38-4 IF frequency alignment help - Printable Version
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38-4 IF frequency alignment help - hffenninger - 04-27-2016
Probably a dumb question but, when aligning the IF frequency of the 38-4, the instructions say to attach the frequency generator to the grid of the 6A8G. Do you disconnect the wire from the grid while doing this? It does not say to do this, but other alignment videos I have seen show that the grid cap is disconnected during IF alignment.
thanks
Frank
RE: 38-4 IF frequency alignment help - Mondial - 04-27-2016
I typically leave the grid cap wire connected when injecting a signal for IF alignment.
If you remove the wire and feed the generator signal through a coupling cap, then the grid is left floating, eliminating its DC bias. If the grid floats without a DC return to ground, it can assume an unstable bias voltage which results in unpredictable performance of the tube and associated circuit.
RE: 38-4 IF frequency alignment help - morzh - 04-27-2016
What Mondial said.
But some times the instructions call for removing the cap.
I simply follow it then assuming the design engineers knew their stuff.
RE: 38-4 IF frequency alignment help - hffenninger - 04-28-2016
Thank you fellas!
RE: 38-4 IF frequency alignment help - John Bartley - 04-28-2016
I do it a bit differently. I inductively couple the signal generator to the radio by draping an antenna wire thru' a coil (random wound on a cardboard tube) connected to the generator. Then I disable the oscillator with a small value cap clipped across the oscillator section on the tuning cap. This way there are no electro-mechanical connections to make between the radio and the generator.
Just a different way ... may not be for everyone.
cheers