The above URL contained the following text regarding the
neutralization of the DX-100B.
The URL was valid as of April 2004.
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Cc: "boatanchors -------------------
From: "JAMES T HANLON" ‹firstname.lastname@example.org›
Subject: RE: DX-100 Loading Cap Question
Date: Fri, 30 May 2003 16:06:41 -0600
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There was a modification to the loading capacitors on the DX100
suggested by Lloyd Jones, W6DOB in CQ, October 1956, pages 34 and 35.
Lloyd had the original fixed capacitor array, and he could not properly
load a 50 ohm line on 20, 15 and 10 meters. He also said that
"the small mica condenser and associated wiring ran extremely hot.
This probably was caused by the condenser and associated wiring being
near self resonance at these high frequencies." His fix as near as I can
read it on my copy is described as follows.
"I removed the 400-600-800 uufd mica, the 200 uufd mica condensers
and the original 250 uufd variable condenser from the pi-output circuits.
A new three gang broadcast-type condenser replaced the original 250 uufd
variable. The three sections were connected together in parallel to give a
total of 1200 uufd and a minimum of about 35 uufd. It was easy to mount
this condenser to the rear plate of the chassis using 3/8" collars and
three 6-32 1/2" bolts. The shaft of the 3 gang condenser was coupled to
the "fine coupling" shaft with a standard flexible insulated coupler. The
coarse adjustment and its associated switch are left in place and are not
used. The wire from C2 on the band change switch was run directly to the
output coax connector. A short wire about 1 1/2" long tapped off the output
lead to the new 3 gang condenser. Now it is possible to load the antennas
on any band all fed with RG/8U or RG/11U through a Johnson low-pass filter
with perfect control, and even after several hours of use, the condenser
and antenna feed wire are cool. With the gain control full on and talking
loud, there is no tendency for arcing between the plates. I have the original
tank coil in the DX-100 and it does not over-heat. It is normally loaded
to 250 ma at 800 volts: 200 watts input.
"Before this modification I had severe TVI in my own TV receiver on Channel 3.
Now there is only a slight amount of cross-hatch lines barely viable at normal
viewing distances." Concerning this mod, Chuck Penson writes in his book,
Heathkit, a Guide to the Amateur Radio Products, "The (DX)100 uses a
combination of stepped loading (for coarse adjustment) and variable capacitor
loading (for fine tuning). This scheme was replaced in the (DX)100B with a
larger gear-driven variable cap less prone to arcing; a modification kit was
offered for DX-100 owners." My own DX-100 has this mod, installed by a former
owner, and it works well as described. So you might want to replace the
loading cap in your DX-100 with a three gang Broadcast variable if you can find
one rather than to go back to the original, postage stamp micas.
Jones also describes a technique for Neutralizing his DX100 final amplifier.
You might be interested in that also if you have the trouble that he describes.
(He does say that not all DX-100's need to be neutralized.)
"My DX-100 would not tune up easily on the 10 or 15 meter bands. When
the final plate tuning was out of resonance by a slight amount, it would
cause the grid drive to drop to less than 1 ma or go to a high value of
about 8 ma. With the knob turned to cw and the grid current adjusted for
about 4 ma, then tuning the "Amplifier Tuning" through resonance would
cause a severe change in grid current."
He added neutralization as follows. "To make my modifications, I slid
the transmitter chassis out of the cabinet. Facing the transmitter front
panel, I noted that the 5763 plate tuning condenser right hand lug of the
stator has a lead going to the tapped vertical coil and another lead
going down through a rubber grommet, through a condenser, to the plate pin
of the 5763. The left-hand lug of this condenser is unused. (See
pictorial 9, page 46, of DX-100 instruction manual.) By looking at the plate
tuning condenser for the 5763, it will be observed that the job should be
simple. I drilled a hole about 1/4" in diameter through the baffle plate
in line with this lug, then soldered a piece of #14 bare wire about 4"
long to this left hand lug. The wire was arranged so that it will not touch
the sides of the hole, and will extend in the direction of the large
RFC, feeding the 6146's. The wire was cut so that it will not reach the
large lower pi of the RFC, to avoid a short in the high voltage. In my
case, complete neutralizing was accomplished when the wire pointed toward the
RFC. Bending the wire closer to or farther away from the 6146 caused
definite unbalance. It required only a few minutes to find a place where
neutralization was excellent, as indicated by tuning the final through
resonance and observing that the grid current did not change at all. Then I
applied power to the final and tuned up. (I take it to mean that he did
the neutralizing adjustment with the final B+ switched off.) Now the
grid current remains steady when the final is tuned from one side of
resonance to the other! Only a fraction of a milliampere change - what a
Hope W8KGI this helps. A DX-100 is a fine rig and well worth
the time to bring it back to life.