Be aware Jack, the anode current dip might be very steep with the cavity,
due to its very high Q.
If you dont have a flawless dip, possibly means a severe mismatch of the
output impedance. ( or input impedance )
Are you sure about the frequency 88 Mc/s ?
At that frequency we used a lot of cavity bandpass filters, but I only
rember using cavity's on frequencies above 150 Mc/s as anode tuning.
Quite a time I didn't handle this stuff, so if anything wrong, give me
shout.
Jos on4kj 68 yy ob
----- Original Message -----
From: "Jack O'Mara" <jomara@erols.com>
To: <amps@contesting.com>
Sent: Friday, October 25, 2002 1:32 AM
Subject: [Amps] 4CX1000A FM commercial Transmitter Problem
>
> 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
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