SB-220 C4 Overheating

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

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  1. WA7PRC

    WA7PRC Ham Member QRZ Page

    That's great for minimizing a few nanohenries of series inductance, which will not help unless the nominal capacitance is very low.

    The actual key is that most handheld multimeters that can measure capacitance aren't very accurate below about 1000pF but, the Almost All Digital Electronics LC Meter II is excellent for measuring small values of L & C (1 nH to 100 mH and 0.01 pF to 1 uF). Other devices such as L/C bridges can be useful. I've also used my GDO (with a known value of L) to approximate small values of C. I'd use an all-in-one DMM if it was the ONLY thing available. And then, try to compare the DUT w/ a known-good part of the same/similar value.
     
  2. NE6I

    NE6I Ham Member QRZ Page

    Replacement choke received and installed today. Amp is back to normal and has full output. However, as I had noticed earlier in this process, I do not see any HV indication on the meter. Who knows how long that's been down. I normally look at Grid and Rel Pwr and haven't had the meter in the HV position in quite a while. A cursory glance at the schematic leads me to believe there may be a blown resistor causing the lack of an HV indication. Grid and Rel Pwr are displaying normally so it's not a meter movement issue. I'll check into this more over the course of the next week.
     
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Heath used three 4.7M/1W Carbon Composition resistors in series from HV to the meter switch (and a 5.6K from the meter switch to ground). High value CC resistors tend to absorb moisture over time, and soar in value. It doesn't help that they don't have a voltage rating. If you measure R6, R7, and R8, I bet you'll find at least one of them has increased in value a BUNCH.

    The last time my SB-220 showed that symptom, one resistor was barely out of tolerance. A second resistor was significantly out of tolerance, and the third was WAY out. The 5.6K (R9) shunt resistor was fine. I replaced the 4.7M resistors w/ Vishay BCcomponents pn VR68000004704JAC00 (Digi-Key pn VR68J4.7MCT-ND, $1.26 each) "Metal Glaze" resistors that are rated 10KV, and verified HV was indicating correct. That was many years ago.
     
  4. NE6I

    NE6I Ham Member QRZ Page

    Looks like R7 is the culprit, measured at 11.3 meg. Replacement on order.
     
  5. KM1H

    KM1H Ham Member QRZ Page

    Replace all 3, HV rated resistors are common and cheap these day instead of the 350V or so 1W carbon composition that Heath used. Also do R9 which can be a generic 1W carbon film type

    Carl
     
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    Like Carl said replace them all.
    I did that on my SB-220, replaced everything on the board with all new.
    It is not that expensive and worth the time and trouble.
     
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    You're welcome, glad to suggest the best part to use. You really should replace ALL THREE 4.7MΩ resistors (if one changed value, the others aren't far behind).

    The 5.6KΩ shunt resistor (R9) is there mainly to prevent the bottom end of the 3 x 4.7MΩ resistor string from soaring to 3KV when the meter switch ISN'T in the HV position. The meter resistance is 1.4KΩ. A change in the 5.6KΩ in parallel won't affect the accuracy nearly as much as a change in the three 4.7MΩ resistors. It's the high-value Carbon Composition resistors that change value by themselves (due to absorption of moisture over time). Low value resistors, not so much. It would have to change a BUNCH to make the meter read incorrect. If it's OK, leave it.

    vy 73,
    Bryan WA7PRC
     
  8. AI6BF

    AI6BF Ham Member QRZ Page

    Metal fatique

    I am attempting to restore a SB-220 and I am finding the many of the connectors, spade, have expanded and just by a little jiggle break. I hope this helps. This may be the raise and fall in voltage.
    George
     
  9. AC0OB

    AC0OB Subscriber QRZ Page

    Overld!1


    If you read a few pages back, in 6I's situation it was a grid choke and a HV voltage divider problem.

    I would check your SB-220 for any grid choke, voltage divider, and bias problems as well while you are restoring it.

    Connectors should certainly be examined for fatigue.

    ---------------------------------------------------------------------------------------

    AC0OB - A Place where Thermionic Emitters Rule!
     
    Last edited: Nov 4, 2014
  10. NE6I

    NE6I Ham Member QRZ Page

    All three resistors replaced today and the HV meter is again operational. The amp is now 100% again. Thanks to everyone for the assist.
     
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