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Repair a Capacitor - Small Air Variable Capacitor in Transmitter

Picture of Repair a Capacitor - Small Air Variable Capacitor in Transmitter
How to repair a small ceramic and metal air variable capacitor like the ones found in old radio equipment. This applies when the shaft has come loose from the pressed-on hexagonal nut or "knob". In this case the nut which is a screwdriver-adjustment, was cracked and would not hold the shaft against the spring pressure of the bearing, allowing the capacitor's plates to touch and shorting out a tuning circuit. The equipment the cap is in is an old vacuum-tube GE VHF FM transmitter converted to ham radio use. If you can spot it in the picture, you are qualified to do this repair.
 
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Step 1: Remove and examine the capacitor

Picture of Remove and examine the capacitor
The removed capacitor is examined. It can be seen that the plates are touching each other. This is bad. I suppose most people would replace the capacitor, but there is no need if it can be repaired.

Somehow the shaft had moved back, forced by the tripod-like spring. The nut on the shaft, which held the shaft in pace, had cracked. This is pointed out because it is important to understahd how the capacitor is held together and figure out why it is broken.

Step 2: Fix the capacitor in a vise

Picture of fix the capacitor in a vise
Notice that the tailshaft is clapmed in the vise without damaging the capacitor's plates. It should only be clapmed tight enough so you can use pliers to remove the cracked knob/nut on the right. The shaft is usually brass so be gentle.

Step 3: Inspect the capacitor's shaft.

Picture of Inspect the capacitor's shaft.
See the little screwdriver groove inside? Normally this would be much closer to the top of the nut. Do not be confused by the inner slot and the nut's slot. They always turn together in a normal situation and this dual slot arrangement is for the convenience of the alignment tech who would be working on the radio set. The little groove is in the end of the actual shaft, of which the other end is clamped in the vise.
This was something they didn't teach me in electronics class, thanks!
Dr.Bill5 years ago
I have need of a 0-360 pF dual gang and several 0-360 pF singles. Any idea where I can get them cheap? DrBill KB1LZL 73's
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