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Hallicrafters S-120 hum

Discussion in '"Boat Anchor" & Classic Equipment' started by K1OIK, Feb 13, 2018.

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  1. K6GB

    K6GB XML Subscriber QRZ Page

    As a former owner of an S-120, 14th birthday, so it was almost new. It always hummed, and didn't hear worth a damn above 20m.
  2. W9GB

    W9GB Ham Member QRZ Page

    LOOK at botton of schematic for AC filament string.
    As noted earlier, four (4) tube filaments and two (2) #47 dial lamps (6.3V) wired in series.

    Voltage on that AC filament string: 50+12+12+12+6.3+6.3 = 98.6 Volts.
    Remaining Voltage (~22 Volts) used by R22, Globar resistor.

    You could consider higher voltage bayonet dial lamps, to replace the #47.
    Last edited: Feb 14, 2018
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Don't get shocked Bert. :eek:
  4. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    I use an isolation transformer.
  5. SM0AOM

    SM0AOM Ham Member QRZ Page

    AC/DC HF receivers with mixer-oscillator stages using "hot cathodes" (in the RF sense) are notorious for modulation hum
    caused by capacitive coupling or leakage between the filament and the cathode.

    When you have ruled out any power supply filtering issues such as dried out electrolytics,
    I would suggest replacing the 12BE6.

  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Oops. Sri.
  7. WN1MB

    WN1MB XML Subscriber QRZ Page

    Take all three sections of C31 out of circuit - then wire in caps of known integrity. That's the better way to rule out the electrolytic.
  8. WN1MB

    WN1MB XML Subscriber QRZ Page

    The 120VAC out of the isolation transformer can be as lethal as the 120VAC feeding it...
  9. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    But when I hook up a scope I won't see sparks fly out of the ground connection.
  10. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    You think the capacitors are shorted?

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