ab.1.4. Circular Dial Wobble
It may be noticed that the main tuning mechanism exhibits an uneven drag or resistance
as the LMO is tuned through its range. This drag may become more pronounced as the
tuning approaches the 5 end of the slide rule dial. If so, here is one possible cause.
Upon investigation, it was determined that the circular dial had a “wobble” or
“eccentricity” as the LMO shaft was rotated. This wobble causes the spiral groove on
the back of the dial to move in and out as the dial is rotated and can result in the spiral
groove follower either coming out of the groove or bottoming out in the groove causing
uneven drag.
Initially, two possibilities were considered: the hub in the circular dial was not
perpendicular to the face of the dial, or the LMO shaft was eccentric or wobbling as it
was rotated. However, neither of these proved to be the case and the actual cause was
more subtle.
It was noted that when the circular dial was placed on the LMO shaft but the set screw
was not tightened, one could wiggle the dial from side to side slightly due to the
clearance between the LMO Shaft and the inside of the hub on the circular dial. When
the set screw was tightened, it pushed the rear of the bushing away from the LMO shaft
resulting in the dial being slightly “cocked” relative to the LMO shaft. This was the
ultimate cause of the observed wobble.
Some clearance is necessary in order to be able to slip the circular dial on the LMO
shaft, but too much clearance results in excessive wobble. Probably when everything
was new, the fit was near perfect and any wobble was minimal, but this may have
changed with time, especially if the LMO shaft has been filed or burnished to remove
burrs or scarring left by the set screw.
The amount of ‘wobble’ was measured by placing a dial indicator on the outer edge of
the dial and rotating the dial through one complete revolution. As shown in the figure,
the dial was furthest forward at the position of the set screw and furthest back directly
opposite the set screw. In the worst case seen, the amount of dial wobble was nearly
0.100” versus another radio where the wobble was approximately 0.020”. If the wobble
is excessive, the nylon spiral groove follower may bind on the back of the dial (causing
drag) or come out of the track (causing the slide rule dial to skip).
It may be possible to insert a very thin shim (.001” or .002”) between the shaft and dial
bushing directly opposite the set screw. It does not matter if the dial runs slightly
eccentric (up and down a bit) as a small amount of up and down motion of the circular
scale is probably not noticeable.
Servicing Procedures Page 74
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