Sb-200 weird thing happening

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, May 15, 2019.

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  1. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Don't remove the plate choke. I have never seen one fail and if it did there would be burn marks or no plate current.

    Wait for the parasitic suppressors, Most likely the problem.

    Without test gear you face a monstrous challenge. I use all vintage Tektronix analog scopes, HP sig gens, function generators, meters etc. I am the wrong person to speak to the current and much easier to use test instruments. What say anyone on that one!

    Regards Jim
    N6UH and KN4CTD like this.
  2. WQ4G

    WQ4G Ham Member QRZ Page

    Get rid of your Tuner. If your Antenna is cut for the band/frequency you are using you do not need an Antenna Tuner. Take it out of the line - uninstall it... It may be causing the weird things that are happening. Does the same thing happen when the Tuner is not in the line?

    If you do not have a Dummy Load get one. I would also suggest an outboard Watt Meter.

    The SB-200 is also equipped with a Relative Power Meter and SWR Meter... What are those meters indicating when the problem occurs?

    Here is a link to W8JI's page on 'Testing for Stability.':

    If you follow the procedure you can determine if the amp is going into oscillation or not...

    Parasitic Suppressors becoming very warm or hot on 10 and 15 meters (I have seen them glow bright red) is an indication that there is too much inductance in the suppressor coils. Too much of your RF output is being forced into the Resistors and is being dissipated as heat. Spread the suppressor coils out, so that there is more distance between the turns, and/or reduce the number of turns. This will raise the cut off frequency of the inductor and allow more of your 10 and 15 meter RF through.

    Please note that 3-1/2 turns over a 47 ohm resistor is a standard design and can be found in many different pieces of equipment by many different manufactures. I think Collins may have been the originator.

    Dan KI4AX
    Last edited: May 24, 2019
    K9AXN and K7TRF like this.

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