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Service bulletin no. 48 (9-8-1979) Some TS-120S users may complain of a hum during SSB operation, which may be caused by RF feedback. There may be various reasons for this complaint. • Bad feedline and antenna system ground. • Poor Mic hookup or shielding. • Vertical antenna with poor or no radial system.
Modifications for the Kenwood TS-120 Hum in SSB TX, TS-120S and others Author: Trio-Kenwood Communication, inc. Service bulletin no. 48 (9-8-1979) Some TS-120S users may complain of a hum during SSB operation, which may be caused by RF feedback. MR艞@ 21294̋L܂B ɋL. Page-1 Page-2 Page-3 Page-4 Page-5 Page-6 Page-7 Page-8 Page-9 Page-10 Page-11 Page-12 Page-13 Page-14 Page-15 Page-16 Page-17 Page-18 Page-19 Page-20 Page-21 Page-22 Page-23 Page-24 Page-25 Page-26 Page-27 Page-28 Page-29 Page-30 Page-31 Page-32 Page-33 Page-34 Page-35 Page-36 Page-37 Page-38 Page-39 Page-40 Page-41 Page-42 Page-43 Page-44 Page-45 Page-46 Page-47 Page.
• For base operation with other than a PS-30 Power supply: Poor regulation and insufficient filtering in the supply. • Linear amplifier creating hi RF field near the operating position. • RF induced back into the power supply causing falsie protection (other than PS-30). • RF pickup by any cables to or from ancillary equipment. Prevention is the best cure.
For those cases where all normal measures have been tried, try installing a.01uF bypass cap across the mic input jack between pins 1 and 4. Some AFSK operators may have similar RF feedback problems with TS-820S or TS-520S. Bypass the mic ground pin to the chassis, pin 3 to 4 and in extreme cases, install a ferrite bead on the Mic coax center conductor at the microphone connector. Please annotate your service manual. TS-120S VFO Stability (Supercedes 7/9/79 bulletin) VFO drift characteristics may be improved by changing component values and location in the VFO. This change applies to serial numbers before 00100001. Note: DO NOT disturb lead dress or component placement while working inside the VFO.
• Remove the VFO unit by four 3mm hex head bolts. • Withdraw out the front panel and unplug the leads. • Remove the VFO cover by 5 phillips head screws. • Remove C17, and C18 from the PCB. Thes may be cut flush to the board.
• Reposition the Ground terminal for the Variabel Capacitor by heating the post and pushing it through to the opposite side of the PCB. • Install paralleled 10pF green and 18pF yellow caps from the bottom tank coil terminal (LH) to the modified Ground post.
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Position the caps near the coil, not the PCB. Make certain the ground lead doesn't touch the coil or coil mounting.
• To reassemble, perform steps 3,2,1. Snug all VFO case screws. A later complaint of frequency jump would indicate a loose VFO shell. New Parts: 10pf CC45SG1H100J 18pf CC45RG1H180J Analog Dial Calibration: • Turn the main dial fully CCW.
The red cursor should line up with the VFO mark on the sub-dial. If it does not, remove the main knob, loosen set screws and align the scale start point with the red cursor. • Turn the main knob to 50 kHz analog. Adjust the aluminum slip sub- dial to line up with any one of the larger black dial marks. • Note the digital error.
If it is MORE than 2 kHz adjust the VFO trimmer cap TC1 (center under the seal tape) to exactly 50.0 on the digital readout. • Turn the main knob to 450 analog. If the digital error is less than 2 kHz it is in spec. 