REDUCING 6-METER SPURS FROM THE ICOM IC-735

By Paul Bock - K4MSG de Hamilton, Virginia

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        In early 1996 I assembled a Ten-Tec 1208 6-meter Transverter Kit which my XYL had given me the previous Christmas. My plan was 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 were fixed frequency and some "stepped" as the IC-735 was tuned, and they ranged 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:

        Although I was able to successfully use the IC-735/1208 combo with the dipole 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 inconsequential.

        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 NOT TO TOUCH THE PLASTIC RIBBON OR ANY OTHER INTERNAL COMPONENTS OR WIRES!!!***

        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 bypassing).

        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 [email protected].

        POSTSCRIPT: Since late 1996 I have been using a 3-element yagi at 25 feet, and there is only one spur audible when operating 6 meters: 50.121 MHz, at about S-2. Consequently, the Ten-Tec 1208/IC-735 combination has netted me about 75 grid squares. 73, Paul Bock, K4MSG

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