SB-220 Troubleshooting

[Chuck KN6H]

My SB-220 works very well on 40 and 80 (1200 watts rf out into a Cantenna
dummy load), but on 20 and 15 meters only about 400 watts out. I also noticed
the plates of the two 3-500Z's running cherry red indicating lots of power
being lost as plate dissipation. But only on freqs above 14 MHz, and all into
the Cantenna.

The SB-220 developed this problem when I upgraded my driving transceiver to a
TS-940SAT.  The former TS-830S had no problem - full power out of the linear
on all bands.

Any ideas what the evident incompatibility is with the TS-940?  (It's solid
state whereas the TS-830 uses 6146's in the output).  Could something have
failed within the SB-220? Plugging that -120vdc antenna changeover relay
voltage into the TS-940  scares me (although it seems to work OK), so I plan
to install Harbach's soft key kit to ease up on the TS-940, but I doubt
whether that would have anything to do with the above problem.

Any SB-220 experts out there? Suggestions appreciated.

73, ...Chuck

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