[R-390] Tightening shaft connection?

[Dan Merz]

Hi,  my interpetation of Bob's comment was that the 47 pf caused too much bfo
into the detector for the agc  - my goal  in putting it in was to increase the
level needed for ssb,  as Jim noted.  I have now taken the 47 pf out and the
set behaves itself now as far as agc goes.  Because I have to crank down the rf
level a bit,  I'm thinking it could use a little more bfo but it's workable
with the just the 10 pf.  It's much better than it was before I put the 2
diodes in,  so I'm happy with that. For some reason (maybe I didn't want to
take the bfo bellows out),  I didn't put the 47 pf in my 390a i.f. chassis a
year or so ago,  and I was happy with the radio without it.  I wanted to try it
in the 390 non-a chassis since the bellows was already disconnected by the
previous owner,  or at least loosened easily.   I'll probably forego
experimenting with tweaking the bfo injection at this point  (chassis removal
fatigue !!)  - it's easier to tweak the r.f. gain on real strong ssb stations.

I finished making the shaft coupler out of 6061 aluminum and it works very
nicely.  I made it  1/4 inch thick instead of  0.2 inch thick like the original
because that was the thickness of my aluminum plate.  All the bending in this
gismo takes place at the thin section at the bottom of the hole opposite the
screw so I just thinned that to 64 mils compared to 80 mils for the original,
so the bending force and stiffness would be about the same.  It should have
less stress at the outer side as a result and may last longer.  I would guess
that all failures of these couplers occur either at the bottom thin section or
at the root of the square corner where the screw head or nut sets.  If they
failed somewhere else,  I would be very suspicious of the type of material
used. I'm only mildly suspicious so far.    I had a failure of each kind in my
two couplers.  My replacement has somewhat longer ears where the screw goes to
avoid the notched/thin section near the screw and the shaft hole.  I'll keep my
eye out for originals to put in the radio when it leaves me, not soon.    Right
now I'm happy to have a beefier version since I anticipate exhanging the two
i.f. chassis occasionally without worrying about breaking them.   thanks for
the comments,  Dan

Jim Brannigan wrote:

> Actually the problem with most AM detectors is too little BFO injection in
> relationship to the IF signal.  Of course these detectors are not build to
> filter the increased BFO signal and it will get into the AGC.
>
> Jim
>
> > My take on the 47pF cap across the original was that there was too much
> BFO
> > injection and that it did adversely affect the AGC. I couldn't neutralize
> > the BFO. YMMV. I found that the two diode attack time mod was the most
> > effective part. A smaller cap across the original is a reasonable cut and
> > try affair.
> >
> > Bob