[MrFixr55]

+1 Arran. Having worked on an 80 which also uses an autodyne oscillator mixer but there were differences. In the 80, theplate connects directly to the IF Transformer primary and is capacitively coupled between the plate and primary of the oscillator coil. In the 89, the 2 coils were in series. I don't know why that made a difference, but it did, IMHO. The 80 performed marvelously for what it was, and especially for its pricepoint. It continued through at least 3 iterations, 80, 84 and 37-84

In addition to all the issues that Arran raises, the tuning cap can be dirty, having bad connection between rotor and frame, and also oxide "crystals" for lack of a better description touching rotor to stator. The "compensating" caps, both standalone and the ones that were part of the tuning cap were also notoriously bad, but I believe that on sets that came from the factory with the 77 pentode came with "mods" to those parts.