Kenwood TS-120S

Discussion in 'Radio Circuits, Repair & Performance' started by KK4HFG, Jan 3, 2020.

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  1. KK4HFG

    KK4HFG Ham Member QRZ Page

    My Kenwood TS-120S just developed an issue that used to be intermittent. When I turn it on, the display shows only two dots on the display, with no receive or transmit. In the service manual, the troubleshooting page says "PLL Unlock" as a likely problem for that issue. I've tried Googling more information on that with little success.

    Has anyone had the same problem in this or a similar model radio? What did you do to fix it, and what do you suggest? Thanks!
  2. KF4ZGZ

    KF4ZGZ Ham Member QRZ Page

    A couple of know issues.
    One is the Kenwood PLL unlock , but I'm not sure if the 120 did it.
    But with the 120, try wiggling the bandswitch.
    There is a lot of slack that can develop over the years in the control shaft set-up for the bandswitch.
    Even though the knob is on the front, the actual switch is deep in the radio and has a gearset, a rod,
    a connector, and another rod before actually moving the switch.
    KK4HFG likes this.
  3. W5RKL

    W5RKL Ham Member QRZ Page

    If the display shows only the decimal points, no frequency, then the PLL has changed from a "LOCKED"
    state to an "UNLOCKED" state. More often than not this is caused by the loss of an Oscillator signal that feeds
    the PLL board.

    Have you checked the presence of the 8Mhz CAR Oscillator signals at the AF board plug J3 pin 2?
    You will need a frequency counter to determine the presence of the 2 CAR oscillator 8Mhz signals
    which are:

    LSB is 8828.5Khz
    USB is 8831.5Khz

    Simply connect the frequency counter to the AF board plug J3 pin 2. Set the Mode switch to either position.
    If you don't see either of those CAR oscillator signals at the AF board's plug J3, pin 2 and you have an RF signal
    generator, tune the signal generator to either of these frequencies and inject the "Unmodulated" RF signal at the AF board's
    plug J3 pin 2, the S-120 should come to life. If this works then the CAR oscillator board is at fault. The CAR
    oscillator board is "below" the AF board and has 3 transistors. If the CAR oscillator signal has failed, it could possibly be
    caused by Q1 on the CAR board.

    The transmitter does NOT need to be keyed to perform the above tests. I worked on an S-120 a few years ago with the same
    problem. The source of the problem turned out to be a bad Q1 on the CAR board.

    You should obtain the S-120 service manual which you can find here:

    Mike W5RKL
    KK4HFG likes this.
  4. KK4HFG

    KK4HFG Ham Member QRZ Page

    Thanks! I took off the covers, and the band switch is moving the switch all the way.

    Thank you for the recommendation on checking the oscillator. I don't have a frequency counter, but am about to look on Amazon to see if I can find one that is decently priced.

    I have that service manual, but needed a more in-depth explanation of what I was looking at. What you wrote made sense of what I was reading. I'll update once I make some headway troubleshooting.
  5. W5RKL

    W5RKL Ham Member QRZ Page

    Amazon is selling a RISPRO VC3165 frequency counter for $72.90. It qualifies for FREE SHIPPING.
    I recommend using a Scope Probe to use with the counter. Amazon is selling scope probes as low as $14.
    I use scope probes with my Sinometer VC-2000, Heathkit IM-4100, and Opto 8000 counters.

    Good luck with the TS-120.

    Mike W5rkl
    KK4HFG likes this.
  6. W5RKL

    W5RKL Ham Member QRZ Page


    The Band Switch doesn't control USB/CW or LSB CAR board crystal oscillator selection. The Mode
    switch wafer S3-1 applies +9VDC to the CAR board's "USB" or "LSB" terminals to forward bias the crystal
    oscillator select diode switch. Diode D2, when forward biased by Mode switch S3-1, selects the LSB CAR
    crystal oscillator and D1, when forward biased by the Mode switch S3-1, selects the USB/CW CAR crystal
    oscillator. USB and CW use the same CAR crystal oscillator's RF signal.

    I would be looking for the CAR board's CAR signal, based on the Mode switch setting, not the band switch

    Mike W5RKL
  7. N5INP

    N5INP Ham Member QRZ Page

    I had the issue with one of these I restored. You want to check the frequency doubler in the PLL unit, specifically Q24 & Q25. The amplification of the transistors may be low; should be between 60 and 120 or so. If it's too low the board function will fail. Just swap them out if you can't test them or observe the signal levels. Hopefully that will fix your problem.
  8. W5RKL

    W5RKL Ham Member QRZ Page


    I respectfully disagree. I doubt the PLL Unit itself is at fault. I am more inclined to believe the
    CAR Unit is at fault.

    If Meryl has access to a signal generator, she can test the failure of the CAR Unit without
    removing any boards. Simply apply either the 8828.5Khs LSB or 8831.5Khz USB RF signal to the AF board's
    plug J3 pin 2 and ground. If the TS-120 comes to life then that's a clear indication the problem is on the CAR
    Unit board. If this is what Meryl finds then I would suspect Q1 on the CAR board is at fault. Q1 is a 2SC460(B)
    Japanese transistor which can be obtained at the various online website. I found them in eBay ad #281736052881
    in quantities of 10 for $9.95 FREE SHIPPING. The 2SC460 will work in place of the 2SC460(B). A 2N3904 will also
    work but if the 2SC460 transistors are available then I would stick with the Japanese transistor. There are NTE
    replacements for the 2SC460(B) but again, if the Japanese transistor is available, stick with that one.

    Mike W5RKL
  9. N5INP

    N5INP Ham Member QRZ Page

    Well you can disagree if you wish but that's what fixed mine when it displayed two dots.

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