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Old 17th January 2007, 03:08 AM   #1
2litre is offline 2litre  United States
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Default Air gap capacitor

Hi All,

Not sure if this question should be here or somewhere else but here goes.

On all my old tube radios (got a couple of simple AM5 AM units and a pair of Maggie console pull AM/FM/Multiplex) there is only one air gap capacitor on each unit. Now the AM units I understand- one band, one air gap capacitor. It's the multiband units I'm wondering about.

-What sets the band (AM or FM) and bandwidth (like FM's 87MHz to 108MHz) of that single air gap capacitor?

Reason I ask is because today I saw a beautiful Motorola console with 2 seperate and independantly operating dials for each band, one for AM and one for FM. That leads me to believe that this tube tuner has 2 air gap capacitors on its chassis instead of the one associated with the big, pulley operated slide bar.

Thanks,
Jim
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Old 17th January 2007, 05:46 AM   #2
lndm is offline lndm  Australia
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If memory serves me, some air variable capacitors have AM and FM caps on one spindle. The AM cap takes up most of the space, the FM section mayn use perhaps only a few of the fins. A dedicated FM air variable capacitor can be roughly as long as an AM cap but much smaller in diameter.
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Old 17th January 2007, 12:31 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the range switching is sometimes done with tappings off the aerial.

Change the L of LC and you change the frequency.
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Old 17th January 2007, 03:13 PM   #4
2litre is offline 2litre  United States
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This was obviously a question posed immediately after observation and without reading.

Thanks for the replies guys.
I'll have to dig a bit on this one.

R/

Jim
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Old 18th January 2007, 02:09 AM   #5
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www.electronics-tutorials.com/receivers/am-radio-receivers3.htm

Maybe this may shed some light on superhetrodyne AM/FM.
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