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Kenwood TS-690SAT S Meter is at full scale deflection with no audio

Discussion in 'General Technical Questions and Answers' started by 4S7FS, Jan 14, 2021 at 8:32 AM.

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  1. 4S7FS

    4S7FS Ham Member QRZ Page

    I have an issue with my TS-690SAT where-by at cold power on, the S Meter was at full-scale deflection with no audio. The RF Gain is fully clockwise. Over a period of about 15~20 minutes, the needle gradually drops to normal with a corresponding gain in audio. After this period operation was normal.

    Originally the radio was functioning fine, with normal gain and audio instantly at power on, then one time I switched it on and it was exhibiting this behavior, so I am not sure if a part has "blown" or it is the result of gradual degradation of a part until it hit some bottom threshold that gradually improved as the radio warms up.

    Now I find it does not give any audio at all and the S meter is fully deflected, even after keeping for 20~30 minutes

    Is it possibly an electrolytic capacitor, Transistor is degraded
    I feel the place to look is the IF board, AGC, RF gain ??

    I have heard this is a common problem with Kenwoods but have not found a reference for the TS690.
    any advice on what to look for appreciated,
    I don't want to go in blind and make the radio a Brick "Hi"
    Sorry if I am reposting for an old problem since this radio is around 20 years old.

    73's
    Farash
    4S7FS
     
  2. WE4E

    WE4E Premium Subscriber QRZ Page

    Check the RF Gain control. Make sure it is adjusted fully clockwise.
     
  3. 4S7FS

    4S7FS Ham Member QRZ Page

    Thanks for the response, previously when the radio worked after the warming-up period it was usually kept adjusted fully clockwise. But now the S meter is deflected with no audio at all even in that position.
     
  4. AA5MT

    AA5MT Ham Member QRZ Page

    I would listen to it on another radio. Does it sound normal, distorted, squeals, none, etc. This will give you a clue. Check it with and without processing. Also remove the mic and transmit. AM/FM/CW/SSB all the same?

    Tom
     
  5. 4S7FS

    4S7FS Ham Member QRZ Page

    HI Tom, TX there is no problem at all. it's only on RX.

    Thanks.
     
  6. KA5IPF

    KA5IPF Ham Member QRZ Page

    Look for info on the TS-450S, same radio except for 6m.
     
  7. 4S7FS

    4S7FS Ham Member QRZ Page

    thanks
     
  8. KA5IPF

    KA5IPF Ham Member QRZ Page

    If you have the service manual go to the part about setting the AGC voltage, usually 3.2v. Measure at the test point and you will find it is at zero volts. Look at the circuit that supplies the AGC voltage and you will find the problem. It sounds like a leaky electrolytic cap pulling the voltage down.
     
    WE4E likes this.
  9. 4S7FS

    4S7FS Ham Member QRZ Page

    Thanks, yes I have got the service manual and as you suggested I suspect one of the caps in this location as in the schematic below must be leaky or dried out, I wish I had an ESR meter to make my life easy, have to pull each one out "Hi". will work on it over the week and see how it pans out.

    Any suggestions are welcome.

    73's

    upload_2021-1-16_22-53-41.png
     
  10. WE4E

    WE4E Premium Subscriber QRZ Page

    I agree with KA5IPF. This sounds like an AGC error. You have the benefit of voltage call-outs on the schematic. Put the radio into the specified test condition, then walk the circuit with a voltmeter. Investigate any areas where the voltages are incorrect. That should help narrow down the cause.
     

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