This is sort of a repost of my plea for help from a few days ago -
hopefully I've provided some additional information so that someone will be
able to help me pinpoint my problem. Any help at all would be greatly
appreciated!
I've got an SB-200 and am unable to get it to load up on 10m. It used to
work on 10m, giving me 500w out or so. But something apparently has
changed in the last few months, and I'm having a heck of a time figuring
out what is wrong. Let me say upfront that I don't know much at all about
tube amps, but I can read a schematic and have a good general idea of how
it all works...
The problem is that I see an SWR of > 3:1 on 10m at the exciter, and I
can't get even 100w output from the amp with 50 to 100w input. The internal
tuner in the exciter can't get a better impedandce match at all. I've tried
different lengths of coax, etc to no avail. It appears that the SWR is
fluctuating very rapidly - the SWR meter in the exciter (FT-920) is an LCD
bargraph, and I can see it going back and forth between say 2:1 and 5:1 or
so, very very rapidly.
When the SWR at the exciter is lowest, I see a plate current of about 100
ma and a grid current of 10ma or less. But less than 100w out for 50 watts
in, and the tubes light up bright red in a heartbeat.
I've just installed a brand new matched pair of Svetlana 572Bs, and can
load about 700 watts out on the other bands; the old tubes still loaded up
to around 500w on all bands but 10m. So I'm fairly confident that most of
the tuner works fine.
I've replaced the 10m input coil and the 68pf capacitor on the 10m input
network, to no avail. Also tried a 39pf and 47pf cap. I verified that the
bandswitch is making good contact on the input and output side.
I've also tried disconnecting the capacitor on the input network - that
gives a much better match (around 1.5:1) at the exciter, but I reach a
point, as I increase input power, where the SWR suddenly skyrockets, the
exciter cuts back it's output power to protect itself, the SWR comes back
to normal and this cycle starts over again.
Beyond that, I have no idea where to look to find the problem. It seems
obvious that there's one or more components somewhere that are affected by
10m operation, but I am stumped.
Any suggestions would be greatly appreciated.
73 & thanks in advance.
-Marty KI0LO
martyt@pobox.com
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