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![]() "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 |
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