The bandswitch in my (ex) SB220 is apparently still alive and healthy
after 23 Years of heavy service and several "bad" experiments.
It survived contest after contest, and also an external power supply
with 4KV DC (full bridge), and a 160m modification including a plate
vacuum variable that doesn't arc, and the replacement of the two 3/500Z
with a single ceramic tube delivering more power, and not counting the
many antenna "accidents" or those happened meanwhile modifying.
By the way, the cooling is forced and no air stream reaches the switch
since 1987.
73,
Mauri I4JMY
> ---------- Initial message -----------
>
> From : owner-amps@contesting.com
> To : amps@contesting.com
> Cc :
> Date : Tue, 26 Sep 2000 11:31:49 +0200
> Subject : Re: [AMPS] wanted SB 220 bandswitch
>
>
> > 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
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