[AMPS] wanted SB 220 bandswitch

[measures]

>
>
>> 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
>
>The failure mode you describe is caused by dirty contacts.
>
My SB-220 bandswitch was clean when I had the open contact arcing 
problem.  When I decreased the Rp (see N7WS' measurements.) of the 
parasitic suppressors, the arcing stopped.   

>Once a contact behaves that way, the plating is pitted. It makes 
>progressively poorer connections, until eventually it has a heat 
>failure.
>
Heat failure is reportedly common in AL-1500s.  The fix is a new switch 
and an added fan.  

>The current is pretty high at HF, as is the voltage. You have both 
>circulating currents and high peaks voltage to deal with in the 
>resonant tank.
>
>The VHF makes for colorful conversation, but isn't very likely from a 
>technical viewpoint.
>
Mr. Rauch has never explained what could suddenly change the value of 
R-supp by a few hundred percent without showing any visable damage.  

end

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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