Rich Measures wrote:
> At VHF, with a conventional Pi-output, there is No VHF load on the
> amplifier, even if a 50 ohm VHF-rated termination is connected to the
> output.
Years ago I acquired a Yaesu FL2100B amp that had a known problem. Hook it
up,
turn it on, and it sat there quietly, waiting for you to use it. But--the second
you hit it with ANY drive, it instantly went to full saturation, a maximum
condition that would soon melt down the 572B's at least, and the first time it
happened I fell all over myself getting to the AC power switch-that was the ONLY
way to stop the meltdown. It would do this on ANY band, every time. It was as
though there was NO PI- network even attached, it was completely ignoring the
PI.
Looking inside, I very carefully checked over each component and connection. ( I
hadn't heard that much about parasitics at the time). Every component I checked
seemed within specs, though I did not check R of the parasitic suppressors. New
tubes changed nothing. Nothing I could do affected the problem at all. None of
the "standard routine", confirm all mechanical contacts and resolder all grid
connections, make sure it has all low impedance grounds, etc helped at all. I
went over it very closely and could find absolutely nothing amiss. I don't
remember now what all I tried but I was interested in anything that I could
think
of that could make the amp become an oscillator.
I was scratching my head over it when an old buddy Bert Harrison, W6NHW
showed
up. Bert, (now SK, rest his gentle soul) was a retired RF engineer and was of
course very interested, and talked me into, reluctantly, letting him take it
home
with him. When he brought it back, it worked fine, was completely normal, and
was
as docile as a lamb. When I pressed him to tell me what he had done to make it
work, he admitted he wasn't sure, one of his "standard routines" had solved it.
His routines obviously included something mine did not, but he was not able to
tell me what he had done to solve it.
When he finished, it was working. He said he used no additional parts and didn't
change the tubes.
This story is necessarily incomplete, and I will never know what caused that
whopping oscillation. Of course, I was never able to measure its frequency in
any
way since my primary concern, once it was present, was to quickly get the amp
turned off before meltdown.
Anyone like to share a similar problem and it's cure?
73, Dick W0EX
I was scratching my
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|