SB-220 C4 Overheating

Discussion in 'Amateur Radio Amplifiers' started by NE6I, Oct 18, 2014.

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  1. KA9JLM

    KA9JLM Ham Member QRZ Page

    I wonder if that amp was ever wired for 220. It does sound like it could be a bad transformer.

    If it is wired for 120 and is connected to something on a different phase in your power panel, strange things can happen. (IE Radio or computer in your shack on a different circuit)

    I would take Carl's advice.

    "As for the current discovery: Each socket has three 200pf caps, one from each grid pin to ground. There is also a 1 mH choke to ground to provide a DC path for the grid.
    The older rectangular caps are very prone to develop leakage, heat up and split the seams. An open choke will render the tube in that socket inoperative unless there is a sufficient path thru a bad cap; it opens during a tube short and tends to protect other components such as in the bias supply. "

    The amp just keying make no sense if you have no radio connected to it. That sound like another problem, that may have smoked something else.

    Good Luck.
    Last edited: Oct 22, 2014
  2. NE6I

    NE6I Ham Member QRZ Page

    First, yes, the amp was once wired for 220 VAC. That was 25+ years ago at a former QTH in another city. I've run it on 120 VAC ever since, at a rental QTH and now this one. For the past eight years, the amp, radio, computer, et al have been on the same circuits as they have always been. (Yes, I've been slow to getting around to putting in a 220VAC circuit in the current shack.) There have been no changes to the setup or anywhere in the house that should have affected phase or that sort of thing in recent years. All of my testing so far this past week have been on the same circuit in the shack. The amp runs on a nearly dedicated 120VAC circuit and the other loads on it total less than a tenth of an amp. I'm pretty confident this is not a power source issue. But stranger things have been known to happen.

    Also, I did not get a chance to check the RFCs, especially RFC4 as discussed by Carl. Read through tonight's test results and let me know if RFC4 is still suspect. I'm now thinking anything and everything to do with V1 IS suspect. But feel free to enlighten me further. This is a good learning exercise.

    Last night's discovery was that putting a tube in V2 and leaving V1 empty resulted in constant and proper voltage across C4 with no C4 heating and no T/R clicking.

    Tonight, I added the second tube to V1 and left the plate choke disconnected. All was okay.

    I then moved the plate choke from the tube in V2 to V1 and left the tube plate in V2 disconnected. The problem was back. In my mind, this rules out the plate choke for V1 which was suspect in my mind previously.

    Next, I moved the tube from V1 to V2, putting the original plate choke from V2 back where it belongs on the tube in V2. No tube was placed in V1. Powered up, and no problem. This seems to rule out the 3-500Z tubes. (Yay!)

    I then put the remaining tube that had originally been in V2 into the socket V1 with the plate disconnected. No problem.

    So now, the tubes have been swapped and the only repeatable thing is that connecting the plate choke to the tube in V2 and leaving the plate choke disconnected from the tube in V1 results in no problem but connecting the plate choke to the tube in V1 results in a problem no matter whether the plate choke is or isn't connected to the tube in V2 (say that three times quickly!).

    This seems to me to further point at a problem with the V1 circuitry somewhere.
  3. AC0OB

    AC0OB Subscriber QRZ Page

    Yep, it sure looks like like RFC4 is either open or partially open, since if V1's grid doesn't get a good DC ground, then that tube gets biased into conduction at "idle."

    Measure the resistance from V1 pin 4 to ground and then measure the resistance across the choke.



    AC0OB - A Place where Thermionic Emitters Rule!
    Last edited: Oct 22, 2014
  4. AC0OB

    AC0OB Subscriber QRZ Page

    Then you might desolder the caps C16, C19, and C26 and check their resistance as well. If the choke openened up, then they might have failed as well and may show some resistance.



    AC0OB - A Place where Thermionic Emitters Rule!
  5. WA9SVD

    WA9SVD Ham Member QRZ Page

    A first SWAG would be to check the rectifier, D-18. If it's shorted, AC will heat the cap real fast...
  6. KM1H

    KM1H Ham Member QRZ Page

    It might help to read the complete thread, including Post # 4.

    Any NEW insights would be welcome
  7. NE6I

    NE6I Ham Member QRZ Page

    Gentlemen, we have a winner! RFC4 is open. C16, C19 and C26 are apparently good as all show no continuity at all with the DVM in the resistance (ohms) position.

    Any idea where I can get a replacement? I cannot find a 1 mH choke at the usual places (Mouser, Allied Electronics, Radio Shack, Frys, eBay or even Google!).
  8. NE6I

    NE6I Ham Member QRZ Page

    Found some at Mouser after finessing their web site. Searching for 1 MH choke results in nothing. 1000 uH produces results. Sheesh!
  9. KM5FL

    KM5FL Ham Member QRZ Page

    I think what you need is right here:

    One of the mods I do to SB-220/221s is remove all those chokes and mica caps and directly ground the grids using short strips of copper..

  10. W9GB

    W9GB Ham Member QRZ Page

    RF Parts in San Marcos, CA is at your backyard .. they should have an appropriate RFCx, Choke in stock.
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