Frank Dresser wrote:

"Arthur Pozner" wrote in message
...
Found specs on TA 7358AP; rated from -25 to +75*C . Obviously it is
not the problem. The PVC1 and PVC2 on the other hand are most likely to
cause the terrible drift,especially now, in the hot summer days. Their
internal structure contains polystyrene dielectric,as the tmperature
rises plastics expand as far as I know. This will inrease the
capacitance very minutely, but apparently enough to be seen on the
digital display . One way to tackle this is to add a component with
opposite thermal rating- capacitor or inductor in the oscillator/mixer
circuit.


I know I'm picking nits on this one, but I'll guess the capacitor's
dielectric is polyethylene, not polystyrene. Polystyrene is actually one of
the best plastics as far as temperature coefficient goes. Very good
quality, low temp coefficient, low loss fixed capacitors are made with
polystyrene. Unfortunately, polystyrene is also somewhat brittle and
scratches easily. The plastic in variable capacitors is polyethylene, as
far as I know. Polyethylene is soft and sort of waxy, and forms a decent
bearing material for the capacitor's plates. It also has a higher
coefficient of expansion than polystyrene.

The expansion will reduce the capacitance, rather than raise it. This will
cause the frequency to drift up. It's possible the radio already has
temperature compensation, and the compensation is over aggressive. If a
radio's IF is above the radio's oscillator frequency, and upward drift in
oscillator frequency will cause a downward drift in received frequency. I
have the impression the s-350 is single conversion, 455 kc or so, however.

Inductors show the same effect. Expansion lowers the inductance.

Frank Dresser

There is also the possibility of frequency drift caused by mechanical
instability (binding) in these plastic dielectric tunings cap's. My
RF-2900 used to drift a few Khz after the tuning was changed, especially
when the new frequency was many Mhz from the previous one. It took a
minute or more for it to stabilize again. I improved the mechanical
stability by disassembling the tuning cap (layer by layer) and carefully
reassembling it, to minimize any binding in the rotation.


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