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GE paging transmitter 4CX250 question.
Hi folks,
I am responsible for the care and feeding of our GE pager transmitter on 150mhz vhf frequency range. I don't have the model number here as I am at home at the moment. It uses a single 4CX250 in the cavity final. The problem is this, 10 watts output with 10 watts input, Plate voltage is 2100, plate current is very low at 100ma (normal is 270) and the screen voltage is around 60 to 80 volts. The screen is supplied through an 80k resistor from the 2100 volt supply and has variable resistors to ground to set the actual screen operating point. I have placed as much as 500 volts on the screen circuit externally with the tube removed to check the bypass caps and they seem fine. They withstand the 500vdc without breaking down. The tube is good, we tried it in another cavity (our spare amplifier). The plate circuit is good also. At first I thought I had lost a plate choke or connection causing the screen to draw excess current. I know the screen is drawing excess current causing the screen voltage to fall off. The control grid current looks normal and behaves normally with tuning controls. I don't get it?? How can I have excess screen current and yet have the low plate current? The tank circuit seems normal, the tuning controls appear to work properly, in fact every once in a while the unit will work perfect for a minute or two making 250 watts output, then it goes back to acting weird again. Anyone ever have a GE vhf amplifier do this? Thanks!! |
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