Ameritron AL-80B out of service

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

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  1. KE8CTG

    KE8CTG Ham Member QRZ Page

    Folks,

    I have an Ameritron AL 80B I bought new approximately one year ago and was tuning up on 40M this morning (7.218) into my dummy load and heard a snap and seen a flash inside the amp then the amp went dead and I could detect a burnt electrical smell. The SWR was 1:1 and the amp was set to the correct band. The transceiver and everything else is working fine on the station. I'm no electrical guru but wondered if anyone has any ideas what went wrong.

    Thanks in advance, Blaine
     
  2. KP4SX

    KP4SX XML Subscriber QRZ Page

    Maybe a bug crawled in there :eek:

    Did it blow the fuse?
     
  3. KE8CTG

    KE8CTG Ham Member QRZ Page

    I haven't looked yet but am checking now, I have my suspicion that its more than the fuse with the flash I seen inside the cabinet and the burnt electrical smell.
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Once the cover is off, almost everything is exposed and visible.

    Look carefully (with good lighting) at the bandswitch contacts, everything in the plate tank area, and rotate the LOAD capacitor slowly through its range looking for any signs of arcing.

    Although they may have changed this over the years, my AL-80B uses a 3PDT relay for T/R switching and obviously that's a very critical component: If the relay contacts get dirty, oxidized or misaligned (purely from lots of use) and the "output" side contacts aren't making solid connection, the RF generated by the tube has nowhere to go and lots of bad things can happen including arcing. Unless an arc carries a lot of energy and is sustained a while, that usually won't blow the fuses. A real "short circuit" anywhere will.
     
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    From your description it sounds like you had some kind of internal short and blew the fuse. At least I take 'and then the amp went dead' as meaning you blew a breaker/fuse. My first bet would be an internal anode to grid tube short which may have done additional damage such as to the B- protection diode. I'd pull the tube after safely discharging the HV supply and reassemble the finals cage so that the B+ shorting switch is happy, turn the rig back on and check the HV supply on the front panel meter. I'd probably also check D117, the B- protection diode with a DVM while the amp is opened up. Since you mentioned a burnt electrical smell I'd also ohm out R132 and R133 the two pulse rated HV glitch resistors in the B+ supply line, ideally these shouldn't blow in the event of a tube short but it can happen.

    You could have some other issue like a failed cap in the HV filter bank but the HV test with the tube removed would show problems like that. IOW, if the amp still blows fuses with the tube removed then there's a problem in the supplies.
     
  6. KE8CTG

    KE8CTG Ham Member QRZ Page

    Thank you all for the quick responses, I did pull both fuses off the back and did not notice any scorched area on them. I will have to check continuity though with a multi-meter but mine is whomped out. Tomorrow I will pull the cover off and look for anything obvious and upload pics if anything is obvious.
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Definitely check for continuity through the fuses, but I don't recall if the AL-80b has a soft start circuit. If so or if one was added then I'd check the resistors on the soft start board and if one or both is blown then check the soft start relay as that's another potential failure point that would result in a dead amp even if the fuses didn't blow.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The AL-80B does have a soft-start circuit with a separate relay for that. But it's only engaged for about 30 mS or something.
     
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, I just took a gander at the schematic. If the relay is doing its job then the soft start resistor and fuse should only be in circuit for a handful of milliseconds but if the relay has failed in its normally open state or has very dirty contacts the OP could easily have burnt up that soft start resistor or blown the associated fuse. That seems like a possibility worth checking out considering he smelled something burning and apparently has no power even though the mains fuses look good.
     
    Last edited: May 19, 2019
  10. AG5CK

    AG5CK Ham Member QRZ Page

    Did you check the safety switch, or at least give it the sniff test? I can't remember if it only kills the mains to the hv transformer or everything.
     

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