[R-390] MHZ gear clamp busted

Thu, 19 Dec 2002 07:02:53 -0500

Walter and Matt - many thanks for your excellent advice!  I suppose
that, given I must at least partially disassemble the gears, it's not a
bad idea to clean and re-lubricate everything, correct?  I have read
somewhere that once this is done the performance of the tuning can be
quite smoothe.

The pictorial for disassembly seems straightforward enough.  We'll have
at it!
-m-

-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Walter Wilson
Sent: Thursday, December 19, 2002 05:58
To: Mark Richards
Cc: [email protected]
Subject: Re: [R-390] MHZ gear clamp busted

> So in summary my questions are (1) how to replace the MHZ gear clamp
and
> (2) how should the MHZ gearing be engaged and (3) how does the
> anti-backlash work.

Mark,
You will have to remove the front panel.  No real difficult steps here.
First, set the KC setting at +000 (that's one KC above 999), and the MC
at
7MC.  Just remember to loosen the 1/4 shaft nut around the dial lock,
and
rotate the dial lock about 1/4 turn to disengage it from the KC disc.
Shine
a flashlight down in there, and you'll see what you need to do.  Then
loosen
the 13 screws that secure the front panel and remove the knobs, and
you're
ready to go.  Oh, and you'll have to loosen the clamps the secure the
Bandwidth and BFO shafts to their respective shafts on the IF deck.  Now
pull the front panel loose from the MC and KC and AGC shafts, and tilt
forward.  It helps if the R-390A is sitting on a platform about 2 inches
higher than your workbench or table, so that when the panel lays
forward,
its handles will rest straight against the working surface.

Now you have a more challenging task.  If you set your frequency at
7+000,
then all your camshaft marks should be aligned with the points of the
camshafts.  This was done just in case you move something during these
next
steps.  You'll have to remove the veeder root counter, and all the gears
in
front of the front gear plate.  Then you will be able to loosen the
screws
on the front plate and "gently" remove the plate.  You should now have
access to the MC gear clamp.  After replacement and reassembly, check to
see
that the cams are still aligned, which will be the case if nothing bad
happened while the front plate was off.

The MC gearing engages both of the split gears to the other wider gear,
with
about two gear-teeth of rotation to stretch the spring.  Both teeth
engage
in the middle of the wider gear, and the spring is stretch if you get it
back on correctly.  The spring tension works in a fashion to pull the
two
gears to fill the entire width of the space between the gears of the
wider
brass gear.  It essentially gives this split gear a variable width
tooth,
that stretches to fill any gap that might otherwise exist.  IF the gear
train moves smoothly, then this amount of spring tension is enough to
overcome any gear friction.  If the gear train has a lot of gear
friction
somewhere, then the anti-backlash features are basically defeated by the
friction (that is stronger than the spring tension of the split gear).

Your 7MC problem is very often due to C327, which is a 100pF capacitor.
Your capacitor kit should contain a 100pF capacitor to replace it with.
While it's not always a problem, I'd personally never go to the trouble
to
remove the RF deck without replacing this capacitor, the three paper
capacitors, and checking all the resistors underneath (there are several
resistors that seem to be persistant problems).

Walter Wilson - KK4DF
http://r-390a.us
(REAL Radios: R-390A, R-390, T-368E, Ranger, Thunderbolt, KWM-2A)

----- Original Message -----
From: "Mark Richards" <[email protected]>
To: <[email protected]>
Sent: Thursday, December 19, 2002 12:15 AM
Subject: [R-390] MHZ gear clamp busted

> Hello to the R-390 group.  I am so pleased that there are still
> enthusiasts of this fine radio active.  May I draw on the knowledge
> herein?
>
> I have stolen an hour from our demanding new twins :) (5 weeks old)
and,
> while my wife and the children were snoozing, managed to begin some
work
> to replace the MHZ Gear Clamp on my R390a.  The MHZ knob never worked
> since I acquired this radio years ago.  I never used it seriously and
> planned to rebuild it someday.  Someday has arrived.  Besides, my life
> has to consist of more than changing diapers and burping babies!
>
> Through the good auspices of David Medley, I've secured a gear clamp.
> Now I need some good advice before I get myself into a deeper pickle.
>
> It appears that it is impossible to replace the MHZ Gear Clamp without
> actually disassembling the entire shaft.  I began this work by
removing
> (actually breaking) the snap-ring on the end of the MHZ shaft and then
> tried to pull the shaft out, using the MHZ knob.  Hopefully this is a
> common ring that I can get at the local auto parts store :)  Anyhow,
> this method failed as it appears that a component of (the very novel)
> mechanical turns limiter is fastened to the shaft.  I cannot determine
> how it's fastened, but it's definitely on there.  Perhaps there is a
pin
> driven to hold it in place?  It does not appear possible to remove it
> with simple tools.
>
> Therefore it appears my only option is to remove the front panel of
the
> radio and then disassemble the front portion of the gear mechanism
(the
> Veeder-Root counter assembly).  Perhaps then I will have access so
that
> I can slip on the new clamp?
>
> I want to remove the front panel anyway and perform some of the
cleaning
> steps you recommend but first wanted to make sure the unit was
> electrically sound before I messed up the mechanics.
>
> Would any of you experienced folks be able to offer me some sound
> advice?
>
> Oh, I did manage to power up the radio and found it to be actually in
> fairly good working order.  It does appear that the sensitivity drops
> off dramatically below 7 MHz, however.  I did not make measurements -
> these were simply done with a signal generator attached to a wire -
yet
> the differences were dramatic enough to suggest that there is a
problem
> below 7 MHz.
>
> I have a capacitor rebuild kit so once I get the MHZ gear back in
> service, I will strip the radio down, clean it up and replace the
pesky
> capacitors.
>
> One more question - regarding the actual MHZ gear.  It is assembled,
as
> you all know, as two thin gears, independent of each other but
connected
> by a small spring.  I understand this is an "anti-backlash" mechanism.
> When I reassemble things should BOTH of the MHZ gears be engaged with
> the gear they connect to or just one?  How does the anti-backlash
> mechanism actually work?
>
>
> So in summary my questions are (1) how to replace the MHZ gear clamp
and
> (2) how should the MHZ gearing be engaged and (3) how does the
> anti-backlash work.
>
> Thanks for your kind help,
>
> Mark Richards
> K1MGY
>
>
> _______________________________________________
> R-390 mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/r-390
>

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