4-400 turning blue.

Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, May 15, 2015.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    I have checked the sockets and the seem fine.
     
  2. KB4MNG

    KB4MNG Ham Member QRZ Page

    I can get the same results on 80, I was using the wrong coupling switch setting.
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Without being there it's hard to tell, but 'sounds' like the amp may be oscillating. That would cause high plate current which changes with plate tuning.

    Usually if that occurs you'd see some RF output on a wattmeter, with a high SWR into any kind of load except a good dummy load.

    It may need a neutralization adjustment. Look at the resistors in the plate parasitic suppressors to see if they look overheated.
     
  4. KB4MNG

    KB4MNG Ham Member QRZ Page

    I will look at the neutralization issue. Is that the chrome device that sits by the tubes that has a screw adjustment on top? It adjust a cylinder going into a cylinder. When I started on the amp, I had adjust (bend) it so the two cylinders were not touching on their sides.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yeah, it is...but do not adjust it without a means of measuring exactly what you're do ing. This is not a hit or miss adjustment and you can't guess at it.

    Takes at minimum an RF millivoltmeter. I've often used an old Boonton 92A for this, which works fine. And you do it with the screen voltage turned OFF, which in the T-bolt takes some special precautions. If you don't have the equipment or experience, I wouldn't recommend making this adjustment.
     
  6. KA9JLM

    KA9JLM Ham Member QRZ Page

    That sounds like the one that UPS destroyed.
     

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