On Wed, 24 Mar 1999 21:39:51 -0600 Marty Tippin <martyt@pobox.com>
writes:
>
>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...
Ive been away for several days so excuse the late reply.
The first thing to check is the output wafer switch contacts, they can
get fried. I have replacements also.
Next check the parasitic suppressors by unwinding the coil. If the R is
more than 10% above 47 Ohms replace them.
Another area to check are the 33 Ohm resistors and 200pf caps at each
grid pin.
Finally, the 10M input coil could be fried and the ferrite core cracked
or damaged.
>
>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.
OK, now that Ive read down to here you can skip a few suggestions!
That bright red leads me to the grid area again so check that all out
carefully. Measure the bias, it should be around -125V in standby and
somewhere around -5V when keyed. A parasitic can blow or crack the grid
caps so do a good visual check. Do the same with the grid bias RFC, make
sure that doesnt look like it has had a meltdown.
Open parasitic resistors could also raise havoc by adding L ahead of the
Tune cap. Does the amp perform 100% on 15M with no instability or strange
meter readings?
GL Carl KM1H
>
>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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>
>
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