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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.

Step 4: Use pliers to twist on the nut to loosen it. It is a press-fit.

Picture of Use pliers to twist on the nut to loosen it. It is a press-fit.
Trying to remove the nut with pliers. The nut was still pretty tight even though it had slipped. It is a press-fit and 50 years old. Since the nut is cracked, it will come off with some slight persuasion. Check the next step before proceeding with this one.

Step 5: Carefully removing the nut

Picture of Carefully removing the nut
The nut was more easily removed when a screwdriver helped to hold the shaft from turning. Even though the back end of the shaft was in the vise, it was not so tightly gripped by the vise for fear of damaging the capacitor further. The little screwdriver helped out. A good quality screwdriver with a tip in like-new condition is important for something like this.

Step 6: The nut is removed. examination.

Picture of The nut is removed. examination.
The nut is removed. There are no internal threads, this was a press-fit from the TELERADIO capacitor factory. The nut is made of brass and chrome plated.

Look at the assembly. Aside from being a little dirty, it's basically OK.

The challenge is to clean this, re-align the axial position of the capacitor plate spacing against the pressure of the spring, and re-afix the nut with some JB-weld while avoiding getting any of the adhesive into the bearing or the thrust bearing surfaces, all in one simple step!

If you get adhesive in between these moving parts, the capacitor will not be able to be turned for adjustment.

Step 7: Disassemble and clean the part.

Picture of Disassemble and clean the part.
The cap was disassembled to check for excessive wear or anything else wrong, and put back together. Once the nut was removed in the previous step, the part will come apart quite easily.

Note the long tang of the three-way spring. This is where a soldered conection was, when the cap was in the radio set.

The parts do not have to be immaculate. They are plated, so just lightly clean them with solvent and possibly a very light application of "scotchbrite" to the shaft and spring bearing and thrust surfaces so that any dirt is removed and a good electrical connection is promoted.

Step 8: Reassemble the cap and apply adhesive to the tip of the shaft.

Picture of Reassemble the cap and apply adhesive to the tip of the shaft.
Note: I did not clean it very thoroughly, but I cleaned the areas that matter. Since shaft nuts are not available for these any more, (were they ever? It lasted 50 years, what do you want?) a very small amount of "JB weld" was put on the shaft end, and an extremely thin layer, inside half the length of the nut, the half being the portion away from the bearing.

The expoxy seen on the shaft here was cleaned from the sides of the shaft and left only at the end.

Only the tiniest amount in the thinnest possible layer was allowed to remain on the circumference of the shaft, in the hope of getting a bit of it stuck between the inner diameter of the nut and the outer diameter of the shaft, and at the same time avoiding getting any between the nut and the bearing end (which also looks like a nut here).

Use alot of care. Due to the press fit, only the slightest amount of epoxy is required between the shaft surface and the inside diameter of the nut. Again, keep that well away from the bearing area so that when the next step is done, which is irrevocable, you won't ruin the work.

Step 9: Press the nut back onto the shaft.

Picture of Press the nut back onto the shaft.
The nut was placed on the shaft and pressed carefully on in the vise. But not too far. There is no turning back. Notice how the cap is exactly straight in the vise.

Step 10: Set the plate spacing.

Picture of Set the plate spacing.
The vise was closed carefully until the pates were centered as they should be. This was left overnight to set.

Notice the plates are quite evenly spaced. Also note the spring is compressed as well. This is how it should be.

Now leave this alone for 2 days. There is about 6 LBs of pressure on that spring and the epoxy must be completely cured before it has to take the strain.

Step 11: Finishing up

Picture of Finishing up
The capacitor was removed from the vise, checked, and reinstalled in the radio.

Notice the epoxy built up in the recess of the nut, where it has filled the slot of the shaft as well as bonded to the shaft and inner diameter of the nut.

It has held for 4 years now and the old tube radio is on the air on 146.7 MHz.
This was something they didn't teach me in electronics class, thanks!
Dr.Bill6 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