[R-390] kilocycle/megacycle movement

[Dan Merz]

Hi,  just a word on what I did so far.  I tried Joe's approach,  being a little
strapped for bench space at the moment,  and fashioned a wire hook and moved the
two cams that were obviously at the wrong spot.  The screw/nuts on the two gears
involved were verrrry tight and I soaked the screws a bit with penetrating oil
overnight before putting some real force on them.  After bringing the cams to a
better position,  the set tuned much better on the 3 to 4 mhz range where I first
noticed a problem.  Once I get the bench cleared,  I'll do a better standard
alignment since this has been on my agenda since I got the set.  I keep thinking
I'll come up with a URM 25 s.g. in my wanderings but so far still have only a
Heathkit "lab grade" generator that I've always used,  not very well shielded.  But
I suppose it might still do a reasonable job?   Again just looking into a 390a is a
pleasure so pulling it out of its cabinet has its own reward.  After all the
backlash talk,  I did a little reading in the spec. documents available online and
see that the only backlash specification had to do with the tuning knob, pto and
dial readout,  at max of 100 cps.  My radio behaves itself well in this regard.
No mention that I could find of the amount allowed for the rest of the gear train
going to the coil racks,  etc.  I suppose this is indirectly covered by the
requirements on uniformity of output over the bands.  I wonder if one can get a
rough idea of how insensitive (or sensitive)  tuning would be to this kind of
backlash by rocking the Mhz knob a bit off the notch and noting whether the signal
level drops. Since I don't really know how well my set is aligned,  I can't go any
farther with this idea yet.  Maybe someone with a carefully aligned set could
describe what happens.  Thanks again for all the suggestions,  Dan.

Dan Merz wrote:

> Hi all/Jim,  I took a look inside and did some reach-in cleaning/inspection but
> didn't find anything obvious.  And the detent spring seems to be ok though
> difficult to inspect.  I can't see all the cam alignment marks but noticed the
> 4-8 mhz cam was off the mark some but was still operating on the right
> surfaces.  The 2-4 mhz cam mark is not too visible but I noted that it didn't
> look right as far as how the cores were traveling at the upper end of 3 to 4
> Mhz -  aha ,  the  Mc/Kc link that started the Mc knob moving occurred when
> near the upper end of the range around 3.92 Mhz where the core rider was
> actually dropping on the wrong side of the cam surface.  The cam is out of
> proper adjustment,  and the cam surface is steep enough on the "forbidden" side
> that it locks the cam shaft which forces the detent on the Mc  knob to give way
> to accomodate the motion of the Kc knob as I tune within the band.  Mind you,
> this doesn't take much force on the Kc knob,  hardly different than regular
> tuning so there must be some gearing that is helping the detent disengagement -
> I didn't quite have the patience to analyze that .   Intuitively,  I would have
> thought it would be the other way around - a lot of force on the cam to hold it
> solidly enough to disengage the detent.
>
> I haven't figured out where the loose connection is that got the 2-4 cam out of
> sync,   I guess the gear on the cam shaft since that's closest.  I seem to
> remember checking these alignments when I first got the radio a couple of years
> ago and I haven't loosened anything in the gear train except to put the pto
> oldham coupler  in the right position after I worked on the pto.  I think
> pulling the front panel is probably the best step to take next so I can see all
> the cam markings and alignment.  Or is there an easier way?  thanks for all the
> suggestions,  some of the split gears were checked and seemed ok.  So far I
> haven't seen a shaft slipping in a gear but that could be happening.
>
> Dan
>
> Jim Miller wrote:
>
> > Sometimes a coil slug will stick at a band edge causing low sensitivity.
> > Look at the slug rack as you tune at the high end and see if any slugs stop
> > moving prematurely.  If you see one, tap on it ot wiggle it by hand to see
> > if sensitivity returns.  If it is a slug that is sticking, you can usually
> > free it up by loosening the screws that hold it to the slug rack and allow
> > it to re-seat itself so it doesn't bind.
> >
> > Dan Merz wrote:
> >
> > > Hi,  I haven't used my  390a much over the last 2 or 3 months and I was
> > > listening on 80 meters tonight and found a problem tuning up near 4
> > > Mhz.  It seemed dead there and I could tune in stations down near 3.9
> > > somewhat better (I started listening on another set for comparison to be
> > > sure the band wasn't dead).  After awhile I noticed that the megacycle
> > > knob seemed to move off its "lock" position and was moving as I tuned
> > > the kilocycle knob.  It did this sometimes and would get off the band
> > > position but not always.  Is this most likely a lubrication problem?  I
> > > really never was able to pick up a known signal at around 3.99 Mhz and
> > > I  tried rocking the megacycle knob to improve things,  or to find if
> > > something else was "hanging up".  I haven't lubed the set since about 2
> > > years ago when I did a pretty thorough job in this regard and everything
> > > seemed very free.  I'll pull it open tomorrow and take a look but was
> > > curious if there was an obvious known component that causes this kind of
> > > symptom. It seemed ok on other bands but I didn't spend a lot of time
> > > checking.   thanks,  Dan.
> > >
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