[Central-Electronics] 100V RF phase/Null adjustment touchy

[Jim Candela]

Jon,

  I never had a 100/200v, but I do own 20a's. On one unit I took the carrier 
null pots and replaced them with a lower value with series resistors on each 
end to get the same resistance as before. Since the original pot was not 
perfectly at the center, I had an offset on the fixed resistors such that 
the new pot was close to center. The result was a smooth carrier null 
without the need to stare at it in order to keep the null at optimum..
   On another 20a, someone installed wire wound 10 turn pots. That was a 
debate on this forum back several years ago, using WW pots in a 9 Mhz phase 
shifter. I had other troubles so I took the WW pots out. Those WW pots did a 
fine job though, and my removing them was because I was stumped. The actual 
problem was the two 9 mhz coils were too far spaced away from each other. 
Not my first guess..

Jim
WD5JKO

----- Original Message ----- 
 From: "k6jek" <k6jek at comcast.net>
To: <central-electronics at mailman.qth.net>
Sent: Sunday, March 21, 2010 7:44 PM
Subject: [Central-Electronics] 100V RF phase/Null adjustment touchy


> I've been fooling with the 100V.  The carrier suppression was lousy.  The 
> usual parts are relatively new and closely matched so I figured what the 
> heck, try the RF phase adjustments.
>
> Using a FLex radio as a spectrum analyzer, with the mod caps out to keep 
> audio from confusing the issue, I adjust those two RF transformers, tweak 
> this one, adjust null pots, tweak that one, adjust null pots.  I was able 
> to make a huge improvement, from a miserable -20 dB to -60 dB.  Pretty 
> good, huh?
>
> Except the way this thing is adjusted, the null pot adjustments on the 
> front are super touchy.  The slightest nudge of the knob and I mean 
> miniscule nudge and I lose 10 dB of carrier suppression.
>
> What have I done wrong, and what should I do to fix it?
>
> Jon
>
> ______________________________________________________________
> Central-Electronics mailing list
> Home: http://mailman.qth.net/mailman/listinfo/central-electronics
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Central-Electronics at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html