IC-735 spurs in T-T 1208 (short version)

[Paul H. Bock]

     The following article is considered to be in the public 
domain and may be re-published, copied, or distributed without 
prior permission.  Credit to the original author will be 
appreciated.

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        REDUCING 6-METER SPURS FROM THE ICOM IC-735


     After assembling a Ten-Tec 1208 6-meter Transverter
Kit and connecting it to my Icom IC-735 transceiver, I noted the
presence of in-band (6-meter) spurs which apparently were coming
from the IC-735.  Some were fixed frequency while others "stepped"
as the IC-735 was tuned, and they ranged in amplitude from near
noise level to S-8.  Removing the microphone eliminated many of the
spurs, so it was obvious that they were being radiated by the
cable. Further tests verified that the PTT, PTT return, and UP/DOWN
pins on the mike connector were the worst offenders.  The cure is
to bypass these pins inside the IC-735.

     To accomplish the internal bypassing, 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.  Three bypass
capacitors should be installed:  0.1 ufd to pin 6 of the mike
connector (PTT Return), and 0.01 ufd to L8 (PTT) and L6 (UP/DOWN).
The free lead of each bypass capacitor should go to chassis
ground via a solder lug under the nearest PC board holddown screw
on the large board mounted to the underside of the IC-735. 

     Install a 0.1 ufd capacitor to pin 6 of the mike connector by
making a small loop in one lead of a capacitor (cut the lead to
about 3/8" long first), bend the loop 90 degrees, then slip it over
the pin.  Reach in carefully with a small soldering iron (25 watts
or less) and "tack" the lead to the pin.  USE EXTREME CARE NOT TO
TOUCH THE PLASTIC RIBBON OR ANY OTHER INTERNAL COMPONENTS OR WIRES! 
Connect the other end of the capacitor to the ground lug.

     RF chokes L8 and L6 are mounted vertically to the board with
the outer-most lead looped back down to the board.  Alternate loops
have insulating sleeving on them, but the loops of L8 and L6 (top
and third from the top as viewed by an observer) have no sleeving.
Make a small "hook" in one lead of a 0.01 ufd 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.  Connect the other end
to the ground lug.  Repeat the procedure for the second RF choke.

     After installing the three bypass capacitors most spurs were
eliminated, while the strongest of the original "carrier-type"
spurs (50.100 and 50.200 MHz) were barely audible at the noise
level.  The synthesizer "step" spurs at 50.121 and 50.225 are no
more than S-3 and there are no other spurs of consequence in the
range from 50.100 to 50.225 MHz.  The modifications have no
apparent adverse effect on operation of the IC-735.

Paul Bock, K4MSG