Problem with the ETO ALPHA 77SX amplifier.

Discussion in 'Amateur Radio Amplifiers' started by DL6AAR, May 1, 2015.

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  1. DL6AAR

    DL6AAR Ham Member QRZ Page

    I have a problem with my ALPHA 77SX amplifier, that i have rebuilde.
    The probleme is : When i switch the amp on, the meter slowly run to a current from ca. 100mA.
    I measure - with a external meter - on the kathode from the tubes in this moment 56V. - Ground meter to chassis and plus meter to kathode tube - .The data sheet for the 3CX1500A7 shows, there cannot run 100mA current with 56V on the kathode.
    I have test this problem with other tubes, it is the same problem. Then it is not a tube problem.
    A secound problem is, when i switch the amp. relais then on - PTT- , i hear the relais, but the U kathode change not to 18V there are still 56V.

    Can anybody help me?
    Please excuse my bad english

    vy 73 de friedrich dl6aar


    P.S. I looking also for spare parts for this amp. Electrical or mechanical.
  2. W9GB

    W9GB Ham Member QRZ Page

    Last edited: May 2, 2015
  3. W1QJ

    W1QJ Ham Member QRZ Page

    That amp has electronic bias. I believe there is a MOSFET transistor perhaps a 2N3055. It is probably shorted.
  4. W9GB

    W9GB Ham Member QRZ Page

    Friedrich -

    I assume you have the manual?

    ETO ALPHA 77SX and 77DX
    Operating and Technical Manual
    circa January 1990

    Standard Electrical Parts, page 32.
    Standard Troubleshooting Hints, page 14.
    Theory of Operation - Low Voltage and Misc. Circuits, pages 17-19.
    Bias Switch transistors are Q202 and Q203
    These transistors are found on Circuit Board, CB2 (plug-in card).
    You mention that you are "missing parts".
    ALPHA 77D / Dx / Sx PARTS LIST

    IF this is a Conversion of an ETO Alpha Medical Amplifier (using 8877),
    THEN some parts and assemblies (PC boards, electrical parts) may require DIY fabrication.
    Alpha / RKR Designs has limited parts for 77Dx/Sx amplifier models.

    Dick Byrd, N4UQ has "converted and rebuilt" some former ETO Alpha medical versions.
    Your BEST SOURCE for Hard to Find parts, with Alpha 77 and 77Dx/Sx models.
    Last edited: May 2, 2015
  5. DL6AAR

    DL6AAR Ham Member QRZ Page

    Dear OMs many thanks for your response.
    W9GB: I hope, i found the problem, if not, i send N4UQ a short email with my problem, maybe i can send him the PCBs to test and service them.
    W1QJ: I have measured the bias switching transistor, but i found no failure.
    The other point is, on the kathode are 56V, when i look in the data sheet from the 3CX1500A7, it is inpossible, that the tube run with 100mA current.
    But she does.

    friedrich dl6aar
  6. KM1H

    KM1H Ham Member QRZ Page

    Remove the tube and see if the voltage is still there.

    Lou, the 2N3055 is a bipolar, a very old and popular PS regulator.

    W9GB likes this.
  7. KO6WB

    KO6WB Premium Subscriber QRZ Page

    Also check the grid for proper grounding. If it is resistive that could produce a problem such as this.

    Hope this helps
    K5RT likes this.
  8. W9GB

    W9GB Ham Member QRZ Page

    Friedrich -

    Focus your component level troubleshooting to the CB2 (Control Board) and CB3 (QSK board).

    Following the schematic diagrams (pages 30, 31) from Transformer secondaries to these
    2 boards and the RF tube --- CHECK Diodes, Zener Diodes, and Resistor values.
    When you stated that Voltage does not change, with T/R operation,
    then CB3 switching of that voltage must be also examined.

    IF this 77Sx amplifier is INCOMPLETE (you are DIY re-building from parts or assemblies) ---
    THEN it is really a DIY Build, rather than a restoration of a factory-built Alpha 77Sx.
    Perform as much component testing/value readings
    with this amplifier disconnected from AC mains and de-energized.
  9. KM1H

    KM1H Ham Member QRZ Page

    MOST 77SX's left the factory as DX's; I converted my share when that was a common occurance. Bypassing the EBS was also popular.
  10. W9GB

    W9GB Ham Member QRZ Page

    Good points. What we don't know -- is what he started with?
    Frederich -

    YouTube video of 77-Sx ... restored by Dick Byrd.

    At 3:00 --- scan the internal layout of amplifier (may be useful if you had parts)

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