Repairing a shorting tuning capacitor

[Refugee]

This job applies to both valve and transistor radios.
This one was shorting over the bottom half of all bands and on close examination I could see that one set of vanes was out of line.
I set up the continuity peeper and adjusted the nut and screw and found I could not clear the shore without causing the other set of vanes to short so I set the nut to the middle of the range that cleared the short at the shaft end.
Then I further examined the vanes to see what end was worst and heated the solder on the mounting insulator and manipulated it to clear the short.
Success

[pwdrive]

If not careful then adjusting the lateral position of the rotor plates relative to the stator plates to clear a short can alter the capacitance value resulting in tracking errors, less of a problem on MW band but on higher SW bands it can be difficult to get it to track, sometimes impossible.

Lawrence.

[Refugee]

What you are saying Lawrence is correct for each side of each vane.
The vane also has another side to it and there is only has one answer to this.
While the change is taking place on one side the opposite change is taking place on the other side so they should balance out.
This even applies to the spacing between the stator and the frame all be it negligible.

[pwdrive]

What you are saying Lawrence is correct for each side of each vane.
The vane also has another side to it and there is only has one answer to this.
While the change is taking place on one side the opposite change is taking place on the other side so they should balance out.
This even applies to the spacing between the stator and the frame all be it negligible.

"While the change is taking place on one side the opposite change is taking place on the other side so they should balance out."

In distance terms yes but surely not in capacitance terms.

Take one rotor plate and the two adjoining stator plates, that is in effect two capacitors in parallel (C1 + C2) If the distance between the stator and the two fixed plates is equal then the capacitances of C1 and C2 will be equal, lets say they are 100pf each when the rotor is fully engaged with the two stator plates, the total capacitance will be 200 pf.
If the distance between the rotor and one of the stator plates eg: C1 (100 pf) is reduced by 50% then C1's capacitance will roughly double to around 200 pf (ignoring edge effect etc) That's the same value as C1 + C2 as originally stated, meanwhile there still remains the capacitance of C2 which has now diminished in value due to the plate spacing being increased by 50% but that value still has to be added to the new value of C1 which results in a different total value to the original.
50% is an extreme example to illustrate things.

Lawrence.

[Refugee]

Well we need to take into account the accuracy of manufacture and note that there are tracking adjustment cuts in the outer vanes to account for this. The difference will be less than what is needed if aligning an accurate instrument with the tracking vanes.

[pwdrive]

Yes I agree.
My previous post was to highlight what can happen to the unwary when trying to do a DIY adjustment of the lateral position of the rotor, it is very easy to to get the capacitance value wrong compared to it's original capacitance value, way beyond what the adjustable outer vanes can compensate for, and sometimes what the trimmers/padders can compensate for especially on shortwave bands.
Distance wise the two distances between the rotor plate and any two adjoining stator plates will always add up to the total of the original two distances no matter how far the rotor is adjusted in the lateral plane because it's a simple matter of addition and subtraction, things change capacitive wise because of variable division numbers (distances between rotor and the two adjoining stator plates) comes into play in the parallel plate capacitor formula.

Lawrence.

[Radio Fixer]

I'm surprised you had to unsolder the fixed vanes of one section Refugee. Perhaps it was dropped sometime, surely couldn't have been shipped out like that.

Anyway, it worked so well done.

Gary

[Refugee]

The set had been dropped and there was also a cabinet repair for me to do too.
So no surprise.
http://golbornevintageradio.co.uk/forum/...p?tid=3896