Help with Alpha 77SX

Discussion in 'Amateur Radio Amplifiers' started by K8SU, Mar 18, 2020.

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  1. K8SU

    K8SU Ham Member QRZ Page

    Have Alpha 77SX that has an issue with Electronic Bias. When amp is turned on getting about 350 to 400 ma of plate current. After seeing this checked the tube bias voltage and getting 0.74 volts with and without tubes.

    Have gone through most on the components in the CB-2 Board.

    I am open to any tests and wondering if anyone has any insight. My understanding is I should have 56 volts of boas to cut off tubes when just sitting. When mic is keyed should go to about 15 volts and then when RF is detected it should drop to 8 volts.

    Thanking the ham community in advance. Amp was working great and only chafe was driving the amp to a different location.

    thank you

  2. W1QJ

    W1QJ Ham Member QRZ Page

    Kevin, have you tried a different tube to make sure you don’t have a bad tube? If the tube is known good, Q 202 is the electronic bias pass transistor. It acts as a switch to turn on the 8.2 v zener operating bias diode. This transistor should only come on when a very low RF signal is present at the input of the amp. The cut off bias should remain on until Q202 turns on and then the bias changes to operating bias of. 8.2 volts. This bias will switch on and off as the RF appears. Your problem seems to indicate that you have operating bias as soon as the amp is in transmit and that it is also way to low. That would indicate that the pass transistor Q202 and bias zener are both most likely shorted. This typically happens with a high energy tube arc which takes out both. I’d focus my attention on both those parts.
  3. K8SU

    K8SU Ham Member QRZ Page

    Thanks for the tips and tested both of those components and both tested OK. The odd thing is I am getting 0.74 volts with tubes in the amp and completely removed from the amp in the unkeyed state. I believe I should be seeing 56 volts at that point then 15 to 18 volts when the amp is keyed and 8 volts when the RF coming into the amp is sensed and converted to DC to activate the circuit with the Zener.
    Will check parts again in question and report back.

    Thank you very much
  4. HS0ZED

    HS0ZED Ham Member QRZ Page

    Sounds like you need to start at the source of the 56 volts. Pin 20 on P201 should be about 40 volts AC, D215 rectifies and C12 smooths. Result should be around 55 volts or so. That goes through R207 direct to the Cathode I think.
  5. K8SU

    K8SU Ham Member QRZ Page

    Thank you very much for the reply. You are correct and did that early on. I am losing the 56 volts on the far side of R207. I have pulled the board and checked components. Everything test fine. I am thinking there may me something with one of the pins when the board is installed that is causing the issue.

    Any additional thoughts always appreciated.


  6. HS0ZED

    HS0ZED Ham Member QRZ Page

    D206 short?
    C201 leaking?

    Pull the board and measure in isolation, that will at least prove it to the board or chassis.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Kevin, Alpha made an extension boards so that one could make voltage tests on the board easily. Comes in very handy with those amps. If you are still watching this thread Kevin, DO you have the 56 volts to begin with? I think you said you do. I learned this the hard way, ALWAYS be weary of these small little -01uf bypass caps that are on any DC voltage line ANYWHERE along its travel. The capacitor can change into a high impedance type resistance and bleed all your voltage off to ground. I found this problem working o a Titan 417 which would blow a fuse as soon as the amp was keyed into transmit. Drove me nuts. I finally traced it to a bad bypass cap on the grid bias line to the tube. The cap was drawing the voltage down so low the tube was going ballistic. It was that damn little .01uf bypass cap, rated a 250v. I measured it at several megohms. N4ATS came on asking about a similar problem on a 417 he was working on at E-HAM and I saw it, told him to check C3XX, he did, that was the problem. SO check all your bypass caps.
    K0UO likes this.
  8. W1VT

    W1VT Ham Member QRZ Page

    Capacitors degrading to a few megohms isn't surprising to me. With a constant DC voltage one is likely to have metal migration issues unless you carefully choose the parts. And a few megohms won't bother most applications. May be useful to study up on parts used for ultra low power applications.

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