>
>> 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.
Sounds like the 10m contacts were previously toasted.
>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.
This is typical. 110MHz energy gets as far as the 10m contacts and
deteriorates rapidly. (see arced bandswitch in "Parasitics Revisited"
Sept/Oct., 1990, QST Magazine. photo on my Web site). The most arced
contacts are those for 10m.
The reason for hot anodes is being out of resonance. With the 10m
contacts vapourized, when the bandswitch is on 10m, the tank coil is
actually resonant near 21MHz.
>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.
My SB-220 delivers c. 400v-pk into 50-ohms (1600w)on 40m voice-SSB. .
>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.
Only an ohm-meter can tell. Unfortunately, this requires unsoldering one
end of
R-supp. .
>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.
Why would the tubes produce only 10w at VHF? They are rated for full
suds at 110MHz in amplifier or oscillator service. The anode-resonant
freq. in a SB-220 is c. 110MHz. //
Contact vapourization occurs whilst the 10m contacts are open. The open
contact withstanding potential is c. 5000v. The anode supply is 2900v.
My guess is that some funny business is at work.
>
"Everything is more complicated than it looks". -- Murphy
cheers, Ian
Mr. Carl Heuther, KM1H, has SB-220 bandswitches for sale. [adr shown in
recipients]. Free, unsolicited advice: In my 220, intermittent
bandswitch arcing stopped when I decreased the Rp of the vhf parasitic
suppressors.
end
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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