Ameritron AL-80B out of service

Discussion in 'Amateur Radio Amplifiers' started by KE8CTG, May 18, 2019.

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  1. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Where there's smoke, there's evidence!
    Flashlight, magnifying lens, nose, eyes.
    Find what burned. Answers will become apparent.
     
  2. KE8CTG

    KE8CTG Ham Member QRZ Page

    20190519_154440.jpg 20190519_154522.jpg

    OK guys, I checked continuity of both the fuses at the back of the amp and one was bad, the other still good. Then i popped the top cover off looking for any smoke residue burnt wiring or any other thing obvious and found nothing. The flash I seen came from the right hand side of the amp as you are facing it, I seen it through the vent on the top. Attached are pictures from the suspect area. Let me know if you see anything obvious. I'm not an electronics expert whatsoever and am not comfortable poking around in there, so I'll probably have to find someone who can work on this for me.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Nothing obvious to me but I don't have a big screen here (remote).

    If a fuse blew, that's either a high-energy arc or a short circuit somewhere. A typical low-energy arc generally won't blow the fuse.

    If only one fuse blew and not both: Are you using the amp on 120V and not 240V?

    Rather than bother with 21 screws each time, it's safe to replace the cover and install maybe one screw in the top to be sure the power interlock is engaged, and leave the other 20 out "each time" you want to test something. (I think that's how many screws there are! -- I'm not with the amp right now.) You just need to engage the power interlock switch which is in the left rear upper corner of the amp (viewed from front) and keep the cover in place to keep your hands and other body parts out of it.:)

    With that in mind, you might try (once the cover is off, and the HV is drained down to "zero," and the amp is unplugged from AC power) removing the anode cap from the tube, then removing the tube altogether and setting it aside someplace safe, replacing the blown fuse, and powering the unit "on" again to see if it powers up.*

    If it does, and HV indicated on the meter is normal (around 3000V), then you don't have a power supply failure -- which is a good thing. That isolates the problem to either the tube itself or something in the RF compartment, outside the power supply. If it's the tube, there goes $239 or so. If it's something else, it might be cheap or even free to repair once it's located.

    [*Always unplug the AC power cord and make sure the HV is discharged down to zero before working inside; after working inside, always replace the cover, put in at least one screw to hold it in place, and plug the power cord in again.]
     
    Last edited: May 19, 2019
    KN4CTD and K7TRF like this.
  4. KE8CTG

    KE8CTG Ham Member QRZ Page

    Thank you for the reply, I am running on 120V household current.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That explains why only one fuse blew.
     
  6. AG5CK

    AG5CK Ham Member QRZ Page

    Even on 240 sometimes only one will blow. I like to use a clip lead from b+ to chassis when working on amplifiers and I've forgotten to take them out a couple of times. :oops:
     
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  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Assuming the panel voltmeter is functional (usually is) I just watch the meter as it drops down to <100Vdc (takes a minute or so) or even down to 0V (takes longer) and consider it safe. The real danger is the AC power line connections which are pretty exposed. Remember to unplug the power cord!
     
  8. AG5CK

    AG5CK Ham Member QRZ Page

    I do the same. The jumper is not necessary but makes me feel better. Just one more mental check before my hands go in there.
     
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    It's just one more thing for me to forget before I turn the amp back on... :eek::p
     
    KN4CTD likes this.
  10. W1QJ

    W1QJ Ham Member QRZ Page

    The 3 BIG Ameritron amps, AL-1500/1200/82, all having the same power supply platform usually will only blow one fuse in a fault. Since the filament transformer has it's own fuse, often times what will happen is that one line fuse will blow and the HV supply will shut down but the filaments will still be lit, the fan still blows, and the panel lights still work. If you have the meter function switch in PO you will not see that the HV is off but you can still key the amp. You won't see any output, and you'll probably see high swr on your radio. This could be very confusing if you are not aware of what happened. So keep this in mind if you own one of these.
     
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