Reducing IC-735 6-meter spurs

Paul H. Bock
Mon, 29 Jan 1996 9:18:14 -0500 (EST)


     Recently, I assembled a Ten-Tec 1208 6-meter Transverter
Kit which my XYL had given me for Christmas, and which I
planned to use with my Icom IC-735 HF rig to return to 6
meters after a hiatus of about 15 years.  The "Blizzard of
'96" having rendered outdoor work unpalatable, and roof work
downright dangerous because of snow and ice accumulation, I
decided to build a simple dipole and install it in the attic.
This done, I proceeded to give 6 meters a try - but what a
cacophony of in-band spurs greeted my ears!

     With the 1208 connected to the dipole, which is located
in the attic and about eight feet directly above the shack,
tuning across the band from 50.0 to 50.250 (14.0 to 14.250 on
the IC-735) revealed a vast array of spurs - some fixed
frequency, and some of which "stepped" as the IC-735 was
tuned - ranging in amplitude from right at the noise level
(about S-2 on the IC-735 S-meter) to S-8.  Several of the
most prominent in the primary range of interest between
50.025 and 50.225 are listed below:

  Frequency (MHz)         Amplitude          Description
  ~~~~~~~~~~~~~~~         ~~~~~~~~~          ~~~~~~~~~~~
      50.047                 S-7             Steady carrier

      50.100                 S-8             Steady carrier

      50.121                 S-6             Step (follows
                                              IC-735 tuning)
      50.200                 S-8             Mixed, steady &
      50.226                 S-6             Step

     Although I was able to successfully use the IC-735/1208
combo with the indoor dipole for 16 QSOs in 5 grids during
the January VHF Sweepstakes, the presence of such a
multiplicity of spurs, at least some of which were apparently
emanating from the IC-735 (witness the "step" signals noted
in the table above), was deemed unsatisfactory.  I therefore
proceeded to investigate the nature of the problem and to
attempt to determine a solution.

     The first thing I noticed was that removing the
microphone from the HF rig eliminated many of the spurs and
drastically lowered the amplitude of the stronger ones.
Using a short length of wire as an "antenna," I alternately
touched each pin in the chassis connector and discovered that
although several pins were "hot" with the spurs, the worst
offender was the PTT ground line.  Touching a small bypass
capacitor between the PTT ground pin and the chassis reduced
the amplitude of the spurs several S units.

     A word about where the spurs come from:  Most are
present inside the IC-735, at least some of them are
synthesizer-related, and they are conducted between the PLL
board and a small auxiliary board mounted behind the front
panel adjacent to the mike connector.  Signals are carried
from this auxiliary board to the mike connector via a small,
thin, plastic ribbon with parallel circuit traces on it; the
end of this ribbon is soldered to the pins on the back of the
mike connector (the ribbon has an eyelet pattern which
matches the pin layout on the connector).  Needless to say,
any signal present on any trace is readily coupled to other
traces, and with the microphone plugged in a ready-made
"antenna" is available to radiate these spurious signals.

     Despite the use of small RF chokes and some bypassing on
the auxiliary board, the spurious signal levels audible in
the 6-meter band when the antenna is located in the proximity
of the IC-735 are quite prominent and annoying.  Obviously,
mounting the 6-meter antenna outside, high and in the clear
and as far away from the IC-735 as practical, will reduce the
spurious pickup.  Some of the spurs were so strong, however,
that I deemed it appropriate to try and reduce the amplitudes
as much as possible; if reduced to the point of being a mild
annoyance with the antenna only eight feet from the rig, I
surmised, then using an outside antenna should render them

     Unwilling to immediately make internal modifications, I
reconnected the microphone and tried clamping a ferrite RFI
choke onto the mike cable adjacent to the connector.  The RFI
choke consists if two 1" x 1/2" x 1/4" thick pieces of
ferrite material, each with a longitudinal semicircular
groove in it.  Each piece is mounted in a hinged plastic
cover and forms half of a clamp which, when placed over a
cable and snapped shut, is supposed to reduce RFI leaving the
equipment and traveling down the cable.  It is intended to be
used on personal computers to prevent radiation from keyboard
and monitor cables.

     The ferrite clamp proved to be something of a
disappointment; while it did reduce the amplitude of the
spurs, using one clamp only reduced the level about an S unit
and using two reduced the level no more than half an S-unit
more.  Given the amplitude of some of the spurs, I considered
this to be inadequate, and so set about to determine what
internal modifications could improve the situation.

     I will spare the reader the details of all the
measurements and combinations tried, but suffice it to say
that internal bypassing is the only way to effectively reduce
the spurs to a point where they become barely an annoyance; I
was unable to eliminate them entirely.  The remainder of this
discussion deals with the final modifications as they were
incorporated in my IC-735.

     To accomplish the internal bypassing, first remove the
top and bottom covers of the IC-735 and lay it on its top
with the front panel facing to the left.  This puts the mike
connector on the observer's left and immediately in front.
Behind the mike connector is a small, vertically-mounted
circuit board, and pointing towards the observer from this
board are several small, green RF chokes in approximate
vertical alignment.  These chokes are, from top to bottom:
L8, L7, L6, L5, L4, and L3, and are in-line chokes for the
PTT, squelch, UP/DOWN control, +8 vdc, audio output, and
microphone "hot" lines, respectively.  L2, the microphone
return, is located on the same board but on the bottom left
as viewed by the observer and is pretty inaccessible.

     Since L7 (squelch), L5 (+8 vdc) and L4 (audio output)
are not connected to any wires in the mike cable when the
standard HM-12 hand mike is used, they can be ignored.  This
leaves L8 (PTT), L6 (UP/DOWN), and L3 (mike) which could be
bypassed.  During testing I determined that bypassing these
chokes reduced the spur levels as follows (tests performed
while monitoring the 50.100 spur):

       L8 bypassed:  Level reduced from S-7 to S-6
       L6 bypassed:  Level reduced from S-7 to S-4
       L3 bypassed:  Level reduced from S-7 to S-6

     I also determined that bypassing L2 (mike return) would
reduce the spur from S-7 to S-5, and bypassing pin 6 on the
mike connector (PTT return, which does not pass through a
choke) would reduce the level from S-7 to S-4.  L-2 is
located such that my soldering iron wouldn't reach it, but
pin 6 of the mike connector was barely accessible from behind
by *very carefully* reaching in.  I therefore determined that
I would install three bypass caps:  One on pin 6 of the
connector (PTT return), one on L8 (PTT), and one on L6
(UP/DOWN).  Since bypassing L3 (mike) had only a minimal
effect at best, it would be left alone.

     The capacitors I chose were all CK05 square ceramic
types, but disc ceramics should work just as well.  The
values I used were 0.1 ufd for bypassing pin 6 of the mike
connector and 0.01 ufd for bypassing L8 and L6.

     The RF chokes are easily bypassed, since they are
mounted vertically to the board (horizontally to the
observer) with the outer-most lead looped back down to the
board.  Make a small "hook" in one lead of a bypass capacitor
(cut the lead length to about 1/4" or so), hook it over the
wire of the loop, and tack it with the soldering iron; the
other lead of the bypass capacitor should be grounded.
Alternate loops have insulating sleeving on them, but as it
happens the loops of L8 and L6 (top and third from the top as
viewed by an observer with the rig upside down as described)
have no sleeving and so the connections are easily made (see
the note below regarding the ground connection point).

     Connecting to pin 6 of the mike connector is a bit
trickier:  First, make a small loop in one lead of a
capacitor (the lead can be cut to about 3/8" long first),
bend the loop 90 degrees, then slip it over the pin.  Reach
in *VERY* carefully with a small soldering iron (25 watts or
less) and "tack" the lead to the pin.  ***USE EXTREME CARE

     The free lead of each bypass capacitor should go to
chassis ground, and this is easily accomplished by mounting a
solder lug under the nearest PC board holddown screw on the
large board mounted to the underside of the IC-735.  This
brass screw is plainly visible on the left front edge of the
board (as viewed by the observer with the radio facing left);
just remove the screw, lockwasher and flatwasher, slip a lug
over the screw shaft, and reinstall the screw with the lug
facing toward the rear of the front panel.  I used one of the
long, tapered lugs.  It's easiest if you install the lug
*FIRST* before installing any of the bypass caps.

     After installing the three bypass capacitors most of the
spurs had been reduced to the point of being completely
inaudible.  Of the strongest of the original spurs, the two
at 50.100 and 50.200 are barely audible at the noise level,
the synthesizer-derived "step" spurs at 50.121 and 50.225 are
no more than S-3 (with an S-2 noise level and the IC-735
preamp turned on), and only the 50.047 steady carrier is
unchanged at S-7 (my guess is that this is an external
signal; I have a couple on 2 meters from the local TV cable).
There are no other spurs of any consequence within the range
from 50.100 to 50.225 -oh, perhaps a very faint, almost
inaudible remnant in two or three places - but nothing to
interfere with weak signal operation.  An outside yagi should
only make things better.

     For those who may be wondering, I actually tried cutting
the UP/DOWN line at L6 but it made almost no difference;
bypassing is the only thing that really seems to work.  Oh,
yes, don't forget to ground the IC-735 to your station ground
via the rear panel ground jack - the difference between
"grounded" and "ungrounded" is about half an S unit (with no

     Incidentally, the modifications have had no adverse
effect on operation of the IC-735, and in fact have made 6-
meter operation very pleasurable.  I can't wait for that
first sporadic E band opening!

     If you have questions or comments about the problem,
tests, or procedures described, please feel free to drop me
an e-mail at or call me at (703) 560-
5000 x 2062 (weekdays) or (540) 882-4745 (evenings/weekends).


Paul Bock, K4MSG