08-08-2014, 01:11 PM
I am not familiar with that particular set but drift in the LO is almost certain to be where the trouble lies. Since it uses an ECH81, presumably it's safe to assume that it's the triode part of that valve which comprises the LO. Your post suggests that the drift is on MW only. If that is indeed the case, then my suspicion would first fall on the mica or ceramic capacitors that determine the LO freq. which are 'in circuit' on MW only. However, if it is not only MW that is affected, but all bands, my first approach would then be to change the ECH81. It's also worth checking the heater voltage of that valve at first switch-on and then later, when the freq. drift has stabilized: a small change in heater voltage can sometimes cause significant changes in osc. freq.
Al.
Al.
