[R-390] Tightening shaft connection?

Wed, 22 Jan 2003 09:12:51 -0800

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

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Dan Merz
Sent: Tuesday, January 21, 2003 9:43 PM
To: Tom M.
Cc: R-390 List
Subject: Re: [R-390] Tightening shaft connection?

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 <[email protected]> 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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