[R-390] Tightening shaft connection?

[Dan Merz]

Tom,  I might need that coupler.  After making the Lankford diode additions and a
47 pf cap to the bfo output,  I discovered the coupler was actually broken on the
bandwidth shaft connection right near the nut so that is kaput - I guess I didn't
see this before and/or possibly overtightened it to cause the break.    I also
noticed the coupler on the bfo shaft is broken on the side opposite the tightening
screw but it will still clamp down enough to work.  I remember discussion of these
being susceptible to breaking.  I looked at one closely and it  looks like an
anodized aluminum alloy - definitely non-magnetic material - so I'm assuming
aluminum.  Has anyone made these out of brass or a material  not apt to do this.
The fracture reminded me of season cracking of some brasses which is a stress
corrosion problem.  High strength aluminum alloys can also have this problem. I am
quite curious about how these shaft couplers were fabricated - does anyone recall
anything about them?.......  It's tomorrow now and I decided to make my own
couplers out of 6061-T651 aluminum plate that I found in  my garage - hopefully
these will be better than the originals - they are about half done - fortunately
the hole is a standard size,  5/16 inch and aluminum is almost like wood to saw,
file and shape so it's not going to take long.  I expect these to be superior to
the originals.  Are the small size type more suscetible to breaking than the larger
ones - it appears to me they would be from the shape?

Another problem as well.  The bfo is not acting very nice now - if I turn it on
with say a broadcast station tuned in,  the gain is reduced immediately to a low
level.  I wanted to re zero it  since I turned it some when I was working on the
circuit under the bellows.  I don't seem to hear the bfo like I did before when it
was tuned off zero.  Do I have too much bfo  getting  into the agc circuit with the
increased capacitor (47 pf across the original 10 pf)  I put in   or is this the
symptom of some other problem?  I explored this a bit to make sure I didn't fry the
coupling caps when I soldered but I don't think that's the problem.  Is the
adjustment of the neutralizing cap critical enough to cause such a big effect.  My
first reaction was to take the added cap out and see if good behavior returned.
Dan.

"Tom M." wrote:

> Dan,
>
> I found the shaft to me exactly the right size on mine.
>
> It's hard for me to appreciate the difference by email, but perhaps you could
> fashion a piece of thin copper plate (strip) to wrap around the shaft, and then
> make it the right length to allow the 390A coupler to fit.
>
> Or maybe get a shaft from a 390 or from a different radio??
>
> I have a spare shaft assembly from a 390A panel in my junque that you could get
> rought with.  If you'd like to try it let me know.
>
> Tom
>
> --- Dan Merz <djmerz@3-cities.com> wrote:
> > Hi,  is there a good trick to tighten the bandwidth shaft coupler if the
> > shaft coming out of the i.f. chassis seems a bit small.  The shaft
> > coupler worked ok on the 390a i.f. chassis but when I put the 390 i.f.
> > chassis in the 390a  radio, the shaft seems a bit small for the coupler
> > even when tightened down pretty tight (gap closed).  More to come on
> > the  successful conversion of the 390 non-a i.f. to go in the 390a,  but
> > I'm trying to solve this problem so I can change bandwidth without
> > taking the i.f. chassis out - it seems awkward to have to do that.  I
> > tried putting a 2 mil shim sheet on two sides of the shaft in but that
> > still didn't give enough purchase on the shaft.  Maybe there's a gritty
> > substance that would help?    thanks,  Dan.
> >
> >
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