[R-390] kilocycle/megacycle movement

[Bob Camp]

Hi,

There is nothing particularly magic about the URM-25 generator. Obviously
it's the one in the TM's and that is a nice thing. Other than that there are
other generators both tube and solid state that will do the job very well.
Several of the HP tube type generators come available on a regular basis at
a price below what you usually see URM-25 going for. For that matter the
radio will wind up working every bit as well set up properly with your
Heathkit as it will with a URM-25.

Now, an analog VTVM is something that you can't easily replace ...

    Take Care!

        Bob Camp
        KB8TQ


----- Original Message -----
From: "Dan Merz" <djmerz@3-cities.com>
To: <djmerz@3-cities.com>; "R-390 List" <r-390@mailman.qth.net>
Sent: Saturday, January 04, 2003 6:28 PM
Subject: Re: [R-390] kilocycle/megacycle movement


> 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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