If you’ve seen a big air-variable capacitor, you may have noticed that some of the plates may have slots cut into them. Why? [Mr Carlson] has the answer in the video below. The short answer: you can bend the tabs formed by the slots to increase or decrease the capacitance by tiny amounts for the purpose of tuning.
For example, if you have a radio receiver with a dial, you can adjust the capacitor to make certain spots on the dial have an exact frequency. Obviously, you can only adjust in bands depending on how many slots are in the capacitor. Sometimes the adjustments aren’t setting the oscillator’s frequency. For example, the Delco radio he shows uses the capacitor to peak the tuning at the specified frequency.
You usually only find the slots on the end plates and, as you can see in the video, not all capacitors have the slots. Of course, bending the plates with or without slots will make things change. Just don’t bend enough to short to an adjacent plate or the fixed plates when the capacitor meshes.
Of course, not all variable capacitors have this same design. We’ve seen a lot of strange set ups.
It is nice to see videos like this.
Part of the (nearly) lost skillset tools, and techniques. .
Mr Carlson makes it look easy.
I have done this as part of the rebuild and alignment of the TRW built Heathkit LMO modules (a 5.5-5.0MHZ VFO. I built a test jig with an external power supply and a spare dial assembly for Heath SB series gear. After getting the end points established, frequency error is recorded every 50KHZ. Now you have the first set of numbers from which you will begin blade tracking, in for lower, out for higher. To get down to less than 200HZ error over 500KHZ takes several tries. This was an after school project when I was a senior in high school (1974) It took 8 days working an hour a day. When I finished for the day the LMO was covered in shop rags completely and left powered on until the project was finished.
Thanks HAD for the memories…..
I agree, great memories! When I was in high school in the 80s I too spent hours and hours tuning and fixing tube ham equipment with variable air caps. I could never afford anything new so I would often see the previous owner’s attempts to bend the plates which is how I realized what the slots were for! (I was lucky to get 200Hz error, mostly I just stayed a few kHZ away from band edges.)
As a person not knowing much but enjoying your video presentation, I would like to ask a childlike possible solution to the problem. Since the dial pointer was off alignment by about an eighth of an inch, could you have adjusted the string loop of the pointer to mechanically correct the alignment? I understand such a solution would defeat the point of the electronic lesson. But is it a solution?