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[AMPS] Re: FL-2100-Z bandswitch arcing

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Subject: [AMPS] Re: FL-2100-Z bandswitch arcing
From: grimm@alison.sbc.edu (Kenneth D. Grimm)
Date: Thu, 17 Apr 1997 20:59:08 -0400
Someone whose name I have lost track of asked:
 
> >Wonder if anyone can provide some advise regarding band switch arcing on
> >a FL2100-Z amplifier?
> >
> >Twice now the band switch has arced the contacts closed for the 160 and
> >80 metre positions.  Don't know how it happend the first time but the
> >second time the circumstances were after repairing the switch and tuning
> >up into a dummy load with very little drive and lots of caution.

Rich opined:
>   At one time, Yaesu factory service said that the problem was due to a
> VHF parasitic oscillation.  For a while,  Yaesu had a
> factory-modification to "fix" the problem.  However, it didn't really
> work and they dropped the mod..
>  IMO, the intermittent bndswitch arcing is VHF parasitic-related.  In
> other FL-2100s, reducing the VHF voltage gain of the 572Bs by installing
> lower VHF-Rp suppressors appears to have fixed the arcing problem.  See
> 'Parasitics Revisited' in the 9/90 and 10/90 issues of QST magazine for
> more info.

Rich may well be right, but I will hazard a guess that the arcing has
nothing at all to do with parasitics, but rather with the timing of the
antenna change-over relay.  If you hadn't said that you experienced the
arcing at low drive levels I would be much more sure of myself, but
still I still guess that the output relay is simply too slow.  Once
drive is applied to the amp, the circuit produces power into a tank
circuit which is connected to nothing and which only slowly changes to
the output load.  Until the load is connected to the tank, all kinds of
nasty things might occur.  

Jack, WB8BFS, solved the TL-922 bandswitch arcing problem by changing
the timing of the relay and I would think the FL-2100, an even older amp
than the TL-922, probably has an even larger, clunkier, and slower,
antenna change-over relay which is probably causing the problem.  The
proper time-sequencing of input and output relays is one of the many
important things an amp builder should pay attention to.  

Solutions?  If it is an open-frame DPDT relay, one side switching the
input circuit and the other side switching the output circuit, you may
be able to very carefully bend the leaves of the relay to allow the
output side to make contact before the input side does.  If it is a pair
of SPDT relays then you will probably need to replace the output relay
with a faster type...A Jennings vaccuum relay will cost a few bucks, but
will undoubtedly be faster than whatever you have in the input.  These
can usually be found at hamfests for $30-$50 or less if you are lucky.

Good luck with your trouble shooting.  

Ken K4XL
grimm@alison.sbc.edu 

p.s.
I really hate relays!  They can be a real pain in the output tank!  My
feelings about them are so strong that I removed the miserable little
change-over relay from my TS-940 and replaced it with a MOSFET... which
is pretty fast...and pretty quiet.  Makes break-in CW a pleasure.


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