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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 just picked up a used sb-200 that someone has added all the Harbach mods to. Complete power supply board, soft key, soft start, new t/r relay (it appears) and new fan. Tubes seem fine. Original Cetrons and a new set of matched Taylor’s. Amp puts out good power 500-700 watts on all bands. Seems to work perfectly on 80 and 40. Anything higher 20, 15, and 10, something quite strange happens upon keying the amp. I’m keying up the amp in the correct sequence by hand using a hand keyer. I’m tuning into a dipole that is made “resonant “ with my Ameritron ATR30 matchbox. On 20, 15, and 10, every time I key the amp the swr meter on the ATR30 goes full tilt until the transceiver is keyed and rf drive applied. Once rf drive is applied, the transceiver “sees” good input swr and the tuner shows good output and low output swr. When I unkey the transceiver and then the amp, the swr meter on the tuner goes full tilt again for a split second. I’ve checked nothing so far. I know a few of you on here have extensive experience with these amps. What’s happening here on 20-10? Thanks!
  2. AA5CT

    AA5CT Ham Member QRZ Page

    Amp is oscillating. I had this happen on an LK-500ZA and it scared the crap out of me. Plate meter against the peg kind of scary ...
    K7TRF likes this.
  3. KN4CTD

    KN4CTD Ham Member QRZ Page

  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I think it has something to do with the t/r circuit.....maybe the relay itself. It appears to have been replaced before. I’ll have to recheck. I don’t think it’s oscillating because when this happens, it’s only shows normal idle current on the plate. 100ma. I can also still hear static through the receiver on the transceiver sometimes also when this is happening. I’ll try cleaning the relay contacts. Hopefully, one of the Heathkit gurus will chime in with thoughts.
  5. KN4CTD

    KN4CTD Ham Member QRZ Page

    Anyone have any suggestions as to why someone would’ve added this inductor and cap to this amp? I don’t see it in the schematics and no other pictures I’ve seen of these amps shows anything like it.

    Attached Files:

  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It's almost certainly oscillating. The SWR meter in your tuner cannot display any forward or reverse readings unless it's being driven by RF power. If you key the amp but have not yet applied drive then that RF power is coming from somewhere and an oscillation in the amp is the most likely explanation.
    WA7PRC, KN4CTD and AA5CT like this.
  7. AA5CT

    AA5CT Ham Member QRZ Page

    Looks like a series tuned circuit of some type. Most likely (99.99% chance) it functions as a suck-out filter.

    Tuned to the second or third harmonic of something (unknown at this point, what band it would be effective on) - do you have a GDO? Or antenna analyzer with a coil fixture - you could check where that little circuit is resonant ...
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Looks to me like someone heavily modified that SB-200 or never assembled it correctly in the first place (or both).

    In your underchassis image it shows what looks like insulated parallel wires (like "zip" cord) running from the large hole in the mid-chassis over to the LOAD capacitor. It's not supposed to be. In fact, the image makes it look like that wiring is pulled right up against the edge of the large hole, which is terrible process (promotes arcing from the wire through the insulation to the chassis and should be very well insulated and not "touching" the chassis at all), and that's not the original wire.

    The original wire is specially formed uninsulated bus wire coming through that hole and stood off from the chassis about 1/2" running over to the two terminal lugs on the LOAD cap, with the 1 mH RFC soldered to the bus wire down near the LOAD cap as shown in the following pictorials from the SB-200 assembly manual.

    SB-200 underchassis BSW to LOAD cap connection.PNG SB-200 underchassis line to LOAD cap connection.PNG

    The large homebrew coil and doorknob cap in your image are not any part of the Heath design and look like they were intended to do "something" but likely caused the amp to become unstable so now it's oscillating when keyed but not driven.

    The "spike" you see on the ATR-30 meter only lasts milliseconds and may not cause plate current to change, or at least not enough that the meter could indicate it (slow responding meters) but it's almost guaranteed you have an oscillation there.

    I have no idea why anyone would modify it like that.
    WA7PRC likes this.
  9. KN4CTD

    KN4CTD Ham Member QRZ Page

    Besides removing that inductor and cap to start, what common things in these amps might cause it to oscillate? I checked again and now I do see it trying to pull more plate current than normal idle I think it’s oscillating also. I just need to figure out why and how to remedy it
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Re my post #8 above: I have to retract some of what I wrote after magnifying your image (thanks for making it so clear! -- and it is) and looking carefully I see that bus wire really is bus wire, it just had a funny reflection until I magnified it.

    It looks like the builder did use bus wire as intended, but the added coil and capacitor should not be there. I'd simply remove that and see if the amp is stable.

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