km1h@juno.com wrote:
> On Tue, 21 Apr 1998 10:04:15 -0500 Richard Carroll
> <w0ex@scan.missouri.org> >> 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,
> Did you try and key it without drive?
Yes. Key the mic without audio, nothing, The instant that you spoke the
first syllable, instant plate saturation, plate current meter to the peg...
> .
> It does not sound like a parasitic to me but instead a run away bias
> problem possibly due to a bad relay contact, loose ground lug or
> something equally mundane.
Which was exactly what I expected to find, and was nonplussed when a pretty
thorough search turned up nothing.
> You said "no additional parts" . Does that mean the same thing as "no
> parts were replaced" ?
No parts were replaced.
> The FL2100B is a known reliable workhorse and the ONLY time that
> instability shows up is when you use Chinese or Russian 572B's. A simple
> bias mod takes care of that problem.
>
The amp had Cetrons in it...and it was the only FL2100B out of several I have
owned that exihibited any sign of instability..
> Sometimes it takes a 3rd party to spot the obvious...happens to all of
> us. Sometimes even an Ohmeter check will not show up a problem that only
> appears with voltage involved. Ground lugs and relay contacts are a prime
> culprit.
Clearly Bert tried something that I overlooked, and since he didn't check the
amp after each "trick" he didn't know which one of them fixed it.
I can't say with certainty that it was an oscillation, but I can't see what
else could have caused such a severe case of drive-induced no-holds barred,
destructive saturation as was present here. I Won't mind at all if I never
see that again.
73, Dick W0EX
>
>
> 73 Carl KM1H
>
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