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sb200 problem

Discussion in 'Amateur Radio Amplifiers' started by WB5OXQ, Feb 24, 2011.

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  1. WB5OXQ

    WB5OXQ Ham Member QRZ Page

    I bought a used sb200 and I have a problem with very low output.
    I have good hv shown on the meter
    both tubes filiments light up.
    with the amp off rf passes through easily and the built in swr meter works
    when you turn the amp on the exciter sees swr at infinity, only tried on 40 meters.
    using the exciters built in automaticd tuner will correct the missmatch.
    driving with at least 60 watts I get almost no output. tune and load controls do seem to work. plate current is very low and grid current is too low for the meter to read. I have cleaned the tr relay and input band switch that was no help. Can the tubes be so bad that all this happens? I use an external relay to protect the ts2000. I do not have the alc connected. the unit is running on 230vac.
    there are no known mods to the amp.
  2. K9STH

    K9STH Ham Member QRZ Page

    Clean the bandswitch including the input and output to the amplifier.

    Check the actual antenna changeover relay in the amplifier and make sure that when it is in the engaged position that the input from the exciter is actually connected to the 572B tube input circuit.

    If you end up "giving up" on the linear, bring it up to Richardson to me.

    Glen, K9STH
  3. W8JI

    W8JI Ham Member QRZ Page

    Try more than one band, otherwise it is a toss up between bad tubes or a tuned input problem. If it acts up on all bands, it is probably tubes. It virtually 100% is NOT the relay by your descrition.

    Could be tubes, could be bias, could be input circuit. Try more bands.

    I don't understand why it is so common to clean relays. That's almost never a good thing to do for problems like high input SWR, and relays almost always are cleaned in a harmful way. The only time a linear amp relay needs cleaned is when the receive is erratic through the amplifier.
    Yes. Relays can't, but tubes can. Could be a bad tuned input on 40, could be bad bias components. Try more than one band.


    73 Tom
  4. KM1H

    KM1H Subscriber QRZ Page

    Do you at least get idle current when in TX?

    Its a rare case when a tuned input causes zero output, its usually just very low output and a high input VSWR.

    Common SB-200 problems are arced bandswitches, open grid resistors and an open plate choke.

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