I am helping a friend in putting an old commercial transmitter
back on the air. After checking the wiring and voltages thru
out we can't get the unit to put out any power to speak of. We
put a couple of new tubes in to no avail. The major symptom
is a negative screen current under all conditions of drive
and tuning. I can see some power out but it is about equal to the
driving power. This unit has a cavity in the plate circuit
and is designed to work on the frequency where we are testing it.
Plate current increases with drive as does grid current, but
there is always the negative screen current. The plate does dip
when tuned thru resonance and the output comes up slightly but
nowhere near where it should be. The unit was neutralized
with no problem. One other area that is questionable is the
plate coupling cap to the cavity. The cavity slides down over
the anode of the tube and the space between the tube anode and
the cavity wall which is an aluminum tube is filled with what
looks like Kapton on similar material. This material is wrapped
around the anode of the tube to fill the gap. It is about 3/16
to 1/4 inch thick. I measured the capacity at about 200 pf. I
should mention the operating freq where we are testing is at 88 MHz.
Does anyone have some ideas? The unit is a Collins Radio 830D rated
at 1000 watts.
Thanks for reading all this and 73
Jack W4AD
|