[R-390] COLLINS PTO

[Flowertime01 at wmconnect.com]

Pete,

Inside secret R388 VFO were never as linear as R390 or R390/A VFO's.
That little tid bit never made the TM. If you needed to receive those 
frequencies you were happy to have a R388 to get the job done. Other than beating 
the receiver against a calibration tone you would never know the frequency 
of the signal being received. This problem lead to the use of split head 
phones. Here listen for this signal where ever you find it around some dial 
number.

No one wanted to try and lay tapered winding on the form with more 
exactness. The specified wind pattern was just the best average to make production. 
I do not suggest you start your education into VFO's by reinventing a R388 
VFO.

First worry not about end points.
End points are second.

In any VFO adjustment, the last thing we do is reinstall the VFO.
The second last thing we do is set the end points.
Logically then linear must come before these two steps.

Every thing that follows is just stuff I have read else where and have 
never had to do my self. 

But R388 VFO's are not real available and thus this restoration must be 
done.

Back when the shim stacks were "factor set" the secret ingredient was a 
high 
viscosity slow drying varnish. This let the shims be set and sort of stuck 
in place while other shims were brought into adjustment. The end results is 
that a good VFO has an S curve to the shim stack. 

You would like the ten turns to be 1 megahertz. You do not care where it 
starts or ends as that will come later. The trim caps, and the end point 
adjustment in about the middle of its range needs to give you 1 megahertz over 
ten turns. Set the end point coil device to about mid range. Add or remove 
caps to get a 1 megahertz span. Sort of close to 455. 

Then the non linear deviation needs to be where the shim stack will bring 
the 100 K hertz points into range. This is a very well aged VFO and exact may 
not be possible.

Pull the stack apart and clean all the shims.

You will need to build a template S curve. (soft pine and sand paper. Bondo 
and resand. A stiff but soft aluminum strip that can be formed. Hammered 
copper.

The template needs to set against the shims to set the stack. With the 
template in place you loosen the shim stack and use the template to give form to 
the shim stack curve. Tighten the stack. Test the VFO. Think about the 
template adjustments that need to be made. Modify the template. repeat the 
process. Once that template was found at Collins. It was just applied to every 
VFO. Not having that original factory jig in hand you get to reinvent and re 
calibrate it.

At the factory, one had a template and slow drying varnish. The VFO was 
determined to be working and needing its shims set for final assembly. 
The shim stack was loose.  The shims were varnished. This stuck them to 
each other. The template was applied. the shim stack was tightened. 

With a slow varnish the points could be checked and the process repeated. 
Likely several templates were on hand in a set so the best fit could just be 
applied and "experienced" tuners could hand set a section as needed.

Then the end point was set.


Select a slow drying varnish (hours) Find a thinner that will de solve the 
varnish. (Un do). 

You have a feel for how far you can move a point with the shims. Put all 
the shims over to one or the other limits. You can add / remove turns to the 
trim coil, and add / remove small caps from the circuit. You have to work 
that mix until you feel the VFO runs ten turns and tracks close enough so that 
you can "shim" the mid points. You have to move the circuit back onto the 
fit of the PTO coil taper. When you get there you will see the mid points draw 
close to where they are expected to be. As you are off the "sweet spot" you 
see one end or the other of the VFO range fans fast out of range. This will 
tell you if you need to drive the trim caps and coil up or down in 
frequency to get the VFO back on to the sweet spot of the VFO coil. 

Take Care. Stay with it. A simple frequency counter and a pencil mark for 
the ten turns should get the job done. Like the R390's you can turn the R388 
up on end and work the VFO while hanging out of the chassis. Do pull it out 
and get the outer cover and heater off it just to make the process easier. 
You ca pull all the cans off so you can work the shim stack.

Last resort you may want to file the shims (holes or ends) to get some more 
range. You may want to invest in a R390/A Collins VFO for parts so you do 
not have to file on R388 shims.

Pete, I do not read my mail every day, But I will get back to you if you 
need more info.

Roger Ruszkowski AI4NI (68-74 R390 Radio Repair Person USASA)



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