On Thu, 8 Apr 1999 17:38:35 -0700 Rich Measures <measures@vc.net> writes:
>
>
>
>>
>>
>>
>>On Thu, 8 Apr 1999 11:13:44 -0700 Rich Measures <measures@vc.net>
>writes:
>>>
>>ALL SNIPPED
>>
>>>>>
>>>>> This is somewhat of the principle behind a load pull test. You
>>>either
>>>>> short out or open circuit the output of an amp and then rotate it
>
>>>around
>>>>> the Smith chart (via Ctune). Also, the tank inductor generally
>>>provides
>>>>> a large enough reactive impedance to VHF energy that it likely
>>>doesn't
>>>>> matter wether you have a load on or not.
>>>
>>>Ä amen. In parasite-arced bandswitches, damage occurs primarily on
>>>the 10m and 15m taps. .
>>
>>
>>Completely subjective and unproven Rich.
>>
>? the photographs lie?
Anthing taken long after the fact is open to question.
>>
>>Another option which I subscribe to is Peters/Ians version when it
>comes
>>to parasitics.
>>
>>In a real amp that is 100% stable to start with, the switch failures
>are
>>primarily due to the mfg trying to save a few $$ and using a poor
>>initial choice .
>
>? The TL-922 and SB-220 bandswitches have a withstanding potential of
>a
>bit under 6000v at sea level. The peak RF potential in both
>amplifiers
>is around 2900v. (see arced bandswitch photos on my Web site and in
>"Parasitics Revisited", Sept./Oct. 1990 *QST*)
>? If such bandswitch arcing is not vhf-related, why is it common to
>see
>>100% increases in R-supp after bandswitch arcing?
Simple I believe. The voltage is not an issue in the SB-220 but the
current is. I have examined almost 200 bandswitches and amps that have
been run hard on 10/11M show a loss of contact tension. This soon leads
to overheating and eventual failure. When the rotor heats up it then also
transfers heat to the 15M contact. It then becomes a domino effect.
IOTH, those amps that use positive spring loaded contact pressure such as
the JV-9000 series do not exhibit that failure mode.
I have modified a few SB-220's with a second output wafer and wired in
parallel with a good or rebuilt original. These amps are still used in
contests and failures have ceased.
No voodoo suppressors, no parasitics.
>>Before the doubters wind up in a snit I suggest that they take a real
>>look at the specs of the commonly used switches and try to understand
>>exactly what they are spending money on.
>>
>? Better yet, use a high-potential tester and evaluate the
>bandswitches
>yourself.
I already have and in-circuit testing does not agree with your results.
The SB-220 switch in a humid summertime enviroment will breakdown at a
hair over 3500V. In a dirty or nicotine stained amp the breakdown is
around 2800V. Reconnect the Tune cap and the breakdown is below 2500V in
extreme dirty condx.
73 Carl KM1H
>
>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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>
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