Topband: MFJ 1026 Phase Adjustment

[W2pm@aol.com]

In a message dated 1/26/2006 8:49:09 AM Eastern Standard Time,  
richroad at telusplanet.net writes:

I have  had some success using this device for cancelling noise, but often 
run into a  problem where I run out of phase adjustment. If I change the noise 
antenna, I  don't get enough noise signal. On W8JI's website he talks about a 
mod to add a  capacitor from the bottom of R16 to ground. Has anyone tried 
this, and if so:  Did it work, and what size of capacitor did you use? Tom says it 
is so long  since he looked at the 1026 he can't remember the  details.


I know two people who did the mods and they resulted in significant  
improvements.  I have been able to make some repeatable observations of  MFJ1025/26 
usage and note the following which may be helpful to some with issues  with this 
box.
 
NOISE LEVEL:
 
IF your local crud noise is very strong from the main antenna - S9  plus 
category  (as is nearly the case all the time in suburbia) - your  noise antenna 
MUST product at least S7 level but usually higher.  It  depends on the noise 
but in the cases Ive been able to repeat I have a Inv L 30  feet parallel to a 
24KV powerline 200 ft out back!  So you can imagine the  crud (many times the 
line is quiet believe it or not < S3 noise).   Typically the noise antenna may 
not give you that signal level and the MFJ unit  without Tom's mod's has 
considerable noise antenna attenuation.  In my case  I use a tunable preamp for the 
noise antennas (MFJ959C) and crank the noise  level to S9.
 
The noise level can be quite critical too else you will only get a null  with 
very VERY low main antenna level settings on the MFJ and the phase adjust  
ment will be super critical. It will result in dramatic, near perfect null, but  
the resultant S/N is unusable due to low main signal.
 
You will have to play with actual noise levels but when these very loud  
local crud conditions are present, I find the optimum setting still results in  
the MFJ unit not being able to set the main antenna level beyond the 10 o'clock  
position of the knob - which is less than half total level. Nonetheless the  
results are nearly always quite usable and the null near perfect.
 
When noise levels are under S6 or so (on my FT1000 and TS850) the nulling  
action usually doesn't require preamplification of the noise.
 
OTHER NOTES:
 
IF you manually reverse the main input with the aux input (DONT  
TRANSMIT!!!!) you can see the effect of one of the mods on 8JI's site. It works  very well 
on Topband but is unnecessary on higher  bands.