[HBR] Fixed a nasty problem (non-HBR)

[Ian Wilson]

[OT but relevant I hope to receiver homebrewers].

One or twice a year I dig out my over-ambitious homebrew
dual conversion receiver, based on the 75S3-C. This uses
crystal-controlled downconverters followed by a ~3MHz tunable
IF tuned by a PTO.

Last time I lost patience with the rig I ripped out the HR-10
frontend and for the moment have a 6BZ6 (biased for low gain)
and toroids / dual-gang variable cap for preselector tuning.

This "sort of" worked but the audio was really horrible. This had
to be intermodulation or something in an early stage.

A few days ago I blew the dust off the rig and tried again. This
time I had an inspiration: listen to the output of the 3MHz bandpass
filter with a general coverage receiver. So we just have the RF amp
and first mixer in circuit. Aha! really horrible signals! Nothing to do
with the second mixer, xtal filter, IF strip....

This time around, I have a Tek 7704A scope that can show VHF
signals. It showed a vicious 200MHz squegging oscillation when
looking at the plate of the RF amplifier (through a tiny capacitor).
This disappeared when I removed the connection to the mixer
(at present a 6DJ8 second-generation Pullen). I had forgotten
to add a stubber resistor on the mixer grid. Added that, and
things improved enormously.

I guess I'll keep this project on the workbench for a while longer :)

73, ian K3IMW