[Amps] 4CX1500B help needed...high screen current in HF-8020 PA
TexasRF at aol.com
TexasRF at aol.com
Mon Jan 14 13:33:53 EST 2013
Hi Paul, I have no direct experience with your amplifier model but do have
a couple of general comments about the tetrode tube in use.:
!> 4CX1500B is rated for 12w screen dissipation, Ay 225v, that is 53 mA so
your 40mA reading should pose no problem, especially in a cw or ssb mode.
2> elevated screen current comes with high drive power and/or less plate
loading. If the auto tune circuit is working then you might try reducing
drive power slightly. Sometimes a small change in drive power results in a
surprising change in screen current.
In a message dated 1/14/2013 11:48:53 A.M. Central Standard Time,
radioboy at telus.net writes:
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
about 7 years ago, but can't remember if I had any issues then or if this
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
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
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
put out full power no problem, over 1KW.
Any help would be greatly appreciated.
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