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. There may be various reasons for this complaint.

  1. Bad feedline and antenna system ground.
  2. Poor Mic hookup or shielding.
  3. Vertical antenna with poor or no radial system.
  4. For base operation with other than a PS-30 Power supply:
    Poor regulation and insufficient filtering in the supply.
  5. Linear amplifier creating hi RF field near the operating position.
  6. RF induced back into the power supply causing falsie protection (other than PS-30).
  7. 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.
  1. Remove the VFO unit by four 3mm hex head bolts.
  2. Withdraw out the front panel and unplug the leads.
  3. Remove the VFO cover by 5 phillips head screws.
  4. Remove C17, and C18 from the PCB. Thes may be cut flush to the board.
  5. Reposition the Ground terminal for the Variabel Capacitor by heating the post and pushing it through to the opposite side of the PCB.
  6. Install paralleled 10pF green and 18pF yellow caps from the bottom tank coil terminal (LH) to the modified Ground post. Position the caps near the coil, not the PCB. Make certain the ground lead doesn't touch the coil or coil mounting.
  7. 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:

  1. 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.
  2. 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.
  3. 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.
  4. Turn the main knob to 450 analog. If the digital error is less than 2 kHz it is in spec. If the digital error is greate, proceed:

    For instance if the digital error is 14.454.0 (plus 4 kHz), multiply the error times 4 (16 kHz) and adjust the VFO trimmer cap to the desired frequency (14.450.0) less the error, or 14.434.0. Next adjust the VFO inductor L10 (front under the seal tape) back up to the desired frequency of 14.450.0.
  5. If the error in step 4 was in the minus direction, reverse the direction of correction adjustment in step 4.
  6. VFO linearity final check: The digital readout and analog dial should agree to within +/- kHz at every 100 kHz dial point.

TS-120S VOX operation pick-up time

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 37 (10-7-1979)

High speed CW ops may inquire about partial loss of the first character when operating VOX-CW. The transceiver will operate up to about 20WPM before this is apparent.

Improvement may be had by changing three components on the RF Unit X44-1260-01.

Production later than serial # 940701 has this change.

Please notate your Service Manual.

TS-120S Final Transistor Replacement

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 38 (10-7-1979)

When ordering single replacement 2SC2290 Final transistors for the TS-120S, please specify the hfe rank, by color. When replacing pairs of finals, always select the same hfe randk for both transistors.

Rank designation will be found printed on the transistor, directly under the transistor number.

Rank:     R    30-65
          O    55-95
          Y    85-150
Please return all defective 2SC2290 finals to the attention of the TRIO- KENWOOD service department, along with the warranty claim form.
TS-120S Ant Input Transformer Replacement

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 40 (12-7-1979)
Ts-120S transceivers subject to strong static fields or similar high current atmospheric discharges may suddenly exhibit low RX sensitivity on all bands. TX will be OK. You may suspect T1 on the RF unit X44-1260-01.

When replacing or ordering T1, L43-0696-35, also check the value of C20.
If it is 33pF, just replace and repeak the transformer.

First production transceivers will have a 15pF cap for C20. When changing T1, also replace C20 with a 33pF capacitor, and repeak the circuit.
Old part number:    L34-0696-05    Antenna Coil
New part number:    L34-0696-35
Old Capacitor was 15pF, replace with
New part number:    CC45RH1H330J   33pf

TS-120 Repair Notes


Symptom                  Repair

No CW, No Audio          Sidetone oscillator Q25 on AF GEN unit 2SC1815
Receiver Sens Down       T1, L34--696-35 on RF unit Shorted
Analog Dial Cal OFF      Mechanical Adjustment
Carrier in SSB           RF unit D2 and D3 shorted see TKC Service Bulletin
Loss of Digital          Check P5/J5 on Carrier Unit
Smoke at Final Unit      Check for long eds of wire shorted to case inside
                         final unit also burned L10 and L11, L40-1011-04   
SSB Audio bad            IF shift control turned max CW
Power on Meter Max       The RF gain control was turned max CCW
VFO freq drift           Replace the TEMP comp CAP C17
TX Audio BAD             Check the Carrier Point adjust and make sure final
                         bias is set to 150 Ma
PLL digital NG           Check D15 PLL unit
High IC in RX            Check the Final transistors
15 Meters only           The BCD rotary band switch has a broken wafer.
RX TX only 10M           The BCD divider Q16, 74LS163 on PLL unit bad.     
                         Check the BCD inputs against the pin 2 frequency.
28.5 HF Osc              Check ground screws and tighten at Relay Unit X41-
No CW Filter             Solder short across pins 1-2 J29 IF Unit
No RX                    After the CW filter was installed the pins were   
                         not cut off shorting the signal to ground.
Analog dial off          Perform VFO tracking adjustment at 50 and 450
TX out 40M only          40M 50W and 10M 10W bad finals
No 15M                   Core on the VCO transformer for 15M broken
3.5 and 7 Band same      Shorted trace at Band switch under IF unit
Low RF out               All bands, Q12 SN16913 on AF gen unit, 2SK19GR bad
No TX all bands          L25 open on the RF unit Small gree inductor
No digital no TX         Q12 mixer on the PLL unit bad
No RX no digital         Q26 mixer on PLL unit bad
INtermitt SSB AUdio      Pin 5 of J5 on AF GEN unit loose
No key or TX             D26 and Q24 2SA562 on AF gen unit bad
No TX                    R8 220 ohm at key jack burned open.  Make sure    
                         customer uses shielded line on CW KEY.
15M osc.                 On 15M band only with antenna HF oscillation when
                         first speaking.  Install Ferrite bead on base of  
                         first mic amp Q18 AF GEN UNIT.
No TX                    Loss of AVB Voltage at RF unit.  Check Q9 2SA473  
                         on the filter unit, also check 6 volt Zener diode.

TS-120S Band PAss Filter Change


For complaints of out of band interference, especially BCI, the BPF (Bandpass filter) terminating resistors may be changed for an improvement of approximately 10dB.

On the RF unit X$$-1260-01 change R3,5,7,9,11 from 1.5K ohm to 470 ohm. No adjustments are required.
TS-120S TX oscillation

The TS-120S may exhibit intermittent oscillation on one or more bands.
Cause may be either external or internal. Symptoms may be no control of Carrier level, full current or ALC in either CW or SSB after initial keying or modulation, or "full outup both forward and reflected".

Externally induced oscillation:

For oscillation which occurs only when operating on an antena, suspect RF feedback coupled through the mic, key, or other auxiliary wiring.
  1. Add a .01mf bypass cap across the mic connector, pins 1 & 4.
  2. If not already present, add a .01mf cap across pins 3 & 4 on the mic connector.
  3. Add a ferrite bead at the base of the mic preamp Q18. This may be installed on R42.

Internal oscillation

  1. Make certain the antenna fitting is tight to the rear panel.
  2. Check for loose screws securing the Relay unit X41-1250-00 (located over the RF unit X44-1260-01). Add star washers if necessary.
  3. On the RF unit, check the DRV (Drive Output) coax for snug, secure connection at both ground and center condutors. Follow this coax to the final unit X53-1250-00 and again check for secure connections.
  4. On the RF unit, check diodes D2, 3 & 8 for hight reverse leakage. Lift one end of each diode and mesure with an ohmmeter. If in doubt, replace.

TS-120S Full CW output in ssb mode

Hight reverse leakage, ou shorting of D2 and sometime D3 on the RF UNIT X44-1260-01, will provide an unwanted DC voltage path which gives full CW output while in the SSB mode. Carrier control has no effect, and RX is not affected.

Replace D2, 1S2588, check D3, 1s1587, and install an additional diodo, D8, a 1S1585 on the foil side of the RF unit.

To install, swing the rf unit up. Remove the shield from under the IF unit, exposing the foil side of the RF unit.
Using a razor or X-ACTO Knife, carefuly cut the land between C4 and C20. Solder the additional 1S1585 across the the cut and observe polarity Replace D2, the 1S2588. Some RF units have a 12 PF cap soldered on the foil side of the board, across T1; C20 will be missing.
Remove the 12 PF cap, and install C20, 33 PF in the board.

Production later than 080001 has this change.
TS-120V: Reduced RX current consumption

The TS-120V is equipped with a two bulb lightshow. Remember that the chassis really warms up above the VFO-scale and the S-meter...
I replaced the two bulbs with green ultra-bright LEDs and matching resistors in series. The resistors give you a chance to adjust a pleasant illumination level (I always felt the lightbulbs beeing to bright). As standart LEDs are a little thicker than the original Kenwood bulbs one need to cut off a part of the VFO-scale lighting rubber mounting sleeve's rear portion. Cut of the bulb's wires close to the glas body, they will perfectly serve the new LED-R combinations.

TS-120S, TS-180S phone patch terminal addition

Author: Trio-Kenwood Communication, inc.

Service bulletin no. 44 (6-9-1979)

Kenwood recommends the PC-1 Phone Pacth for all of our radios. Most other phone pacthes will work satisfactorily without any modification to the radio, requiring only an external speaker connection, and that the Mic line be run through the pacth.

For those operators who desire a Pacth input similar to the TS-520S or TS-820S, an input connection and terminal must be added at the Mic input preamp circuit.

Fixed Divider

Use a 100Kohm in series, with a 10Kohm to ground on the input side of the 100Kohm resistor. Use shielded line, and connect as follows:


On the IF Gen unit X49-1110-01 install the fixed divider at the junction of R43 10K, C42 100pF, & C43 1uF (input of Q18). Add an RCA jack, or use remote pins 7 and ground for input.


On the IF Gen unit X48-1240-00 install the fixed divider at the junction of R167 10K, C128 .001uF, & C129 1uF (input of Q32). Add an RCA jack, or use remote pins 7 and ground for input.
TS-120S Final protection

Author: Trio-Kenwood Communication, inc.

Service Bulletin no. 51 (27-7-1979)

If for any reason work is performed in the Final unit, install two additional parts to the Filter unit X51-1200-00. This will improve protection action.


  1. Remove top and bottom covers.
  2. Remove Final unit (6screws).
  3. Filter unit X51-1200-00 will be exposed. Install between Q3 Base, and TP1, series wired diode and resistor D14 and R34.

No adjustment are necessary

New part number D14 1S1555    V11-0076-05
                R34 4.7Kohm

Please notate your Service Manual

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