Suggestions on Al-82

Discussion in 'Amateur Radio Amplifiers' started by N8ZJ, May 6, 2018.

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  1. N8ZJ

    N8ZJ Ham Member QRZ Page

    I am trying to fix a AL-82. I purchased the amplifier not knowing to much history other then it is a few years old (basically new) and it blows the step start resistor. I went through what I normally would as I have fixed many amps before and this one has me pulling my hair out. Everything I have tested good. Relays, filter board, rect board, etc .. When amplifier was shipped to me the blower fan broke nylon screws and and bounced around and nicked up the plate choke some so I replaced the plate choke. This problem was there before that though. So anyways , if I disconnect the red wire off the cap filter board (HV) that runs over to plate choke etc it turns on fine and shows 3600 volts around and step start resistor does not blow. The second I put the red (HV) wire back on the filter cap board its fireworks from the step start resistor. It torches and burns up quick. The plate choke is installed right as I have checked it a bunch of times (never hurts go back over your work). I have not bothered to put tubes in as I dont want to hurt them. So its not the tubes. I checked Zeners and they are good. Also went over the wiring on relays etc and they are to the schematic. Anyone ever had this problem before? Any suggestions to where to look? I am open to any idea's as this one has me stumped.. Thank You, N8ZJ Blayne
     
  2. NK7Z

    NK7Z Premium Subscriber QRZ Page

    I had an AL-82, and it lost a plate chike once, but never the step start... Caps maybe shorting at HV?
     
  3. AG5CK

    AG5CK Ham Member QRZ Page

    Looking at a picture of an al82 I see the hv from the cap bank is connected to a circuit board with the parasitic supressors and plate blocking capacitors.

    If your amplifier is built this way I would disconnect the hv from the parasitic suppressor board and see what happens and/or test the plate blocking caps to make sure one isn't shorted.

    Does the amplifier have the proper fuses installed? My al1200 will just pop a fuse if the hv is shorted to ground.
     
    Last edited: May 6, 2018
  4. N8ZJ

    N8ZJ Ham Member QRZ Page

    I forgot about most the time checking a doorknob cap usually its not going to show up on a multimeter or cap tester etc. Usually they wont show a short unless under load. My mind is very forgetfull so thank you for reminding me. I do see a small burn circle in one of the letters of the cap and when you look real hard you see a small crack on it. So later on today I will replace them as I have spares and update on here.. Thank you for the advice :)
     
  5. W1QJ

    W1QJ Ham Member QRZ Page

    By disconnecting the B+ hv at the power supply you prooved that the power supply section is ok. Be advised though that with tubes in the socket or even without if you use a cheap sand type resistor they sometimes are not good enough and will open up as it absorbs theninitial transient. You need to use a good quality enamel resistor. If you are using the correct resistor then ok. So the problem has to be from the base of the plate choke to the blocking cap. That doesn’t leave much. You have a bypass cap at the base of the plate choke to ground that you can suspect and maybe one of the blocking caps. The parasitic board would not be the problem nor zener diodes. There is no consequence to a shorted zener other than elavated idle current. So there isn’t too many things it can be. If you had tubes installed I’d suspect a bad tube. Please report what you find.
     
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If you're using the correct type of resistor in the step-start circuit and it blows with power on, that's a short circuit or very close to a short circuit following the power supply (since the power supply itself doesn't blow it).

    I'd suspect a HV bypass cap, like the one to chassis at the cold end of the plate RF choke. Or possibly a pinched wire, if there's any opportunity to pinch a wire in that path.
     
  7. N8ZJ

    N8ZJ Ham Member QRZ Page

    Update... after replacing the the 7.5kv 500pf caps the amp is up and working! I don't know why I forgot that those caps usually fail under high voltage and checking them when no voltage was applied and believing they were good was not very bright. So anyways amp works almost like it should. I am now trying to figure out why I am getting arcing around the tube area/parasitic board etc.. happens when tuning up , but usually around 1000-1100 watts etc.. I then think I fixed it and running legal limit talking it will arc randomly (on ssb btw).. Well I will get back on it tomorrow. I just wanted to update to let everyone know what it was and thank you for all your input. Really appreciate it !!
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Small arcs like that (often non-destructive) are often caused by an imperfect connection such as a solder joint that isn't 100% solid or any connection that isn't very tight.
     
  9. K0OKS

    K0OKS Ham Member QRZ Page

    Turn down the lights and figure out where exactly it is arcing. Then do some further investigation.

    Hope to hear it on the air soon.
     
  10. N5DMC

    N5DMC Ham Member QRZ Page

    Do these small arcs happen when connected into a dummy load?
     

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