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I would suspect, that the triode in the LO circuit, with its inherent Ca-g capacitance prefers to look at the 3uH at its anode as the perfect candidate for a Colpitts oscillator, ... and here we go.
So you should probably switch to Colpitts from the start.
... But now, if I see right the anode blocking capacitor to be of leadless type soldered directly to ground plane, I take back the explanation.
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I thought exactly the same and these were the first obvious things I checked. I removed the 3uH choke as it was not really needed but it didn't cure the problem. I added double chokes for filament leads etc. All ground solder joins and RF bypass caps were triple checked. The EC81 is a solid UHF oscillator valve. In this chassis, the unstable oscillation stopped at plate voltage 17.5V and it still oscillated strongly and stably at plate voltage of 12V! Once I replaced EC81 by ECC85 without changing anything else, voila problem solved! I still think there is more to do with the weak coupling of the oscillator coil with other coils with the absence of shields ( I tested the oscillator with both E88CC removed.). Some UHF triodes are prone to self-excitation but i have many successfully experience of using EC81 as oscillator in FM homebuilt sets.
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Looking at the spectrum picture in the second photo in post 48, that looks like your oscillator may be squegging, or if it's in a PLL, the loop could be unstable. The spacing of the spurs will tell you the frequency of the instability. The number of harmonics of the self-modulation says that something is hitting the end-stops, or going into complete cutoff.
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These peaks have spacing of about 200kHz. As I said it is the strangest thing i have seen.
I read an article about the use of shorten transmission line in a Hartley oscillator for the Leak Troughline. So I built similar co-axial design but I have not got the chance to test it as I have been busy:
http://44bx.com/leak/troughline.html