[ARC5] OT? bc-348 bfo question

[Fernando - LU2DFM]

On Fri, Sep 13, 2013 at 07:58:23PM -0500, Bill Carns wrote:
> Just checked the gimmick wires on my minty never-been-molested Q and it is
> three turns of insulated wire wound around the stub wire close spaced and
> tight wound. 
Thank you so much for doing that, it's very appreciated.

I've just had tried two options:
1) move the BFO injection one early stage to the 1st IF: big improvement
in CW (there were no strong SSB signals early this morning).
2) augment coupling with a bigger gimmick: this is a little bit soft
than 1), but I suspect that should be insufficient. Read along please.


>But.....I am in agreement with the folks who have come up
> saying that the BC-348 is not a SSB receiver and the symptoms you describe
> would be what I would expect with a receiver that you could not turn the
> audio all the way up and reduce the signal into the BFO mixer by varying the
> RF Gain.

The BC-348 sets the audio at full level and varies gain at RF/IF when
it's in the Manual Volume Control position. 

> 
> I am not sure it is "broke".

I'm not sure it's broke either, I'm trying to understand how it was
designed. Perhaps the designers where more worried about a BFO driving
too much hard the AGC detector than strong signal capabilities: CW copy
is possible with strong signals and no BFO letting the receiver block,
the silence become the marks.

It's just I'm not sure :)

No other '30s receiver is a SSB receiver either, but none of my others
era's rx (SP200 and SP400, ARC-5, Halli) are that bad at handling strong
signals.

The plate voltage of the BFO oscillator is derived from the RF gain
controlled line (MVC, no AGC); this makes the BFO amplitude progressively
diminish when RF gain control is turned down, in a way that (in this
receiver at least) there is a nearly constant relation between IF signal and
BFO output for most of the RF gain range, so for strong signals the RF
amplitude in the second FI plate is several times bigger than the BFO
amplitude _before_ the gimmick injection, for at least the upper half of
the RF gain control. At approx. 1/3 RF gain setting and low, the BFO
output sets on some 3vpp and stays there, so from this point downwards
it's fine, but the audio level is _way_ down at those lower RF gain levels.

For a setting of say 3/4 RF gain level, I need to pad the antenna with
30-50 dB of attenuation to maintain the BFO work.

I'll continue to analyze and report back.

Thanks a lot.

-- 
73 de Fer, LU2DFM