[AMRadio] neutrailzing


W5OMR/Geoff w5omr at satx.rr.com
Fri Oct 21 13:02:52 EDT 2005


Seems I have issues, still. 

Personal issues notwithstanding, I'm talking about the transmitter ;-)

John/WA5BXO was here before and his diagnosis is that the final tank 
coil is too close to the grid tank coil, and there's some capacitive 
interaction between the two. 

After reading Don/K4KYV's method of using a sensitive wattmeter on the 
output of the final, I thought "hmmm... this 20MHz scope will take 40vpk 
- surely I can drop the output of the Viking II (used as an RF only 
exciter) to get a reading, and then start from there", and that's what I 
did.   I disconnected the plate B+ supply to the final, disconnected the 
AC input voltage to the final power supply (don't want that 'snake' 
whipping about and striking me, with a couple of thosands volts of DC in 
it's fangs!) applied grid drive to the final, and reduced the drive from 
the Viking II exciter as much as possible.  Getting a reading wasn't a 
problem.  Neutralizing worked 'ok' also, except.... the stray 
capacitance my hand created when near the final, would change the scope 
reading.

I said "Self (said I), what you need is a long, non-inductive 
screwdriver to adjust that final with."  Failing to actually posses one 
of these critters, I decided to -make- one.  Glanced around the shack 
for something that would be suitable, and found one of those fiberglass 
rods, like what you find at Home Depot for driveway/curb marking.  
Checked it real quick, but flopping on the exciter again, watched the 
scope, and stuck the fiberglass rod in the area of the final tank.  
Absolutley -no- change in scope readings.  PERFECT!  Now, to find a way 
to make a 'slot' screwdriver on the other end. 

Enter into the picture the World's Famous, Harbour Freight $9.95 dremel 
tool kit knockoff.  I slid a grinding stone in the end of that little 
dude, and grabbed the end of my fiberglass rod, and proceed to make me 
an insulated, l-o-n-g screwdriver, for nuetrailzing my final.  I decided 
that 36" was acutally too long, so I cut the thing in half.  A Foot and 
a half long is far enough away from the final, as to not affect the 
tuning of it.

THAT worked beautifully.  Got the output from the tank with now 
'up-to-normal' exciter drive to show much less RF leakage through the 
final and on to the scope than when I started  (well, it's down as far 
as I can get it, anyhow) but, when all voltages are applied again, the 
grid meter doesn't 'peak' where the plate meter 'dips' (resonance)  The 
maximum output power and minimum plate current used to be around 100mA 
apart, now they're around 20mA apart and I can't get it any closer.

Ideas?  Suggestions?

Here's a picture of the topside of the RF deck..  the Tank coil is obvious.
http://w5omr.shacknet.nu:81/~w5omr/hamstuff/AM-Stuff/Titanic/Aug17519.JPG

And, the bottom, where the grid coil sits
http://w5omr.shacknet.nu:81/~w5omr/hamstuff/AM-Stuff/Titanic/titanic3.jpg

It's a steel Chassis.  I wouldn't think that there'd be interaction from 
the grid to the final, but apparently there's enough stray capacitance 
to make things kinda haywire.

Perhaps, moving the grid wires where the 'cross' back underneat the 
chassis, and only allow them to come up at the grid pins of the 250TH's?

Thanks in advance.

--
73 = Best Regards,
-Geoff/W5OMR





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