Installation of the Collins R-390A BFO and remarks on the IF module

A remark concerning the correct installation of Collins R-390A beat frequency oscillator in the IF module. There is a compressible sleeve that pulls out the rotating shaft of the coil core. The two extreme positions shown in pictures below.

There is no spring inside the BFO that would pull the shaft backlash to one end. The sleeve will have to be slightly pretensioned (see the top picture) when the shaft is fully out in order the oscillator tuning to work. Otherwise, it is completely crazy and gives  pretty much a random beat frequency.

To understand this, I had to open the hermetically sealed oscillator covering, take apart some components and think. I thought there was something wrong with the unit. However, it only had been installed incorrectly by some previous owner, probably only using the radio for AM reception.

Another point requiring carefulness in the reassembly of the IF unit is the shaft of the filter change switch. It is easy to align the shaft so that the contacts do not always engage properly. This leads to intermittent faults that may appear as if one of the expensive mechanical filters is nonfunctional.

So as to the alignment of the IF module, instructions by Chuck Ripple [1] seem most well thought out. Please observe that that there seems to be some confusion in the part concerning mechanical filter alignment. Either that, or my unit has some parts in different positions. However, the right top and side trimmers for each filter can be found by some experimentation.