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TS-120S MODS | Radioaficion Ham Radio



Kenwood TS-120S Modifications

Ant Transformator Replacement: New Part.

bulletin #40 7/12/79

Transceivers subject to strong static fields, high RF, or similar high current atmospheric discharges may suddenly exhibit low sensitivity on all bands. TX will not be effected. Suspect T1 on the RF unit, X44-1260-01.

When replacing T1, L43, 0696-35, check that the value of C20 is 33pF. Early units may have a 15pF for C20, and may have T1 as L43-0696-05. In either case, replace the transformer with an L43-0929-15. If C20 is 15pF, replace it with a 33pF capacitor. Since T1 is a broadband transformer, no adjustment is necessary.

In addition, add two 1S1587 diodes after the transformer as shown to further protect the receiver.

Installation time for this procedure is approximately ½ hour.

Kenwood TS-120S Modifications



phone patch terminal addition

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

For those operators who desire a Patch 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:

Kenwood TS-120S Modifications


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.



Final protection

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

Kenwood TS-120S Modifications

Please notate your Service Manual



VFO Stability

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
Kenwood TS-120S Modifications

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.

Kenwood TS-120S Modifications

Final Transistor Replacement

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.



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 X44-1260-01 change R3,5,7,9,11 from 1.5K ohm to 470 ohm. No adjustments are required.



TX oscillation

The TS-120S may exhitbit intermittent oscillation on the 15M, 10M and/or 20M bands. Symptoms may be no control of Carrier level, full current or ALC in either CW or SSB after initial keying or modulation, or "full output both forward and reflected".

  1. Physically disconnect power from the radio.
  2. Make certain the Antenna fitting is tight to the rear panel.
  3. Remove covers. Unplug speaker lead.
  4. Check for loose screws securing the RF unit X44-1260-01 and the relay unit X41-1250-00 (locarted over the RF unit).
  5. On the RF unit, check diodes D2, 3 and 8 for high reverse leakage. If in doubt, replace.
  6. If not already present on the RF unit, make the following changes:
    R47 (33 ohm) change to 10 ohm Radial            (RD12CY2E100J)
    R50 (560 ohm) change to 680 ohm Radial (RD12CY2E681J)
    C61 (220 pF) Remove
    Add 2 1S1587 (V11-0370-05) diodes in series, anode to ground from the junction of C4 and D8.
    Note: Diode I.D. 1S1587 = 1S1585. Added diodes may induce RX IMD. If se, delete.

  7. As illustrated, carefully cut the cable ties on the wire harness (without cutting thr wires) as it rounds the corner behind the PLL unit X50-1490-00.
  8. Seperate the AVB line, a white/brown line from the filter unit X51-1200-00 to the RF unit X44-1250-01.
  9. Adjscent to the rear of the PLL and to the left you will see a five pin terminal strip. The two right-hand most terminals are bridged. Without breaking the terminal or strip, remove the heavy yellow, and light red and green lines and transfer them to the adjacent terminal. There will now be seven lines, plus the positive end of the C1 on the second terminal. Be sure to cut the wire jumper between these two terminals.
  10. The free terminal will be a tie point for the AVB line bypass capacitor. Solder a .01uF 50V disc ceramic (C52-1710-36) between ground and the first free terminal. Use spaghetti to insulate the leads. Then solder the AVB line to the .010uF cap.
  11. Rerie the cable herness with nylon cable ties (J61-0401-05).
  12. Replace the covers and speaker lead. Power-up and verify correct operation.

Kenwood TS-120S Modifications

Note: Installation time for this procedure is approximately ½ hour.



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.

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