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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?
     

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  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 Ham Member 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 Ham Member 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
     
  11. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    When I received the amp that's the way the bandswitch and the TUNE Cap was. I repaired the bandswitch. The previous owner said he had nearly 700 watts output on 10 meters and I said no way not with the contacts the way they were. So he refunded me part of the payment I made.

    All the testing I have been doing has been into a dummy load. As for antenna VSMR less than 1.5:1 I have a Titan DX.
     
  12. W1QJ

    W1QJ Ham Member QRZ Page

    I think I have a good SB-200 tune cap here. if you don't win that one on EBAY maybe we can swap a good one for that one and a few bucks. The burned plate won't bother me. Lou
     
  13. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The old "turned the bandswitch when I meant to turn the TUNE or LOAD cap" is a common problem with SB-200s and other amps that put the controls in line like that and use the same size knobs for everything.

    That was a really silly thing for Heath to do. Ameritron's done a far better job of placing the bandswitch at a different level than the tuning controls and giving it a different sized knob so this mistake is far less likely to happen. I've often wanted to give the Heath designers twenty lashes for silly stuff like that.
     
  14. K1ZJH

    K1ZJH Ham Member QRZ Page

    When my black front panel comes in from Martin I'll be using the Ameriton style knobs. That will
    stop with the knob confusion. Adding a vernier drive to the plate tune cap was a must do.

    Pete
     
  15. KJ4AQU

    KJ4AQU Ham Member QRZ Page

    Hey Lou, I would rather deal with you than eBay. Let me know if you have a tune cap. I can always cancel my bid!

    Also I am contemplating just selling it as is too.
     
  16. WA7PRC

    WA7PRC Ham Member QRZ Page

    Interesting. In 40 years, I've never done that with my SB-220. The '220 has the controls Tune-Load-Bandswitch, while the '200 has the controls Load-Tune-Bandswitch. It certainly could happen often if the bandswitch was in the middle.
     
  17. K1ZJH

    K1ZJH Ham Member QRZ Page

    Bryan,

    You only have to do it once :) I admit to mistakingly grabbing the bandswitch while retuning
    my SB-220 amp -- being distracted by a DX pileup. I had owned the amp for over 30
    years. Yeah, my stupid mistake, but it does happen.

    Pete
     
  18. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I think you're more careful than most. Lots of people have turned the bandswitch accidentally while transmitting, with typically bad results.

    Heath made a design error when they mounted the bandswitch to the front panel with a nut behind the knob, also. Dumb, dumb, dumb...such critical components should always be rigidly mounted to something behind the panel and not the panel itself and either use multiple attachment points or at least Loctite on the threads. I've repaired a lot of Heath amps that were damaged simply because the nut came loose and allowed the whole switch to rotate, not just the shaft and contact on the wafers.

    I think the SB-220 was a great design electrically and not so smart mechanically. In fact, using the four screws that hold the feet on for captivating the whole slide-on outer cover was kind of Novice-level, as was using sheet metal screws that self-thread into holes in aluminum to hold the inside shields in place (PEMs were available and cost five cents each; way better idea that allows repeated re-uses without damage).

    But they sure sold a lot of them, including one to me.
     
  19. K1ZJH

    K1ZJH Ham Member QRZ Page

    duplicate this forum is extremely slow today??
     
  20. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I agree with that, too. The SB amps tune way too fast for comfort.

    I know the goal was to make it powerful and cheap and they cut every corner they could to keep the materials cost down, but I doubt they would have lost a single sale if the price was $25 higher, which would have allowed them to use $5.95 (at the time) planetary reduction drives on both capacitors.

    There was absolutely nothing else on the market in that price range...the world was their oyster.
     
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