Info on variable gap condenser/capacitor

ErnieM

Joined Apr 24, 2011
8,006
It's the simple basic physics that makes these things work.

For you youngsters who never saw an analog tuned radio front end the tuning cap was a odd number series of half circles. As you turned the knob the middle pieces would rotate in and out of the stack. Many pairs were used to work in parallel and get more capacitance (or were they in series to get less? I never checked lol.)
 

crutschow

Joined Mar 14, 2008
23,494
It's the simple basic physics that makes these things work.

For you youngsters who never saw an analog tuned radio front end the tuning cap was a odd number series of half circles. As you turned the knob the middle pieces would rotate in and out of the stack. Many pairs were used to work in parallel and get more capacitance (or were they in series to get less? I never checked lol.)
Yes, I remember looking inside our old AM radio when I was a kid and observing those variable caps move as I tuned the radio. (Those old radios had the back open so you could easily observe the innards.) It was turned by a heavy string wrapped around a pulley which was wound around the tuning know shaft and also moved the dial indicator. I remember the string broke once and we had to take it to the repair shop to have a new string installed.

Here's an example of one of the caps. This one has two separate caps. One tunes the RF input and the other tunes the IF mixer oscillator frequency. All the fins are in parallel.
 

BillB3857

Joined Feb 28, 2009
2,501
Many of these caps in old radios also had "trimmer caps" attached. These were rectangular in shape with plates separated with mica and adjusted by a screw. This was used to help adjust the local oscillator and receiving RF section to produce a more accurate IF.
 
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