>
>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.
>>>>
>>>>? amen. In parasite-arced bandswitches, damage occurs primarily on
>>>>the 10m and 15m taps. .
>>>
>>>
>>>Completely subjective and unproven Rich.
>>>
>>Ä 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 .
>>
>>Ä 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*)
>>Ä 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.
? So it is the loss of contact tension that indirectly causes the
resistance of the suppressor resistors to substantially increase without
showing an outward sign of resistor overheating?
>This soon leads to overheating and eventual failure.
? only open bandswitch contacts arced.
>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.
? The arced AL-82 bandswitch shown in the photo on my Web site is a
JV-9000.
>
>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.
>
? low Q = voodoo?
>
>>>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.
>>>
>>Ä 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.
>
? A spilt cup of tea will also reduce the breakdown V. . . My
measurements were mostly done on unslimed, removed bandswitches that had
vapourized 10m and 15m contacts. Measurements were taken across the
remaining good contacts.
? It's beginning to look like you were right, Dick.
- cheers, Carl
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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