[Amps] 4CX1500B help needed...high screen current in HF-8020 PA

[Paul]

Hi Guys,

 

I have been struggling trying to figure out why I have high screen current
in my Collins HF-8020 1KW PA. It uses the 4cx1500B and is an autotune PA
built like a tank. This one was built in 1990 and I only had it working once
about 7 years ago, but can't remember if I had any issues then or if this is
new.  I just reconnected the whole system and it works fine, although one
day I went to tune it up and I got a fault light for high screen current. I
went thru a rough alignment of standing idle current, set to 300mA, and
checked all voltages and this is what I have

 

Plate 3000V

Screen 225V

Bias -35 keyed/-75 unkeyed

Standing idle 300mA

Keyed full poop .75A

Screen 40mA  and this is what concerns me. The fault light is off now but it
is still way high, I think. Should it be about 15mA?

 

Anyway, any help would be appreciated. The tube is NOS 1983 with very few
hours on it. All relays, contacts, switches and everything I can see
mechanically is spotless inside.  I do have full manuals for this and have
read thru them but holy cow, they read like a science lesson. I'm sure I
could eventually figure it out after weeks of reading and studying...It does
put out full power no problem, over 1KW.

 

Any help would be greatly appreciated.

 

73

Paul

VE7KHz