Jack Schmidling ) writes:
tchrme wrote:
Jack,
Try the S 38 website. He has lots of info.
http://www.the-s38-guy.com/ 73 KF6KXG Mike

I found that site about 5 mins after posting the question. The first
time I went there I got side tracked in a bra site... like size 38 b?

It was very strange.. S38 guy but whenever I clicked on the link to s38b
I got this stupid bra site.

Anyway, I tried his fix and it worked like a charm, only too well. I
now have a BFO but the sig strength suffers dramatically even in the am
mode.

Wish I understood what was going on. Before I "fixed" it, the sigs got
louder when I switched to CW.

JACK K9ACT

Unless I'm confusing it with a different version that did have a real
BFO (which does't seem the case given your comments), the "BFO" on
that radio is actually the IF amplifier going into oscillation. The
wire you mention is positive feedback, so the stage will oscillate
and thus beat with the incoming signal.

It's a cheap way of getting a beat signal, since it doesn't require
a separate tube. On the other hand, it doesn't give you much control.

The stage is being used like a regenerative receiver, ie positive
feedback. A real regen detector has something to control the
level of feedback. So it goes from merely being an amplifier to
where there is regeneration (the same stage amplifies the signal,
and the feedback means it amplifies it again and again), so the
gain goes up, and selectivity should also improve. But, when
the stage kicks into oscillation, that isn't necessarily the best
place for amplification (I've heard conflicting views about this,
and some have said it's merely that one isn't controlling the stage
properly and good gain and selectivity can be had while the stage
is oscillating).

What it sounds like is you had the feedback wire set so the stage
did have some positive feedback, but not enough for the stage
to go into oscillation, so you noticed the increased gain.

I don't know if there is a control to adjust the regeneration, but
if there is, that might help. Turning it down from where it oscillates
would simply give you gain and better selectivity.

When using the IF stage as the beat oscillator, you really don't get
much control since the same stage is doing two different things. On
the other hand, at the time of the receiver's design, a BFO was really
mostly so you'd hear some tone when tuning a CW station, and it was
sufficient. It was only as SSB became more popular that such schemes
might not provide enough of a beat signal for proper demodulation.

Michael VE2BVW