adding such a stage. Any additional gain would be
at a cost of reduced dynamic range.

I would be more inclined to think it was a product detector circuit, using a
6BE6.

Colin K7FM

On Nov 18, 7:44 am, "COLIN LAMB" wrote:
Although I am not familiar with an additional if stage, it seems there would
not be much advantage in adding such a stage. Any additional gain would be
at a cost of reduced dynamic range.

I would be more inclined to think it was a product detector circuit, using a
6BE6.

Colin K7FM

A product detector was also my first thought, but why the IF can?

73,
Jeff

wrote:
On Nov 18, 7:44 am, "COLIN LAMB" wrote:
Although I am not familiar with an additional if stage, it seems there would
not be much advantage in adding such a stage. Any additional gain would be
at a cost of reduced dynamic range.

I would be more inclined to think it was a product detector circuit, using a
6BE6.

Colin K7FM

A product detector was also my first thought, but why the IF can?

Because a product detector works at IF/BFO frequencies and puts out
audio. Most of the high performance product detectors of that time
frame used a scheme that required at least one center tapped transformer
that works at IF frequencies. Look up some circuits for diode ring
demodulators.

Plus, the passive balanced demodulators are usually a bit lossy, so
there might be a need to pump up the IF gain a bit to compensate for
the demodulator loss.

-Chuck