[R-390] speaking of AN/URM's

[Roy Morgan]

At 01:43 PM 10/22/2007, you wrote:
>My AN/URM-25F has always has severe backlash since I bought it, I took it 
>apart and found an adjustment that should have taken care of it, (it 
>presses the worm gear against the round gear driving the variable cap).

Bob,

AH!  The dreaded worm gear.. heheh

NOTE: the D and F model have different drive mechanisms if I remember 
right.  The F model is quite similar electronically to the D, but quite 
different mechanically.

Backlash in the D model can sometimes be cured by adjusting the ONE-BALL, 
rear tuning shaft adjustment. This is the shaft that carries the single 
knob on the front panel.  There is a single ball bearing at the rear end of 
that shaft, with a slop-adjusting mechanism.   The tell tale symptom is 
that when you push and pull gently on the frequency set knob, the frequency 
changes as you do.  Use CW mode on your receiver.  I suggest hat when 
adjusting this thing, you get the thing plenty warmed up before you take it 
out of the case.  Mechanical play may change when it's hot.

There may also be a spring or adjustment that adjusts the mesh between the 
worm gear on that shaft and the main tuning large gear. This seems to be 
what you found in your F model.

The large gear MAY be of the spring loaded anti-backlash type.  (I can't 
remember).  These would be two different situations.

The F model has a different shaft arrangement on the tuning knob, but I 
can't remember the details.

I have a D model that needs to be taken apart for attenuator 
repair..someone smoked it on one position.  I have another URM-25 handy, 
but I'm not sure if it is an F.  Does anyone know if the attenuators can be 
swapped?

>  It was not even touching so I adjusted it down a little bit, put it back 
> together and I have no RF output now,

Uh oh. Something went awry. Do the tuning cap plates now rub?  Was the 
adjustment on the tuning knob shaft or on the main capacitor gear 
shaft?  Maybe you lost a ball bearing and the cap rotor and stator are now 
rubbing.

>...The modulation still works.

For now.  The D model is susceptible to modulator failure.  The cause is 
drifted resistors and/or leaky capacitors (paper most likely) in the 
bridged-Tee circuit in the oscillator. Dallas Lankford wrote a multi-page 
description of how to overhaul the D model while he was editing Hollow 
State News. The HSN publisher can still supply a paper copy of that. I 
think also that it has been transformed into digital form but I'm not sure 
of it's availability. No owner of a URM 25D should be without one.  (He 
does not deal with the tuning mechanicals. Also, he did not write a similar 
one for the F model.)

>Worst of all a little set screw fell out of it or it was a huge 
>coincidence and I can't figure out where this came from. Anyone have any 
>ideas or does anyone know a place where I can get this reconditioned?

Do you have the errant set screw?  If you report the size of the thing, it 
may help locate the place it belongs.  It may well be left over from years 
ago and there really is no missing set screw.

There has been one person who worked on these things and has sold 
refurbished ones on ebay or mailing lists. Can't remember who.  That may be 
in my notes file.

Roy




- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
13033 Downey Mill Road, Lovettsville, VA 20180
Phone 540-822-5911   Cell 301-928-7794
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