Hallicrafters S-120 hum

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

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  1. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    It has a hum, I substituted capacitors for all the filter capacitors, replaced the diode. Still hums. The schematic I have found has differences, for example the 1000 ohm resistor is marked 500 ohms and I measured 20 ohms across it, resistors I have never found to go down. The 50C5 shows no pins 3 and 4 on the schematic but there are components connected to it. Ideas?


    s-120 hallicrafters schematic.JPG
     
  2. KP4SX

    KP4SX XML Subscriber QRZ Page

    Are you lifting one end of it? Otherwise you are reading the primary of the audio xfmr.
    Its not unusual to use unused pins on a socket as tie points. Consider subbing the 50C5 to confirm that its not heater-to-cathode leakage causing the hum.
     
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Pin 3 and 4 is the filament.

    C31 is bad.
     
    K7TRF likes this.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Well if by 'the 1000 ohm resistor' you mean R23 in the filter circuit then I'd start by replacing that as if that's actually 20 ohms you're not getting the intended ripple filtering.

    Pins 3 and 4 of the 50C5 are the filament pins as shown at the bottom of the schematic. Is one of the components the 500 ohm 5watt resistor R19?
     
  5. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks on the filament, I missed that. I paralleled capacitors across all legs of C31 to little affect.
     
  6. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    The primary of the audio transformer is connected to the plate 50c5, there is no path to read the resistance it via r23
     
  7. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    Re: R 19, you could be right, I will double check
     
  8. W9GB

    W9GB Ham Member QRZ Page

    Hallicrafters S-120 Service Data
    http://www.radiomanual.info/schemi/Surplus_Radioamateur/Hallicrafters_S-120_serv.pdf

    The Hallicrafters S120 is basically an All-American Five
    • No AC isolation transformer
    • An isolation transformer is highly recommended during service (and usage)
    • Tube filaments, Pilot lamps and Glowbar resistor are wired “in series”
    • Safety Capacitors (X/Y) recommended.
    Have you checked the R20 Glowbar resistor, Hallicrafters part # 023-000327 ?
    880 ohm COLD; 100 ohm HOT

    The S-120 uses a selenium rectifier (CR1), which has a built-in resistance. That resistance is vital to properly maintaining the filament voltage, so you can’t just replace it with a silicon diode (1N4007).
    You must put back some of that resistance by adding a resistor after the diode.

    Antique Radio Forums, March 2005
    http://www.antiqueradios.com/forums/viewtopic.php?t=5247
    ==
    Good general reference for Antique Radio restoration
    Badrestorer’s Illustrated Guide to Restoring the Zenith Trans-Oceanic H500
    http://www.renovatedradios.com/articlePages/H500RestorationGuide.pdf
     
    Last edited: Feb 14, 2018
  9. W9GB

    W9GB Ham Member QRZ Page

  10. W7UUU

    W7UUU Super Moderator Lifetime Member 133 Administrator Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    Really nice to see a Burt thread that is so positive and contributes to a dialog about ham radio stuff. I like your "for sale" ads too - they're very often fun to read, with fun videos to go with them. I suspect this radio will end up in one of them - and that's totally cool!

    Kudos, Burt. :)

    Dave
    W7UUU
     
    K1OIK likes this.

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