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756PROIII - no RF output

Discussion in 'Radio Circuits, Repair & Performance' started by VE3TMT, Jun 2, 2018.

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  1. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    RF output on the PROIII went from 100W to less than a watt. I've replaced the drivers (2SC1972's) with a matched pair from Eleflow. No change.

    With the power supply on, but radio turned off, I get 0V on the bases of the drivers and 13.8V on the collectors. Bias is set at 0.68V as suggested.

    I'm now leaning towards the pre-driver, 2SC1971. It also measures 0V on the base on 13.8V on the collector.

    LCD looks as normal when transmitting, peak shows on screen and I can hear my signal on my RSP1A receiver very cleanly. So I've read that the UPC1678GV is still good in this case.

    It is possible for the pre-driver to show good voltages and still be faulty?

    Other things to check??

  2. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    My measurements in BLUE

    final amp voltages.jpg

  3. G3YRO

    G3YRO Ham Member QRZ Page

    I wouldn't go replacing Transistors at random !

    You can SO easily test them with the rig off, just using an analogue multimeter on a resistance range.

    Apart from a complete short circuit (which would cause fuses to blow, the most usual fault is the Emitter - Base junction goes open circuit.

    BUT - you need to lift the Base tab on ONE of the transistors to do a test, as being in push-pull they are almost connected in parallel (as far as DC testing is concerned)

    Roger G3YRO
    AI3V likes this.
  4. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    Hi Roger,

    I have already installed the new drivers with no change in problem. Voltages seem normal on the pre-driver. Although the RX state does give me a negative voltage reading. Is it possible the pre-driver is still bad, even though the voltages look right?

  5. G3YRO

    G3YRO Ham Member QRZ Page

    Perhaps, yes . . .

    I would certainly measure across the junctions with the rig turned off.

    And have you done the same with the PA Transistors? (ie checked EACH of them, by unsoldering the Base tab on one of them)

    And if the transistors all measure OK, I would be checking RF voltages in and out of each stage with a simple home-made diode probe, before changing any transistors.

    Incidentally . . . are you sure the new Drivers you bought are genuine? There are a lot of fake ones being sold now . . . they just mark any old similar-sized transistors with the Motorola logo and numbers. (Edit - I see you bought them from Eleflow, so hopefully they're OK . . . but I certainly wouldn't throw your old ones away - have you checked them with an ohmeter? If they are faulty it will be very obvious . . . transistors don't go slightly faulty!)

    Roger G3YRO
    Last edited: Jun 2, 2018
    AI3V likes this.
  6. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    Step 5: (Collector to Emitter)

    "Hook the positive meter lead to the COLLECTOR (C) and the negative meter lead to the EMITTER (E) – A good NPN or PNP transistor will read "OL"/Over Limit on the meter. Swap the leads (Positive to Emitter and Negative to Collector) – Once again, a good NPN or PNP transistor should read “OL”."

    With the transistor completely out of circuit, positive to collector and negative to emitter, reads "OL", positive to emitter and negative to collector reads 0.639V, not "OL".
    Last edited: Jun 2, 2018
  7. G3YRO

    G3YRO Ham Member QRZ Page

    Not quite sure where those instructions are from . . . your test meter? Is it a Digital one?

    I've often found those to be very misleading for all kinds of tests . . . as they're too high impedance!

    Personally, I would always use an Analogue Meter to test a transistor . . . as then you know you are passing some current to properly bias the junction.

    A Transistor will often show a resistance between Emitter and Collector . . .

    What matters is testing between the Base and the Emitter . . . and the Base and the Collector.

    On each of the above tests, if the Transistor is OK, with the leads one way round you should see a very high resistance . . . the other way you will usually see about half scale (the actual resistance reading is meaningless, as it depends on the voltage being applied and the actual current flow through the internal resistors in the meter)

    A blown transistor will either show a dead short circuit . . . or just as often, the Emitter - Base junction will be open circuit (ie no reading with the leads either way round) - this often happens with RF Power Transistors (as used for Drivers or PAs)

    Roger G3YRO
  8. W9GB

    W9GB Ham Member QRZ Page

    Did you bother to check the T/R circuit (relays)?

    R101. It's located on the rear of the RF B unit, which is the board that contains the XVRT and RANT jacks.

    The Band-pass filters are diode switched.
  9. VE3TMT

    VE3TMT XML Subscriber QRZ Page

    TX/RX is switching just fine. Also checked the regulators on the PA board, all test within spec. With the 2SC1971 pre-driver out of circuit, I'm getting just a little over 2V out of IC201 on RF-B unit, this feeds the input of Q!, the 2SC1971. Nothing after that.

  10. CX3CP

    CX3CP Ham Member QRZ Page

    Hi Max ... If you decide to find the problem and repair it, I would suggest you incorporate some test equipment to your bench to help you in this issue. So I would start first by isolating the RF-B unit from PA stage, then check the signal from the RF stage with an oscilloscope or better yet with a spectrum analyzer if available. You have to know first if there is any signal coming from the RF stage and determine the quality of it ( about 0dBm in max CW mode). Confirming that the signal is good, then continue with the PA stage. If not, then you must see backward from the PA unit to the RF-B unit and the previous stages.

    Put the Icom in CW mode, unplug the J201 in the RF-B board chose 20 meters band, go TX and looking for CW signal from the RF-B unit.

    Another interesting test that can be done is to excite the final stage with a signal generator. This test can also give us an accurate idea if the PA stage works correctly or not. do not forget to select the appropriate band to which we are doing the test. This is by chosing the correct LPF with the band switch in the transceiver. useful article about RF probe

    An RF probe also give you some idea of RF presence.


    Good luck,

    Albert CX3CP .... 73´s

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