On Tue, 21 Apr 1998 13:44:44 -0500 Richard Carroll
<w0ex@scan.missouri.org> writes:
>
>
>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
That is a real head scratcher Dick. Your initial idea of a missing tank
circuit may be the reason after all. Maybe just a RF crystallized solder
joint; touching plates in the load cap, carbon arc on the switch, etc.
Guess we will never know for sure!
73 Carl KM1H
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