AM Tuning Drift in Telefunken Jubilate 1260

[Old Sparky]

Several years ago I comprehensively repaired one of the above 1950s table radios, including replacing both the mains and output transformers, along with the rectifier, reservoir & smoothing capacitors,and all the waxed paper(or similar)types. In te last few days I've replaced the resistor I fitted in series with the 4 x 1N4007 bridge rectifier assembly I fitted. The radio is working well, but it takes the best part of an hour to 'settle down' on MW - i.e., if I tune it to a strong local station (BBC Gloucester on 1413kHz), the tuning will very slowly drift a few kHz LF of the original setting, after which it remains stable, giving as good aquality of reception as is possible on MW.
This is a conventional valve radio, using an ECH81 F/C and internal ferrite rod aerial, and voltage readings are pretty much in agreement with the service sheet. Any suggestions as to what may be causing the problem?

[Skywave]

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.