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Ameritron AL80-B blowing fuse on power up

Discussion in 'Amplifiers' started by mattsowders1989, Nov 8, 2017.

  1. mattsowders1989

    mattsowders1989
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    Ok..... This has me scratching my head. I was using the amplifier and all of a sudden it was like i unplugged it from the wall. i bled the tube and checked it, it checks good. i dont see any burnt components. i just dont know where to start to figure out what happened. when this happened, i had a different antenna hooked up that has a little standing wave in it (1.9:1). thats what i believe may have got it. i dont know. any help would be greatly appreciated.


     

  2. Tallman

    Tallman
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    Do you have an ohm meter?
    Ameritron AL80-B power supply.jpg
     
  3. nomadradio

    nomadradio
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    Checked the fuse yet?

    First knee-jerk thing to suggest is the filter capacitors in the high-voltage section. There should be a four-digit number on the filter capacitors, possibly preceded by a three-digit manufacturer's ID code in the format of "235-xxxx". Seeing the first three digits "235" would indicate caps made by Mallory. Might be a different three digits depending on the factory they came from. What we're really interested in are the following four digits. Typically two digits for the year, followed by two digits for the week of that year, 01-52.

    So, caps marked "9225" would have been produced the 25th week of 1992.

    Depending on mileage, they can go bad as soon as ten or fifteen years after they were made under heavy use, or might last for 25.

    Might.

    Tricky part is testing them. You didn't hear any "snap!" or "crack!" sorts of sound come from it, did you? That sound, along with a date code over 20 years old would suggest that it's time for a new set of filter caps. If one of these shows a short with continuity test, it's bad. But the chemistry inside can cause a short to "heal" and test okay with the low voltage from a meter. Typical behavior when a cap is just too old will be to break down when you power it up and apply normal voltage to the cap. You'll hear that "snap!" again if so. The entire set of eight caps should be replaced if one is bad. Changing just one of them will become a game of "electronic whack-a-mole" as the rest of them fail one or two at a time.

    Or maybe you just had a defective fuse and the amplifier itself is perfectly okay. Sometime fuses just fail from wear and tear even when there is no fault to trip them.

    It's not just the years, it's the miles too. If the amplifier saw a lot of regular use, the life of the filter caps tends to be shorter than if it was used say, once a week.

    73
     
  4. mattsowders1989

    mattsowders1989
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    i know it isnt just a faulty fuse. every time i replaced the fuse, it immediately blew it. when i get home from work, i will check those caps. since the tube tested good on the tube tester, im hoping it is those caps. if not, ill be coming to see you with Forrest. Thanks nomad!
     
  5. Tallman

    Tallman
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    Be sure to unplug from the wall and short each capacitor before attaching the Ohm meter. Leave the shorting probe or the jumper lead attached at least for one minute or longer to bleed the voltages off.
    Diodes can go bad too. You might need to desolder one end of the parts to get true readings.
    I had one amp come in that had the bridge rectifier replaced five or six times because one side of the bridge was shorted out. It was NOT the bridge shorted, it was reading the transformer winding.
     
    #5 Tallman, Nov 8, 2017
    Last edited: Nov 8, 2017
  6. mattsowders1989

    mattsowders1989
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    Thank you tallman!! I appreciate you too!! Lots of good info!
     
  7. nomadradio

    nomadradio
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    When you said you didn't see any bad parts, that included the fuse.

    It's a part.

    And turns out it's bad.

    A blown fuse indicates a breakdown.

    Makes it sound more and more as if maybe those filter capacitors are original after all.

    73
     
  8. wavrider

    wavrider
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    so many different troubleshooting techniques can be used.
    I prefer using an DVM and resistance checks, no power applied.
    Or removing each cap and testing individually on the capacitance tester.

    Short sweet and easy.
    Check each diode using the diode function on your DVM.
    Then check your caps to see if they are good.
    ..

    There is LETHAL voltages in that amp, if you do not know how to work safely around HV then take it to a qualified repair facility.
    Not worth getting hurt.
     
  9. mattsowders1989

    mattsowders1989
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    Well it has became "fuses" now. still no visible component damage, other than the fuses. i replaced the fuse blown and powered it up again to see if i heard any sounds. nothing.... this is when i noticed that another fuse had blown. when i powered it up again, it didnt blow the fuse on the outside, but it didnt light up either. i checked the fuse on the inside and it was blown. last thing i did was replace the fuses and powered it on again. still no sounds of any kind. just blown fuses. im at a loss.......
     
  10. BlowinSmoke

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    Have you taken the tube out of the amp and turned the amp on with no tube in it???? This is the first thing you do when something like this happens. YOU MUST remove the tube completely and turn the mp on and see if the fuse blows. You CAN NOT test for a bad tube with an ohm meter, Can't! Turn amp on with no tube, if the fuse doesn't blow then you need a new tube. EOS
     
  11. mattsowders1989

    mattsowders1989
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    The tube was checked with a tube tester that I know for a fact is good and accurate. Yes the tube was taken out and amp still throws fuses.
     
  12. nomadradio

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    You can check for a BAD with a meter. If the filament is open, or if it shows a short from cathode to grid, that's a conclusive test. That tube is bad.

    What you CAN'T check with a meter is a good tube. Gotta put operating voltages on it for that.

    Even a tube tester has limits. If it says the tube is bad, you should believe it.

    If it says the tube is good, this only means it's safe to try in the equipment. If the radio doesn't like that tube, this is the only opinion that matters, whatever a tester might say.

    73
     
  13. mattsowders1989

    mattsowders1989
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    Well I still have no idea what could be causing the issue. It does it with the tube out completely. I'm thinking rectifier or caps. At least I've got my problem narrowed down. Thanks for your alls help.
     
  14. BlowinSmoke

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    Check the diodes, that is easy enough to do, takes 2 minutes. Then isolate HV secondary from the rectifier and see what happens. If fuse blows then, its a bad transformer. If not then probably a bad filter caps.
     
    Tallman likes this.
  15. wavrider

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    I do not have schematic in front of me
    I once had a SB1000 the input voltage relay was bad and blew the fuses.
    once again use ohm meter to find the direct short to ground.

    With the power switch turned on start tracing input voltage, you will eventually find your short, the cause of the blown fuses.
     

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