[R-390] Oldham Coupler Maintenance

[Flowertime01 at wmconnect.com]

Fellows,

Bob wrote about having to readjust an Oldham coupler after a maintenance 
procedure to the VFO.

The VFO should run with the KC shaft with ease. The KC knob in a good clean 
gear train should almost free wheel when spinning the knob and the cams are 
dropping. That was long ago and your mileage may vary in an older receiver. This 
implies it takes very little torque to turn a VFO shaft.

If you have an original strength Oldham coupler the spring should not stretch 
when the KC shaft is moved. The resistance of motion in the VFO should be 
less torque than the torque needed to stretch the coupler spring.

If you have a rubber band on the coupler, or a very weak spring (ball point 
pen spring tailored to fit) then the springy thing you have employed may 
stretch.

An over stiff spring may not help as it may place side stress on the VFO 
shaft as they go around their respective corners.

You may not want to run the coupler end faces to tight. The angle of the two 
shaft alignments forms a high pinch point some where around the shaft axis. 
The coupler faces may not be true to the axis. You need to open up the faces. 
Some where between the coupler spacer falls out and the coupler binds at some 
point. Run the shafts around a few turns and find that close point between the 
two coupler end faces. Then adjust the end faces up close at that pinch point.

With the spring off, any where in the rotation of the shafts, the coupler 
spacer should be a bit free between the two coupler end faces.

With the spring on the coupler the resistance to rotation of the VFO shaft 
should be less than the force needed to stretch the spring. If not your VFO need 
its shaft cleaned and lubricated. Not a hard thing to do. Most of that effort 
is to have a good oil on hand for the purpose. Mobil Synthetic lube is OK 
hear. Good sewing machine oil was used in the 70's Watch oil or whale oil is good.

To oil the shaft, just set the VFO up on end and oil the shaft. Run it by 
hand both way for a bit to see if that solves the problem. Most times in younger 
receivers it was not necessary to open the VFO and clean out the crud. In 
these now aged receivers, who knows what it will take to get a shaft and bearing 
cleaned up. Worry not it can be done. The job is just time consuming and not 
high technology.

Getting the back lash out of the VFO Oldham Coupler is mostly just attention 
to cleanliness, lubrication and mechanical spacing.

Roger AI4NI

> Hi
> 
> Check your PTO for bad lubrication. The spring on the Oldham coupler  
> can only deal with a limited amount of torque. The same thing is true  
> of the screw driving the tuning core. Since the PTO is sealed they  
> often do not get a regular clean and lube treatment. The result can be  
> concrete where there used to be lubrication. You also may have  
> everything so tight that the PTO is binding up as it rotates.
> 
> Bob
> 
> 

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