> In my case the switch was damaged by accidently rotating it while diping >
> the plate. Wrong knob...
This is probably the no 1 failure mode in SB220s, apart from natural old
auge in the bandswitch. I have on occasion also fingered the wrong
switch, but fortunately not being a contest op, twigged the difference
while the pulse rate was nominal rather than 170 ppm.
For those interested in amp failure modes, here are the indicators:
The SB 220 made minor, occasional "pfffst" noises which I ignored, as
they were seen to be at the joint between the body of the tuning cap and
the stator, easy to see at 400 W RTTY on 10m. In fact the previous owner
had also seen this and attempted to improve the DC contact at that joint
by soldering the pinch stock securely to the body of the tune cap, but
of course the friction slide on the stator remained "loose", and was
left alone as a safety device!.
The failure mode of the bandswitch was interesting - it was accompanied
only by lack of RF output on 10m, with nice hot anodes. No sounds. DC
input power and drive remained normal and simply unkeying and changing
bands several times used to clear the problem (but not any more!)
This particular rig is old, apart from changed HT caps and rectifiers is
stock and produces 1300 W at 7 MHz with 100 W drive and a 220 v AC
supply (2800 V on the anodes) so would surely produce around 1500 W at
240 V mains supply, indicating high gain. The tubes are 1971 EIMACS
which replaced the first set in recent years. I haven't bothered to
check the state of the suppressors, except to confirm that they are
still in place, and they are the originals. Visually they look OK.
I'm currently using the amp as a monobander by jumping the 10m output
contact on the bandswitch, seems to work OK like this.
Regarding this business of legitimate bandswitch failures, methinks the
bandswitch has such a high SWR at even moderate VHF power levels (say 10
W - 21 dB down on carrier level), that the contacts heat up,
loosen/corrode, and fail as the years go by. Nothing to do with
explosive VHF power levels, etc.
For what it's worth,
Ian ZS6BTE
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