[AMPS] Band Switch arcing SB-1000/AL-80

[measures]

>
>
>> >The protective choke opens when the HV is applied to it,
>> >preventing HV to go all the way to your antenna.
>>
>> Faulty electronic circuit analysis.  .
>
>Yes, it sure was! I was on the tower all day...I should never send
>a post when tired. Apparently, you did not read my correction.

There was none when I wrote the reply. 

>The choke should be more robust than the breaker/fuses. It should
>NEVER fail open when the HV appears across it, the breaker(s)
>or fuses should pop first.
>
>> >It is caused by one of two things....leaking/shorted plate block
>> >capacitors, or some critter or conductive object shorted the
>> >plate block cap out momentarily.
>> >
>> Such capacitors are not damaged by being shorted by a foreign object.
>
>A short across the block cap from ANY source; animal, vegatable, or mineral
>will apply Ep to the choke. How else can it get there? The choke is a long
>way from any HV in the SB-1000.  Measuring the block cap with an ohmmeter
>is an exercise in futility. 

Capacitors with barium titanate dielectric do not heal.  When a short 
occurs, that's all she wrote brother, period, good night, adios. 

> You need your famous hi-potter for this, or at 
>least the voltage the cap is rated at. 

nope.  An ohm-meter tells all.  

>In the SB-1000 et-al, the plate block cap is
>way too small for the job... its not even a door knob.
>
>What is the RF rating on the SB-1000 block cap, Rich?
>
Beats me.  Disc ceramic caps. with wire leads are not current rated.  In 
the real world, a 1000pF, 6kV disc ceramic cap gets pretty hot carrying 
one ampere @ 3.8MHz.  . 

>> >I would change out the plate block cap(s) while you have the thing opened 
up.
>> >
>> If a ceramic plate blocking C does not indicate a short on an ohm-meter,
>> changing it will have no effect whatsoever
>
>It is obvious that the block cap is not permanently shorted. 

I doubt if Cb was ever shorted.  


>He has since fired
>up the amp with the new RFC and it works fine. An ohmmeter is a waste of 
>time.

€  The lowly ohm-meter finds  a short in a barium titanate dielectric 
capacitor. 

>If it is getting hot, it may have intermittant leak...it may not check bad at
>ambient temp.

€  This has not been my experience. 

>The bottom line is...SOMETHING put a voltage on the protective RFC that
>was in excess of its rating. It had to be a foreign object or critter, or 
>the block
>cap. I assume he cleaned out all critters and tools from the area before 
>he fired
>up; so all that is left to suspect is the block cap.
>
>I hope he used a replacement RFC that is more robust than the fuses this 
>time!

€  The load cap. will arc to gnd if you put 3kV on it, thereby limiting 
the potential to c. 20v. 

>We have discussed this before, Rich. HV where it is not expected to be is not
>a good idea.
>
>There was an on-going problem with the band switch on the SB-1000. There are
>two types of SB-1000 owners; those who have had an arc on the switch, and 
>those who will do so if they work enough 80 meters. Ameritron improved the 
>situation by using mica padders instead of door knobs. 

€  Doorknob caps that are intended for coupling and bypass apps. are not 
for use in tuned circuits. 

> It has nothing to do with VHF
>parasitics. It is how the two tank coils are placed on the chassis, and  the
>placement and use of the door knob padders. 

€   bzzzzzzzzzzzzzt

>To get an arc that long on the band switch
>takes a small Tesla coil, and that is what you have here. 

€  please explain

>No other magic involved.

€  the anode circuit resonance is c. 155MHz.  The tube has feedback C.  
Eimac rates the tube for amplifier and oscillator service.  .  There is 
no magik.  
>
later, Phil

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


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