REPAIR AND RESTORATION OF THE SR-400
22
WDØGOF 6/22/20014
TECH NOTES CONTINUED
5-2 PA NEUTRALIZATION
Proper neutralization will enhance the proper operation, efficiency and life of your final tubes. Theory and
opinions on the effects of interelectrode capacitance are as numerous as the writers of such articles. So to be
very basic, we are attempting to neutralize the effects of the interelectrode capacitance of the PA final tubes.
HERE ARE A FEW SITES THAT HAVE DISCUSSIONS ON NEUTRALIZATION.
http://www.somis.org/
http://www.vias.org/basicradio/basic_radio_28_04.html
http://www.w8ji.com/neutralizing__amplifier.htm
http://www.kk5dr.com/Tuneup.htm
The neutralization process in the book is ok, but not very precise. It will work, but I prefer a more
precise process. There is nothing new or revolutionary about this process. It is a proven process that has
be in use for over 60 years. All I have done is specifically adapt it to the SR-400. Before starting the
process you need to tune the TX as best as you can at 21.3 MHZ.
NOTE
KEEP THE DRIVE LOW AND THE POWER OUT BELOW 30 WATTS INTO A DUMMY LOAD.
After tuning up be careful not to move the PRESELECTOR, load or plate controls throughout the rest of
this process. Power down and remove the top and bottom covers and the P.A. cover.
1, Disconnect the plate voltage at the bottom of L26 be sure the lead is out of harm’s way.
2, A. Cyclone and Cyclone II--Disconnect the screen voltage at the bottom of R104. Be sure the lead is out
of harm’s way.
2, B. For the 400A—Disconnect either end of R301 and put the lead out of harm’s way.
3, Replace the bottom cover and the P.A. cover. Turn the RF LEVEL fully counter clockwise.
4, Connect the transmitter output to the scope or RF voltmeter. An RF sampling ‘T’ should be used
between the TX output and dummy load to maintain a 50 ohm load on the TX output.
5, Turn on the rig and let it heat up for at least 20 minutes.
6, In the tune position key the TX.
7, Advance the RF LEVEL control until you get about 1 vpp on the scope or meter.
8, With a nonmetallic tuning wand adjust C171 (cyclone and cyclone II) or 117 (400A) for a minimum
signal on the scope or meter. Adjust the scope sensitivity and RF LEVEL controls to maintain a good
presentation of the minimum point.
THIS PROCESS OF NEUTRALIZATION HAS SERVED ME WELL. THIS PROCESS CAN BE
ADAPTED TO MOST ANY TRANSMITTER. THIS IS THE MOST PRECISE METHOD OF
NEUTRALIZATION I HAVE FOUND. IF IT DOESN'T WORK THEN YOU HAVE SOMETHING
WRONG WITHIN THE P.A.
