Heathkit SB-230 Amp

Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, May 26, 2012.

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  1. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Today I got to try out the cheap SB-230 amp I scored at Dayton. I brought it up slowly on the variac, and the HV looked fine. Since I am running it on 110V, it was a little low, but as expected. No problem there.

    The Time delay circuit seems to work, but not much happened after that.

    The relay works, and with my scope, I can see that the turned circuits tune correctly, but no output, or Plate or grid current. NOTTA!

    So don't know if the TUBE is blown, or what. Just no response when keyed. All circuits have been carefully examined and no problems seen.

    I am trying to analyze the tube circuit to see where and what happens. With my scope, I DID see drive signal on the cathode coming from the resistor dropping bank. So RF is making it to the tube.

    Haven't tried taking out the tube and doing anything. Not sure how I can check to see if there is oxidation in the tube socket pins without taking it all apart. What is most distressing is the grid connection, which is 2 feedthrus into the final compartment and then a brass strap around the tube grid ring. That looks hard to take apart without damaging things. But with no plate or grid current, there might be some connection issue rather than a bad tube. I have no way of knowing if the heater is even heating without putting an AC Ameter in series with the filament line.

    This might be tough to fix, and 8873s are rare as Hen's teeth!
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    No IG and no IP sounds like the toob is in cutoff. What does the bias voltage do at the tube? Does it go below (above, actually) -120V when you toggle into xmit mode?
  3. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Guess I gotta tear it apart and see. Thanks!
  4. WC5P

    WC5P Platinum Subscriber Platinum Subscriber QRZ Page

    You know the heat sink on that thing is hazardous, right ??

    Be careful......
  5. KO6WB

    KO6WB Premium Subscriber QRZ Page

    There is a .75A fuse in series with the bias diode. That may be the only problem.
    Hope this helps
  6. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    YOUR RIGHT!!!! That fuse is Blown!!!!

    However, if it blows again, something else if fishy.

    Gotta go score some fuses.
  7. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    THANK YOU VERY MUCH GARY!!!!!!!!!!!!!!!!!!!!!!!

    I soldered a thin wire jumper across the blown fuse (need to replace it with a real fuse someday) and tried it out. The meter is a little sticky, but I use external metering anyway. The meter light has been clipped out. Not sure what is going on there. Uses a funny light bulb in series with a 300 ohm resistor across the 110VAC. Hmmm, wonder why they just didn't connect it across the 6VAC on the filament??

    Anyway, I hooked up the Kenwood TS-830S and at first I didn't have the kenwood tuned up right, but for 25W in, I saw 125W out. Gain of 5. Sure enough, when I messed with TS-830S enough to get 100W out, I then saw 500W out of the SB-230!!!!

    That means the TUBE IS GOOD!!!! YAY!!!!!

    Bet the seller didn't know about that fuse either! It is on the back panel, clear as day, but with the circuit breaker in the AC, I never thought to check it.

    Do need to see what I can do about the meter and light. Then this Amp will be ready for PRIME TIME!

    I see a MOD to convert it to 6 meters! I had planned to do that to the SB-200 I have, but maybe I oughta save that one in case I blow up the Drake L7, and use THIS for 6 meters! A 8873 would be a much better choice for 6 than the pair of 572Bs.

    also this one by Terry N6CW
    Terry is in our DX Club and I know him very well. Now I need to scrape up the parts needed for the mod.
    GREAT!!!! That means nearly 100% of what I brought from Dayton is WORKING! (still need to replace the SX-140 front panel with the one I scored and replace the HG-10 VFOs dial drum.)

    Last edited: May 27, 2012
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The SB-230 would be a way better choice for 6m than an SB-200.

    The 8873 is good to at least 400 MHz and should be a much more "stable" choice than 572Bs.

    I've found UHF tubes are more stable at lower frequencies than HF tubes are.

    You might be able to swap the original PLATE TUNE cap to make it a new 6m PLATE LOAD cap, and score a good 25 pF or so wide-spaced cap for the new TUNE device; make a new tank coil out of very heavy copper wire or tubing; and remove a bunch of turns from the plate RF choke; make a new input tank circuit; and bingo.
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    I wonder if it might be better to leave it intact, sell it to a collector, and use the money to buy/build something else.
  10. W1QJ

    W1QJ Ham Member QRZ Page

    WA6MHZ, ask K2XT about 6 meter amps.
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