Ameritron AL-572 loss of HV reading question

Discussion in 'Amateur Radio Amplifiers' started by N0OHL, Sep 7, 2020.

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  1. N0OHL

    N0OHL Ham Member QRZ Page

    A week ago one of the fuses opened when I turned on my Ameritron AL-572. The fuse was replaced and I was able to power-up the AMP. However,,, When I attempted to use it I am greeted with zero-0 output and high SWR . I started to look around and notice that the HV multimeter reading is 0zero when the amp is turned on. And Grid and Plate readings are also zero when Plate or Load dials are turned.

    I have not opened it up yet but I see the tubes glowing inside. Next to open up and start looking. After grounding of course.

    Any thing you can think of I should take a close look at ? It's a used AMp that I have had and using for 5 years now....
  2. N0OHL

    N0OHL Ham Member QRZ Page

    Forgot to say --- meter lights on and TX red light comes on when PPT
  3. W1QJ

    W1QJ Ham Member QRZ Page

    You lost HV somewhere, somehow. You have to follow the path from power supply to tubes. My guess is if you don’t have any reading on the meter that the power supply isn’t making it. Probably no secondary voltage getting to the rectifier or a problem in the rectifier itself.
  4. N0OHL

    N0OHL Ham Member QRZ Page

    that's what I was afraid of. Any simple way to check Power supply secondary output? I don't think it is HV there is it? I probably need to get out the schematic. I don't have any HV testers.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    You can easily and safely check to see if you have an open diode in the rectifier with the amp off and checking each diode with your VOM on diode check. Since the HV is metered right off the PS board it is generally unlikely that you would not have both no reading on the HV meter and no HV on the tubes. Usually it is one or the other but rarely both. That does not mean it is impossible. NEVER EVER believe that because the panel meter does not read HV that there is none. That is a good way to get killed real quick. Did you check both fuses? Although with only one transformer if the filaments light up and you have 12v then the transformer is getting primary voltage but fr some reason you may not be making HV. Sometimes your meter may not read HV but the amp makes power so that means you do have HV but the meter circuit is the problem. You may have a problem with a capacitor being open somehow. You just have to take it step by step and see where there is a loss of voltage. Just be careful!!
    KH2ZZ likes this.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    I presume somebody hasn't modified the Amp and put a fuse in the HT line (which has blown) ? (but it would have to be BEFORE the HT metering takeoff point)

    If not, then - as mentioned - you may just have an open-circuit diode in the HT rectifier circuit - if so, this is a very cheap and simple fix ! (however this is pretty rare . . . and most silicon diodes go short-circuit when they fail, not open)

    Roger G3YRO
  7. N0OHL

    N0OHL Ham Member QRZ Page

    Txs guys.... Will open today -- ground out, and start looking first... Hope I find a big burned out Cap , Diode or something easy like that.
    I should have waited to post until I had taken a look inside, but haven't heard of a partial mostly "sort-of-dead" AMP, so thought I would ask.

    Roger -- wouldn't it be nice to find a fuse in there!!!!

    Will let you know what is found.... TXS

    Chuck N0OHL
    KH2ZZ likes this.
  8. KH2ZZ

    KH2ZZ XML Subscriber QRZ Page

  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Easiest way is with a second (external) meter rated for safe use at 5kV, and measuring with the cover off and the interlock defeated so the amp will power on with the cover removed.

    That can all be safely performed by someone who knows what they're doing and has a 5kV (or higher) rated test meter with appropriate test leads. Not recommended for anyone who would actually need to ask about this.:)
    K4AGO, W4NNF and KN1M like this.
  10. N0OHL

    N0OHL Ham Member QRZ Page

    Open ---
    Nothing stands out as bad--
    Both fuses OK
    interest - found part of old 2N 3906 Diode lying inside -- Q2 appears to be replaced -- see pic-
    Cap - distorted -- board and 270 ones - see pics
    Power Diodes, resisters tested OK

    Will change input RIMG0001.JPG RIMG0001.JPG RIMG0002.JPG RIMG0003.JPG RIMG0004.JPG RIMG0005.JPG RIMG0006.JPG RIMG0007.JPG RIMG0008.JPG voltage to 125 - I only have 125v variac
    I see three secondary transformer leads -- r/g r/y and red -- the HV AC will be RED?

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