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Kenwood TS-440 Distorted TX Audio

Discussion in 'Radio Circuits, Repair & Performance' started by W4FBI, Jul 12, 2018.

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  1. W4FBI

    W4FBI Ham Member QRZ Page

    Transmit SSB audio is perfect on all bands except 20 meters.

    So, I'm thinking it has to be BPF diodes (or associated relay) for 20 meters. - Nope. removed and checked those two diodes on the RF board (not too much fun doing that). Checked good. Replaced the BPF relay board (I have 3 or 4 of these), No difference.

    Standard MC42 mic transmitting into a dummy load.

    Again - works just fine on all other bands. Rules out cabling/power supply/mic/antenna, etc...

    Any thoughts?

  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    Go to the RF unit and check for leakage on the RX BPF diodes (see pic). With the radio on 20m check the voltage on each of the green chokes. It should be zero except for the 3rd from the left (D18 choke). If any except that one show voltage clip the lead on the diode just behind it. It could have several shorted diodes. After clipping the diode check for the distortion. If none replace the diode(s).

    Note usually it is the 1st and third one from the right.

    Attached Files:

  3. W4FBI

    W4FBI Ham Member QRZ Page


    Thanks for the response.

    I did check voltage on all those coils and they read virtually zero except the 20m diode location (L52, D18, D19). It reads 8v (as it should).

    I will double check them again

    BTW, distortion is also on CW and AM. Doesn't seem to care what mode.
  4. KA5IPF

    KA5IPF Ham Member QRZ Page

    Did you check for distortion on 30m? It uses the same VCO as 20m.
  5. W4FBI

    W4FBI Ham Member QRZ Page

    I'll check...
  6. W4FBI

    W4FBI Ham Member QRZ Page

    30 meters works perfectly - no distortion.

    Also, all VCO's cleaned out and adjusted perfectly.
  7. W4FBI

    W4FBI Ham Member QRZ Page

    OK, since I have another TS-440, I swapped out the RF boards. Same result! Distortion only on 20 meters

    I'm beginning to think there is a gremlin involved...
  8. W4FBI

    W4FBI Ham Member QRZ Page

    OOPS. The gremlin was me.

    Transmit is set into a dummy load.

    Receiving is set up on my Mag Loop. It was 'tuned' to 7.2Mhz. Mag Loops are very sensitive. When you try to receive @ 14.2Mhz (when it is tuned to 7.2) it attenuates and distorts. After I tuned the Mag Loop to 14.2 - all is well. Duh.

    Perhaps other can learn from my mistake.
    AA7QQ likes this.
  9. AA7QQ

    AA7QQ Ham Member QRZ Page

    Thank you for posting the (Somewhat embarrasing) outcome.
    Just goes to show that there are so many factors at play in the RF world.

  10. G3YRO

    G3YRO Ham Member QRZ Page

    Can't imagine how ANY antenna could cause distortion on a received signal . . .

    Roger G3YRO

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