sb220 Help

Discussion in 'Amateur Radio Amplifiers' started by N9DON, Dec 28, 2010.

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  1. N9DON

    N9DON Ham Member QRZ Page

    I have a 220 that when turned on the plate meter shows 2 on the scale and begins to heat up.This just started this a.m. I shut it off and cooled it down w/ a muffin fan.I removed the cover (powered down) and cleaned the ant relay,but that wasn't it. Any help greatly appreciated.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sounds like the keying line is shorted. Does it do this with the keying line unplugged from the rear panel?
     
  3. W8JI

    W8JI Ham Member QRZ Page

    The only thing a dirty relay can ever do in an amp like yours is cause intermittent receive.

    Break the habit of cleaning relays. It's a bad habit to get into!! :) It does the relay no good, and actually can be harmful, and a dirty relay would never cause the problem you have now.


    The most likely cause of your problem, if the tubes are getting hot, is a filament to grid short in a tube. This would normally be accompanied by a negative going grid meter reading. It would also result in no gain.

    Another cause would be any filament to ground short, which would also cause a negative grid current meter. This is unlikely but possible.

    A high resistance or open plate current shunt can cause a problem like you describe, but plate current would pin hard when the amp is keyed. There would also be no undue tube heating.

    As WIK says, you might unplug the relay line from the amp. It could be shorted, maybe a welded relay contact in the exciter. In this case you would see no reverse grid current.

    73 Tom
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree with Tom about cleaning relays, it's risky business unless you're very careful.

    I only use a dollar bill wiped between relay contacts to clean them, and nothing more abrasive than that.* A piece of ordinary printer paper soaked in WD40 also works. Any sort of "tool" is probably damaging.

    *Of course if you're feeling rich, a $20 bill also works fine.;)
     
  5. W9OS

    W9OS Ham Member QRZ Page

    SB200

    Is 2 on the plate meter the normal current with mic keyed with no audio?
    If so the tubes may not be shut off in standby mode.
    In the “receive mode” the (–) 135VDC is present at the grids of the tubes in more or less full force since there is virtually no current flowing. When the push to talk switch on the exciter is activated the external transceiver relay or circuit grounds the bias through its various resistances – the relay coil in parallel with the 2,000-ohm resistor, and the 33-ohm resistor in series with the lead to the antenna relay terminal. The effect is to drop the bias voltage significantly so that the tubes begin to draw about 80 milliamps without drive, or at idle. Once the drive is fed to the amplifier the 572B’s will draw about 600 or so milliamps of plate current.

    My Swan Mark II has the same problem .I need to dig into it.

    Hope this helps.Jim W9OS:)
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Putting 572Bs in an SB-220 would be an unusual modification.
     
  7. W8JI

    W8JI Ham Member QRZ Page

    The bias in the 220 is also not applied to the grids. :)
     
  8. W9OS

    W9OS Ham Member QRZ Page

    SB 220

    Ok ,I read it as a SB 200.

    Jeeze you stick you head out of the fox hole to help some one and you get it shot off!!!!

    Back to my point.

    In standby on a SB 220 there is a 10K resistor in series with the center tap of the filament transformer. This should be enough to shut the tube off in standby.
    When you go key down the relay shorts out the 10K and puts the CT at B- or close to it.
    My Swan Mark II is the same except they put 10K in the grid circuit.Same effect..
    If you think the tubes are good I'd look there first to get the standby current to zero.. You could try one tube at a time to see what happens..
    Best Regards, Jim W9OS
     
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    If the SSB/CW switch is in the SSB position (HV ≈ 3200V), Ip = 200 mA isn't too high, according to the Eimac 3-500Z datasheet. There's going to be some heat given off (3200V * 0.2A = 640W).

    If the plate current comes up when you turn it on, you have something causing the T/R relay to pull in. Disconnect the plug from the ANT RELAY jack and try it again. If you don't see any plate current when you turn it on, the problem isn't in the SB220. What rig are you using with the SB220, and are you using a soft key unit/module with the SB220?
     
  10. N9DON

    N9DON Ham Member QRZ Page

    sb220 relay

    I should have mention it,the first thing I did was unplug the relay conector.I put it back together after a light cleaning on contacts and it still goes back to 200ma on plate?
     
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