6 things to recommend, Jack:
1) Could the output connection to antenna or load be loose or broken,
i.e., the tube is running unloaded? In this case, the screen current
would be wacko, but would also trip overloads or fuses if driven very
much. Screen current in the 4CX1000A can be negative, or positive,
depending on the operating point. Yours sounds too much negative
though. Check the metering too, to make sure something isn't open in
the shunt.
2) Is that rig one with grounded screen, and the screen v is applied
to the cathode? Collins/Continental did that in their FM rigs to get
the most stability with tetrode. However, it does make
troubleshooting deadly, as there is screen v under the socket on the
filament leads! watch out!
3) If #2 is true, could there be a breakdown in the socket, screen
voltage being shorted? I would expect blown fuse by now.
4) The anode blocker sounds fine. A little bit thick in my
estimation, would 200 pF is fine.
5) Post your question to the broadcasters discussion group at
www.broadcast.net. I cannot get into it this minute, but you want to
get to the technical discussion group, where the engineers chat about
things like who came up with the first Doherty linear amplifier style
transmitter. (answer : Doherty at AT&T)
6) contact Continental Electronics in Dallas, J. Fred Riley is the
man to ask for. (800) 733-5011. They continue to make the Collins FM
transmitters.
>From: "Jack O'Mara" <jomara@erols.com>
>To: amps@contesting.com
>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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