Help with SB-200 input circuit

[Jim Mullen]

Hi and thanks for reading this.

I have an SB-200 that will not present a good load on 10 and 15 meters. The
input coils seem to have no effect when tuning them. Checking the setup
with a dip meter, reveals good, in-band dips on 80, 40 and 20, but a real
strong dip on about 28 and 35 MHz when the band switch is set to 15 or 10
meters.

I pulled the entire coil & switch assembly out of the amp, just sitting on
the bench, I get a very good dip meter dip at 28 MHz.

Am I chasing my tail here, because the circuit stinks, or am I missing the
obvious? Beats me!  My best guess at this time is the whole assembly is
self-resonant at around 28 MHz, and is swamping out the tuning ability of
the individual 10 and 15 meter circuits.

The switch and all contacts look fine, coils are in good shape, capacitors
test OK, and are the right value. Coils and switch are wired correctly.

I'm driving the amp with a Kenwood TS-440. Amp tubes aren't the greatest,
but shows about 500W key down on 80, 40 and 20.

Any help or ideas would be greatly appreciated.

Jim Mullen - WA1ZUH
144 Tower HIll Road
Brimfield, MA  01010
(413-245-3228