Sb-200 weird thing happening

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

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’m going to. I’m just guessing (I hope I’m wrong) that the extra coil and cap was added to try to remedy the same problem. I already screwed up and hot switched it not paying attention so now I’ll be removing the bandswitch to repair the 15 meter contact!! I’m gonna remove that coil and cap and update
     
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    I removed that extra coil and cap. The amp acts fine on 40 and 80 meters but still does the same thing on 20 and 10. I’ve never dealt with an oscillation before so I’m kind of at a loss where to start looking for the problem. I don’t have any special test equipment or such. Any suggestions on things to check that could possibly cause these “oscillations”? I have verified now that sometimes when this is happening, it is pulling more plate current.
     
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That would be my guess as well, the previous owner ran into oscillation problems and tried to fix it with a series resonant trap circuit at the output. I'd try to discover the root cause and fix that instead.

    Check all the components and wiring near the grids of the tubes. That circuitry is accessible from the rear of the chassis at the bottom of the tube sockets. Also check the parasitic suppressors on the anode caps of the tubes. You need to desolder one end of the inductor wound around each suppressor resistor to check the resistor values. I'd also consider the HV bypass caps associated with the anode choke. Check that and things like HV band switch wiring and contacts.

    Personally I hard grounded the grids in my SB-200 which is what I'd do with most vintage amps that floated the grids as that's one likely place for problems like this to occur. But that change involves simple changes to the grids (literally pull the existing components and strap the grids to chassis ground with short, wide copper straps), some fairly easy changes to the bias circuitry and some changes to support grid current metering after you make the changes above. So it's not a super quick fix but among other things it can help minimize parasitic oscillations.
     
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I understand how to directly ground the grids. If I thought this would help, I’d do the same thing. Can you describe what changes would need to be made to the bias circuitry with directly grounded grids? I understand the bias voltage should be -2v when the amplifier is keyed. The t/r relay looks like a new Harbach replacement and has the resistor swapped out for the correct value to account for the difference in the resistance of the relay coil.
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    On the higher bands where the instability occurs, since it seems to accept and amplify drive okay and is unstable only when keyed but not driven:

    Try "tuning up" normally, then increasing the LOADing by turning the LOAD control more clockwise (which reduces the load capacitance) and leaving it that way without re-tuning anything.

    That could help improve stability sometimes, even though output power will be very slightly lower (maybe 5%).
     
  6. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’ve tried that already....no dice. I did notice that C10, a .001 3kv bypass cap that’s supposed to be at the bottom of the plate choke is not even there lol! I don’t know how that would cause oscillating, but it sure could let RF find it’s way into the power supply.....correct me if I’m wrong. I’m gonna pull and check the values of everything at the tube sockets I guess
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    It’s oscillating as the relay switches for that momentary second that there is no load on the tank. Soon as the relay closes it stops the oscillating immediately. I’d have to see your suppressors to see if they are voodoo magic types or if original the resistors may have changed value. Normally these amps are ok if the suppressors are right. The amp has been modified by an unknowing person and needs a full look at to make corrections. As far as grounding the grids. That’s a pretty major modification and if you have to ask how it has to be done, you’ll need a full schematic and step by step instructions on how to do it. It’s involved needless to say.
     
  8. KN4CTD

    KN4CTD Ham Member QRZ Page

    They’re not the voodoo magic types. They are original resistors. I’m assuming one neeeds to desolder one side of the coil to get an accurate resistance measurement. I’m in the process of doing that now. I do have a nice full schematic of the original design. Someone has put quite a bit of time and money into all of the Harbach upgrades but that doesn’t fix the current problem. It seems to me that it is oscillates until rf drive is applied.....but even then the “output” SWR tries to go through the roof, but not always. When keyed and no drive is applied, it oscillates on 20, 15, and 10. Works fine on 40 and 80. I’ll post a pic of the suppressors.
     
  9. KN4CTD

    KN4CTD Ham Member QRZ Page

    Old and burnt looking
     

    Attached Files:

  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    The OE SB-20x/SB-22x parasitic suppressors were 3-1/2T 14AWG solid tinned copper wire (about 80 nH), wound on 47Ω/2W carbon composition resistors. The inductor adds some series reactance to reduce VHF-up gain. The resistor functions to reduce the inductor Q, and the value can be anywhere from about 27Ω to nearly 100Ω.
     

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