[Boatanchors] Seeking Advice on Colpitts Oscillator Mystery

[Al Klase]

Hi Scott,

Is it possible that you have enough tuning range to get to the second 
harmonic?  That is, you might be tuning the oscillator down to 3525 KC.

Al

whitebear1122 at comcast.net wrote:
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> I am seeing an odd signal on my 80m/40m self oscillating Colpitts oscillator.  It runs a pair of 10's in push pull and I'm getting up to 12 watts output. 
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> On 40m there are two resonances in the tank as I set the receiver to 7050 Kcs and tune the variable cap from min to max I detect two signals, one sounding louder than the other.  I don't have a spectrum analyzer so I can't determine amplitude differences other than what I see and hear on the receiver. 
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> I wondered if a leaky fixed transmitting cap might cause multiple resonances so I changed both 400 pf caps out with different ones.  No change.  I tightened all the connections and made sure the connections were sound.  All tank LC connection between the coil, fixed transmitting caps, variable cap, are made with 1/4" copper tubing so they are beefy.  I replaced each of the 10 tubes with a different one.  I also experimented with reducing the plate voltage from 400v down to 200v as well as adjusting the grid drive.  I even removed one of the tubes and let it run as a single tuber.  Also moved the antenna link coil away and reduced the output power.  And after all of that, that 2nd signal is still there.  
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> So there must be something else that I'm overlooking. 
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> In the Colpitts tank configuration there is a 100 pf split stator variable capacitor across the tank coil.  The center of the two caps is grounded.  I have a 400 pf capacitor in parallel with each section of the split stator.     I measured each of the 400 pf fixed caps with my brand new AADE L/C Meter (wow what a fine meter!) and one is 394 pf and the other is 440 pf, as best I can recall from last night.  Just how close do these need to be?  
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> Can anyone offer any additional insight into this problem of apparent multiple resonances? If the caps are not leaky, then I can't imagine where the other smaller resonance comes from.  The only thing I can think of is something on the drive side.  Maybe those differences in 400 pf fixed cap values. 
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> 73, Scott WA9WFA
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/