On Sun, 18 Dec 2011 00:23:24 +0000, Ken Fowler wrote:



The reason that it suggests to inject the 467 KHz IF signal into the
mixer grid is that the tube will serve as a capacitive coupling for the
signal into the IF amplifier. If you try to inject at the mixer plate,
you have to worry about the high voltage at that point.
73. Ken, KO6NO

A better reason to inject the signal at the grid is that if you couple a
signal generator to the plate, it will detune the IF transformer and
prevent getting an accurate alignment. Using grid injection insures that
the plate circuit is unchanged from the way it will be in normal
operation.

The general rule is that connecting test equipment ALWAYS disturbs a
circuit. You want to connect it in such a way that the disturbance can
be ignored.

A burned out resistor could be the result of a bad tube but more likely
would be caused by a shorted bypass capacitor. Since the resistor
decreased in value, perhaps it applied enough current to the capacitor to
cause it to burn open (metalized type capacitors are designed to do
this), which is why you don't see a short now. It could also be the
result of a loose wire touching something, a stray solder blob from a
previous repair job, or anything like this.

Good luck,

--
Jim Mueller

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