[R-390] backlash on URM-25

[Roy Morgan]

On Feb 15, 2009, at 12:22 PM, Bob Young wrote:

> My URM-25 bought from a vendor who sells them on epay, it came with  
> some back lash and it has gotten worse,

Bob,

(With all due respect to Keilbasa):

If yours is a D model (not an F) then the fix may be easy.

The frequency knob is on a shaft that goes all the way to the back of  
the oscillator compartment casting. There, at the rear, is a single  
ball bearing and end play adjustment assembly.  The assembly consists  
of a threaded bearing seat, the bearing, and a lock nut.  (Note:  I  
just looked at the digital copies of the manual I have here and I  
cannot: a) see where the end of that shaft is in the  picture  b) see  
whether the shaft goes to the rear cover of the casting, or if the  
bearing is inside so that you can see it only once you get the rear  
cover off.)

If the end play is more than it should be, the knob will have backlash  
as  you turn it to adjust the frequency.  The simple test for this is  
to push and pull on the frequency set knob while listening to the  
generator with a receiver set for CW.  The note should not vary much  
at all (especially on the lower frequencies), and when you let loose  
of the knob, the note should return pretty much to what it was.  If  
the beat note disappears altogether, or varies a lot, your shaft has  
end play.

My notes include an email from a fellow who found a tension device  
that holds the worm gear part of that shaft in mesh with the gear  
that's on the tuning cap shaft.  He solved backlash apparently by  
adjusting that tension.. It's not clear that we are talking about the  
same thing or not.  I do know that the F model has a different drive  
shaft mechanical arrangement.  I would expect to see grease at the  
worm and main gear.

To fix this, run the generator out of its case till it's plenty warm.   
This makes sure your adjustments will not be thrown off due to warm  
up.  (If you care to, clean and re-lube the whole frequency set  
mechanism first.)  Then loosen the lock nut and turn the inner bearing  
seat in a bit, and tighten the nut.   You'll have to learn how to  
preload the adjustment, or allow for the effect of the lock nut  
tension after you make a change.  You should arrive at a point where  
the shaft has no perceptable end play, and runs free, both.  If  
there's a separate adjustment that sets the depth of engagement of the  
worm gear in the main tuning gear (that is phenolic, I think), then  
set that for little or no backlash.

If I remember correctly the main phenolic gear is not spring loaded  
for anti-backlash.

While you are in there, apply a drop (only) of Caig De-Oxit to the  
tuning cap grounding wipers, and just a touch of oil at what I assume  
are ball bearings on the shaft.

Do report what you find.  My memory of all this is some years old  
now.  If you run into trouble or some mystery, I may be able to dig  
out a URM-25 here and have a look at the thing to report more clearly  
what's there.

Roy

Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073