Alpha 374A or 76 series power switch

Discussion in 'Amateur Radio Amplifiers' started by AD0AC, Sep 8, 2019.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    This thread has been going on for a while now and it seems no resolution as of yet. I had a look at the schematic at this amp just now. I had worked on a couple of these amps and one actually required the replacement of K1 and K2. They were burned out badly. Here is a set of possible problems I can see could prevent K1 (the on contactor) from turning on. You should be certain NONE of the following are the culprit.
    1. On switch bad
    2.Off switch bad
    3.F1 or F2 open
    3.R4 open or out of spec
    4.R3 open or out of spec
    5.K1 coil open
    6. Contacts on K7 shorted closed
    7.S5 interlock bad
    8. Both phases of 240v line not present
    9. Amp not properly strapped for 240v operation

    Those are a few causes I could find with a few minute examination of the schematic, there may be others but for sure you MUST check ALL of the above because ANY of or combination of the above will keep K1 from pulling in.
  2. KM1H

    KM1H Ham Member QRZ Page

    All the ones I repaired had relay problems

  3. AE1P

    AE1P XML Subscriber QRZ Page

    Any updates ?
    Following with interest as my 76A has just showed the same power up when power button pushed on, both F1,F2 good...
    Haven't pulled cover yet...that's next...
  4. AD0AC

    AD0AC Ham Member QRZ Page

    Not yet. One of my other hobbies is woodworking and I’ve been building some stuff in the evenings and weekends for Christmas presents. Plan is to pull the cover again and test from the mains forward to see where the voltage drops out without the turned on.
    AE1P likes this.
  5. AE1P

    AE1P XML Subscriber QRZ Page

    Finally got around to taking the cover off and doing some digging...
    here is at least one confirmed culprit...
    Alpha1.jpg Alpha2.jpg Alpha3.jpg
    N8ZI likes this.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Now to find the reason the resistor blew up.:)
    AE1P likes this.
  7. AE1P

    AE1P XML Subscriber QRZ Page

    I agree, Steve...start with testing relay coils...:)
  8. AE1P

    AE1P XML Subscriber QRZ Page

    So it was definitely R3 that blew up on the Guardian K1 power on relay...
    I'm not finding anything obvious as to what may have caused this...
    R1,2 & 4 test ok, and all resisters on control board are looks like one
    side of the R3 resister is tied to K7, the overcurrent protection relay...
    Thoughts? suggestions?
  9. W1QJ

    W1QJ Ham Member QRZ Page

    Is there anyone still following this thread that actually has a 76a amp? I have one question. When you key the amp up into transmit with no RF drive at all, do you see the grid meter go up to about 10 ma?
  10. N8ZI

    N8ZI XML Subscriber QRZ Page

    Yes, I'm still following this thread and yes I have the Alpha 374A which is a part of the 76 series of amplifiers. Not to mention my amp is having intermittent issues with switching into TX.

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