08-22-2015, 02:43 PM
I don't think the drift is normal, as the oscillator circuit is basically the same as many other Philco's of the 1940's. I could not find a schematic for the 48-206, but looked at the 47-204 which Ron says is similar.
The first thing I would try is a new 7A8 mixer-osc tube. If the tube is gassy, its characteristics could change as it warms up, affecting the osc frequency.
There is not much else to the osc circuit, just the osc coil and the tuning cap. The only other part I see is the 47 pF coupling cap to the 7A8 osc grid which you might change. You could also try hitting these components with a bit of freeze spray and see if the tuning changes.
The first thing I would try is a new 7A8 mixer-osc tube. If the tube is gassy, its characteristics could change as it warms up, affecting the osc frequency.
There is not much else to the osc circuit, just the osc coil and the tuning cap. The only other part I see is the 47 pF coupling cap to the 7A8 osc grid which you might change. You could also try hitting these components with a bit of freeze spray and see if the tuning changes.
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