My SB-200 won't load up on 10m at all anymore... Used to be I could get
around 500 watts out but something has apparently changed. It also used to
show a SWR of around 2:1 to my FT-920 exciter which was high but workable
with the ATU.
Now I can't get more than 50 watts out of the amp, even for 100w of drive,
and the SWR is through the roof on 10m ( > 3:1). The tubes light up cherry
red in a hurry too - plate current never goes about 100 mils or so; don't
know about grid current. With the auto tuner in my exciter, I can get the
SWR down to around 1.5:1 or so on 10m but it makes no difference on output -
plus it looks like the SWR is oscillating violently between 1.5:1 and 3:1 or
so - the leds on the indicator flash wildly between those two points.
I just replaced the 10m input coil (old one had a badly mangled ferrite -
cracked in several places, thanks to the previous owner) in hopes of curing
the problem. No dice.
I can load up just fine on the other bands - about 700w out on 20m, 500 or
so on 15 and 40m; 75m has always been a little strange but I can get about
500w out there. 10m used to work about 6 months ago and nothing significant
that I can think of has changed.
I'm going to order a new set of Svetlana 572Bs just for good measure (I have
no idea about the history of the tubes currently in the amp) - but I don't
expect that to solve the problem either. I'm thinking maybe the capacitor
on the input circuit is flaky
Suggestions please on where to look next?
Thanks & 73,
-Marty KI0LO
martyt@pobox.com
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