[Kenwood] TS-700SP VFO problem

[Gary Peach]

> I have seen the same problem.
> I think it is little whiskers of aluminium growing between the
> plates of the capacitor.
> 
> I have sprayed electrical contact cleaner in through
> an inspection hole on the VFO liberally. And
> by rocking back and forth on the worst patches seem to have 
> resolved the problem, at least for a couple of months....

Quite a problem;
  in the olden days I used to deal with this problem using
   a very high voltage source.
The high spots will arc and you can see the sparks at those points.

The temperatures generated by the sparks is sufficient to melt
 the Ali and the high spots will disappear.
 
Moving the shaft about ensures that all the high spots in all
 the alignment positions are flattened down.

A current limiting resistor of sufficient Wattage should be placed
 in series with the voltage source to protect the capacitor
 and power source.

DC or AC may be used just so long as sufficient Joules are available
 to melt each high spot.

Try a 1,000 Volt Megger for starters just to prove to yourself that
 it does work.
I have no idea if TV EHT can provide sufficient current to do the job,
 but the addition of a high working voltage fixed capacitor and
 a charging resistor may just provide the extra punch, sorry melt. :)

The capacitors so produced seemed to give good smooth tuning and
reliable operation provided they were shielded from 
the ingress of dust and dampness.

So how can this be done with the capacitor in situ?
That depends upon each individual installation.
If it is possible to lift the connection into the circuit of 
at least one side of the capacitor then the process will work in place
and is probably mechanically better as the configuration of 
the variable capacitor is not further distorted by 
the installation process.

Of course as the new methods of construction have, 
to a large extent displaced variable capacitors, 
this tip is like me of historic value only.

If the capacitor is to work in a very high voltage circuit
then the voltage used to clean up the plates 
 should be at least 50% greater than 
the maximum expected voltage across the plates. 

Example
This means in practice that a capacitor in an AC circuit 
using 1,000 V RMS the clean up voltage should be 2 x Peak plus 50%

Peak voltage is root two times mean voltage, 
 thus
   2 x 1.4 x 1.5 x 1,000 Volts = 4,200 Volts

The minimum usable would be just over 2 PIV or about 3,000 Volts

Gary7SLL, hoping that was coherent enough.