[AMPS] parasitics

[km1h@juno.com]

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, 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.  


Did you try and key it without drive? From your description I would
suspect that you would see the same results. 
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. 
You said "no additional parts" . Does that mean the same thing as "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. 


>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.

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.

73  Carl  KM1H

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