[Boatanchors] WANDERING 6K8 OSCILLATOR

[Bill and Liz]

A couple of weeks ago I posted about the 6K8 mixer stage in a homebrew receiver which was wandering up and down in frequency randomly when listening to CW or SSB.  The problem is now cured; after trying all the obvious fixes (looking for bad solder joints, bad resistors, voltages which were off etc) I did the following:

1-I placed the filaments of the 6K8 and the 6J5 BFO tube in series and connected them to 12V DC regulated-this almost did it.

2-I installed a 105V zener for the 6K8 screen and the 6J5 plate-this reduced the wandering to almost zero-I guess the VR tube was not holding the voltage closely enough to deal with our constantly changing AC line voltage.

3-After reading every HBR receiver article I could find looking for ideas I found some notes by the late Ted Crosby indicating that coils which are coupled too closely can react in the manner I was experiencing.  I made some slight adjustments in the spacing between primary and secondary of the oscillator and mixer coils-this did not make any noticeable difference but at least now I know that the coils are correct and winding spacing should not cause any problem in the future.

Thanks again to all who provided input on the problem.  All I need to do now is find some low value NPO, N500-1500 and P500-1500 caps so I can experiment with further reducing the warmup drift (which is not really all that bad).

Bill VE3NH