Quote:
Originally Posted by crackle
Hi
I have just found this thread after reading the one posted by Coopzone.
I made a Minimod and found the frequency drifted, after a few hours it was almost off station altogether.
I used mica capacitors for the tuned circuits, could this be the cause, are Polystyrene better.
Or could it be something else causing it to drift.
I even fitted a zener diode in the supply to the VFO.
Mike
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Hi Mike,
They're not particularly known for drift and no one else has mentioned it, although it is possible with a simple L/C oscillator. I found on my prototypes that the frequency only drifted by a couple hundred Hz from 12V to about 4.5V so I didn't bother with VFO voltage stabilizing with such a simple circuit, so I guess the zener made little difference on yours? Drift at normal temperatures didn't seem to be a problem, and the circuit itself doesn't generate any significant heat.
The article is the first link in the following thread:
https://www.vintage-radio.net/forum/...68&postcount=1
The inductor cans should be grounded as should one pin of the unused coil in the VFO inductor - that will help. (See article.) Maybe also just check that L1 is the right way around, with the tap towards the earthy end - it's not centre tapped.
Assuming it is the MiniMod that's drifting, it is most likely to be C1 or C2, and the mica caps you're using here should be fine. But try polystyrene if you have some of the right value. Is the circuit sitting somewhere that's getting warm?
Drifting right off the tuning suggests a change of a few kHz. That seems unusual at normal temperatures.
Ian