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sb200 no/low output on 10 meters

Discussion in 'Amateur Radio Amplifiers' started by WX7G, Sep 29, 2019.

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  1. WX7G

    WX7G Ham Member QRZ Page

    I recently purchased my SB200 & tuned it into a cantenna dummy load. 2400 volts hv & 500 watts on 80,40,20, & 15 w/50 watts drive. On 10 M only 30 watts out. I am NEW to linear amps so any thoughts????
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    What rig are you using to drive the amp and what does the rig (not the amp) see for SWR on 10 meters when the amp is operational? IOW, is it possible your rig is seeing high 10m SWR and folding back its output power (assuming it's a solid state rig with high SWR foldback) when operating the amp on 10m?

    If your rig lacks a good built in SWR meter and you have access to an outboard SWR/watt meter you could temporarily place the outboard meter between your rig and the amp to make sure your rig isn't seeing high SWR looking into the active amp and is still delivering rated power into the amp's input.

    If the input side of the amp is fine and your rig isn't folding back power looking into the active amp then you could have output tuning or tube problems leading to low power on 10m. I'd double check that there are no modifications to the output tank circuit tap or anything else unusual inside the finals cage that could be indicative of an 11 meter mod or other circuit changes. If all that checks out I'd suspect the tubes are getting old which will typically show up as 10m problems before it impacts the lower bands.

    The other thing to check is the condition of the contacts on the open frame T/R relay. Those often get charred and oxidized with use and that could very easily impact 10 meters (either as input or output matching problems) before it became apparent on other bands.
    W1TRY likes this.
  3. KG7FIU

    KG7FIU Ham Member QRZ Page

    If you're using 50W drive, you're getting less power out with the linear turned ON. Mind as well just keep the SB-200 off on 10m(!)

    Can you compare the power out in "standby" mode vs. "transmit" mode on 10m? Does the power out actually go down when the relay is engaged and the SB-200 is in xmit mode?
  4. W1QJ

    W1QJ Ham Member QRZ Page

    SB-200 amps are known for poor input swr on 15 and 10 meters. They are pretty good on 80-20 and therefore those bands make decent power because the drive power is forwarded instead of reflected. You need to check the swr seen on 10 meters to the radio. It is almost a given that a solid state radio will see a poor match. The input circuits can be reworked to improve the match and hence more power out. The radio antenna tuner can be used so long as it does not "seek".
    WA7PRC, K7TRF, W2VW and 1 other person like this.
  5. WX7G

    WX7G Ham Member QRZ Page

    Unfortunately, yes.
  6. WX7G

    WX7G Ham Member QRZ Page

    I had a friend install new parasitic resistors & replace the L- match with a Pi-network on 10 meters. No change.
  7. WX7G

    WX7G Ham Member QRZ Page

  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Can you measure the SWR looking into the amp by placing an SWR meter between the rig and the amp. That will tell you right away whether you have a power fold back problem due to high input SWR.

    It's quite possible your input SWR is fine and you have an output network or more likely a problem with weak tubes but measuring the power into the amp and the SWR looking into the amp is a good troubleshooting step that can eliminate or help you focus on specific concerns. As always when troubleshooting it's best to divide and conquer.
    K2XT likes this.
  9. K6BRN

    K6BRN XML Subscriber QRZ Page

    Another possibility is a burned output tank circuit contact on the band switch wafer - this is very common on the SB-200's, especially on 10M. However, output power in that case is about zero.
  10. W2VW

    W2VW XML Subscriber QRZ Page

    Yep, the bandswitch shorts progressively so a blown away 10 meter position might leave the output network electrically on 15 meters.

    Cathode current could still be comparable to the other bands but output would be poor.

    A complete set of readings from the OP may be enough to put together a more useful diagnosis.

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