[Amps] SB 200 Puzzler

[Phil Clements]

>. It appears to me that It's picked up some
> added capacity on 40 and 75
> meter positions and thats the reason the plate tuning capacitor is tuning to
> less capacity ( 20 mtr.area ) for resonance in the 40 and 75 meter bands. I've
> checked about everthing, I've cleaned (ink eraser) and completely checked the
> band switch (continunity), removed and checked the values of the plate
> blocking cap C24, 1000uuf, C26, 100uufd  parallel cap, and the 500uufd 
> switched in
> loading cap for 75 mtrs. All were within 10% on my cap. meter. I lifted the 
> 1.1
> mhy. RF choke and output lead to check the loading cap. tested perfect. With
> the plate tuning cap fully meshed ( max. capacity) attached to the total turns
> of the coil assy. the capacity reading was 215 uufd, with the plates fully
> open the reading was  65 uufd verifying that the plate tuning cap was at it's
> proper value of 150 uufd. My questions are ---
> 1. Is 65 uufd a reasonable amount of stray capacitance for just the final
> coil assy ?
> 2. Does anyone know if there is a minimum capacitance  value for the plate
> tuning cap ?
> 3. Any suggestions / help would be much appreciated.


If you are sure it is not the padder caps changing value when power is applied, 
I would check for shorted turns on the plate choke, and its bypass cap. It is 
hard to trouble-shoot via E-Mail! While you are at it, unsolder one end of each 
resistor on the parasitic suppressors, and make sure they have not changed 
value. I do this at annual spring-cleaning on my amps religiously.

GL!
Phil, K5PC