Need help troubleshooting Omni VI Plus

Discussion in 'Radio Circuits, Repair & Performance' started by K5DH, Sep 11, 2020.

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  1. K5DH

    K5DH Ham Member QRZ Page

    I recently bought an Omni VI Plus. It was an estate piece... "lights up, receives, no antenna to test transmit." Yeah, I know. Despite the big red flag, I bought it anyway. Everything seems to work fine... except for one little thing...

    Transmitted RF is getting into the receiver! I have a problem with the LPF board that I can't pinpoint, and I'm sure I must be overlooking something obvious. Using my Astron RS-35M power supply, Kenwood SW-2000 Wattmeter, and Drake DL-1000 dummy load for testing. Key down, start dialing up RF power, and output stops increasing on my Wattmeter at about 40 Watts, but PS current continues to climb to about 23 A. The little resistors on the Front End board which form the Attenuator got toasted. Disconnect the coaxial cable from the LPF board at J17 (the feed to the receiver) and the transmitter works as it should, with all 100+ Watts going out the Antenna jack. Below is a snip of the schematic with my voltage measurements added in. When I first started working on the rig, I found D6 literally blown off the board! (lightning strike?). I have replaced D6-D8 (1N4007) and D9-D14 (1N4148)... diodes are cheap. All of the measured voltages seem reasonable and I don't see why the diodes are not blocking the transmitted signal from the receiver signal path. Here is the circuit description from the TT manual:

    "During transmit, relay K1 is closed and passes the filtered PA output to the Main Antenna connector. A small sample of the transmit signal is passed through C12 to voltage doubler D11 - D14. They develop about 150 volts of reverse bias for diode switch D6, D7, and D8 to protect these diodes and prevent the transmitter signal from leaking into the receiver front end."

    I assume the 150 V developed by D11-D14 is with 100 Watts applied. At 40 W, I get 90 V. Seems reasonable.

    What am I missing here?

    [​IMG]

    I must be missing something obvious... can't see the forest for the trees, so to speak. Because someone will ask... the switching between Main and Aux RX antennas works fine.

    I sure wish TT had included a voltage chart in the Omni VI Plus manual, like they did for the old PTO-era rigs.

    Any suggestions?
     
  2. K5DH

    K5DH Ham Member QRZ Page

    No one has any ideas to offer?
     
  3. N7EKU

    N7EKU Ham Member QRZ Page

    How are:

    L4
    Q1
    R10
    D9
     
  4. K5DH

    K5DH Ham Member QRZ Page

    L4: Checked good (100 uH coil measures about 102 uH)
    Q1: Switching like it's supposed to
    R10: Checked good (47k resistor measures about 46.6k)
    D9: Replaced with new part, so presumed good
     
  5. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    What is the voltage at the junction of R10 and D9 during transmit?
    Also, what is the voltage at the junction of R10 and the collector of Q1 during transmit?
     
    Last edited: Sep 16, 2020
  6. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    Also, you must be transmitting ... just like you did with the 40 watt measurement.
    The other tx voltages have no meaning; because, you will not have any TX bias voltage developed at the the point of measurement.
    This voltage is what reverse biases the diodes D7 and D8.
    You should measure the voltages I requested and repeat the other transmit voltage measurements.
    Once you have done that, it will hopefully point to the problem.
     
  7. N7EKU

    N7EKU Ham Member QRZ Page

    Yes,

    At what point is the -90V gone?

    73, Mark
     
  8. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    Exactly! Once he measures the transmit voltages at the request points, we will be able to tell.
     
  9. N7EKU

    N7EKU Ham Member QRZ Page

    Did you install D9 in the correct direction?
     
  10. KF4ZGZ

    KF4ZGZ Ham Member QRZ Page

    I'm not board level proficient .... but just reading along ......
    Sounds like relay K1 may be bad or leaky?
     

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