SB-200 Damaged plate on "TUNE" capacitor

Discussion in 'Amateur Radio Amplifiers' started by KJ4AQU, Apr 1, 2012.

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  1. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Should have posted this earlier but I noticed the tune cap has a damaged moveable plate like it had been arcing to a fixed plate. See picture. I know blurry but it is the 6th one up from the front. Can anything be done besides replacing the whole thing?
     

    Attached Files:

  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    It's easier to replace the capacitor than to rework it.

    Depends how much you value your time.
     
  3. WC5P

    WC5P Premium Subscriber QRZ Page

    FYI, I just saw an SB-200 tuning capacitor on E-Bay. I think the current bid was about 20 bucks.
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If it's new and undamaged, that's a very good deal, I'd grab it.
     
  5. AG6K

    AG6K Ham Member QRZ Page

     A thin flat file can be used to smooth up arced plates. However, it might be a good idea to figure out what caused the Tune-C plates to arc and correct the problem before you lose a tube.
     
  6. W1QJ

    W1QJ Ham Member QRZ Page

    unfortunately those plates are not removable. Plates that can be removed can be replaced with new ones. Those do not come apart for repair. You might try to file it and see what happens. It may still arc there. If it does, pull that plate right off the shaft. One plate won't make a big deal. It may just arc somewhere else? There is a good one on ebay now, if you buy it, I will buy that bad one from you so you can recoup a few dollars. Lou
     
  7. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    I think I will try that Lou. But of course only if I win!
     
  8. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    As to what may have caused this I am not sure. But my suspisions are the band switch. On the 20,15 & 10 meter band switch settings the contacts were burned. I aquired an old ceramic rotary switch with similar size and and contact ratiings and drilled out the old contacts and replaced with those from other switch. 10 meter contacts were burned out on both sides. Being held in place by 2.5 mm screw, washer and nuts. Works pretty good. Band switch that is. I have seen the tune capacitor "coronaing" 1 time at 15 meters though.

    But I will wait and get a repacement tune cap and test again. Or i'll try to lightly file down rough edges and see if that helps.

    But won't have a way of testing though. I had to sell my 746Pro for financial reasons.

    Maybe just sell as is.
     
  9. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Easy enough to fix file the plates smooth again and take Rich's advice and find out why the arcing happened in the first place.
     
  10. K1ZJH

    K1ZJH Ham Member QRZ Page

    I'll confess to have taking a jeweler's file to the plates on my SB-220 tuning cap. So far, so good.... make sure you use a Q-Tip
    swap to fully remove any metal particles/dust between the plates afterwards, or you will have another light show.
    I reached for the tuning knob and grabbed the bandswitch by mistake.

    You shouldn't be losing bandswitches or having the caps arc unless something else is going on in that amp. Is your antenna
    SWR reasonable?

    Pete
     
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