I thought restoring the missing 150V to the plate of the 7A4 oscillator would solve my BFO issues, think again Kimosabe!
The Hallicrafters SX-42 uses a 7A4 as a BFO oscillator, both for the 455 kc and the 10.7 mc IF sections.
During alignment of the BFO I can barely hear the tone as I null it with the BFO coil. The 10.7 tone is loud and clear.
I have:
cleaned the mode switch which restored the plate voltage
Checked/replace associated resistors and caps.
Replaced the micas in the BFO coil.
Replaced the entire coil/can with one out of a known good part rig.
No change in behavior throughout.
The BFO oscillator appears to feed into the output of the third IF at the detector. With my scope I can see a nice signal there that varies with the pitch control, both 455 kc and 10.7 signals.
The plate of the plate osc is resistively coupled to the local oscillator. That signal is hard to see and looks weak. I don't have a good enough understanding of how it works to know whether it is ok or not.
So even though I can hear a weak tone during alignment the bfo is as good as absent during normal operation.
Would going back and re-peaking the 455 kc IF have any effect? I doubt it.
Any ideas and/or wisdom would be greatly appreciated.
73
Rich
PS I am certainly not ruling out anything stoooopid like having a switch in the wrong position etc.....