Hallicrafters S-120 hum

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

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  1. WN1MB

    WN1MB Ham Member QRZ Page

    No. A short could be confirmed with an ohm meter. One or all sections might be very leaky, though, and just paralleling a good cap across a very leaky cap isn't necessarily going to clear the hum and id the culprit. That's why I'd take each section out of circuit and tack in the known entity cap.
    KP4SX and K1OIK like this.
  2. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    The suggestion that the chassis was not ground turned out to be correct however I find that one side of the AC line is connected directly to the selenium rectifier and the other side of the AC line is connected to the switch which when engaged grounds that side but not to chassis ground, but to electronic . Does this sound correct and why didn't the schematic show that. When I added one 40uf capacitor from one of the B+ legs to the electronic ground the hum disappeared but I noticed a slight odor and a whiff of smoke.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Does your headphone jack float with respect to ground, for instance via a fiber washer, or does it make a direct DC connection to the chassis ground? As a test you might disconnect the lead running from the speaker to the grounded side of the headphone jack. Your B- to ground cap test suggests that your hum stems from the speaker being referenced to chassis ground when it ideally would float off the speaker transformer secondary.

    Hope that whiff of smoke wasn't something important ;)
  4. KP4SX

    KP4SX XML Subscriber QRZ Page

    I think it does. Two different ground symbols are indicated.
    K7TRF likes this.
  5. N2EY

    N2EY Premium Subscriber QRZ Page

    Good advice, but the S-120 can be used safely without an isolation transformer. The trick is to install a 3 wire grounded cord. Black wire goes to S4, white wire goes to B- (V3 pin 3), green wire goes to chassis.

    Note that the "ground" symbol in the schematic is B- and not chassis, nor ground. The "three prong garden rake" symbol is the chassis. Note R18, C29 and C30.

    I think you mean the voltage drop of the selenium rectifier is needed to give the correct plate voltage.


    There are several common sources of hum in a receiver like the S-120:

    1) Bad electrolytic capacitor (C31, all four sections)
    2) Note that R13, R14, C17 and C18 are part of a Centralab "Couplate" unit, and may need to be replaced with individual components.
    2) C29 and/or C30 leaky or shorted.
    3) Increased resistance of various resistors in the 50C5 or 12AV6 circuits. Note how there are some very high value resistors used.
    4) Heater to cathode leakage in any of the tubes.
    5) Heater to grid leakage through the socket. Note that pins 2 and 5 are the grid of the 50C5, while the adjacent pins 3 and 4 are the heater of that tube. Anything less than perfect insulation will result in some AC on the grid. Similar situation with the 12AV6 diodes.
    6) Wiring mistakes, undocumented mods, etc.

    Important test: Does the hum vary with the volume control? Or is it there all the time, regardless?


    Trivia time:

    1) The 12BE6 converter stage uses a form of the Hartley oscillator circuit, electron-coupled to the mixer section.

    2) Directly-heated tubes have filaments. Indirectly-heated tubes have heaters.


    btw, be very careful when posting references to Centralab "Couplate" units. Spellcheckers see it as a mistake and try to auto-correct to something very different.
  6. K1OIK

    K1OIK Platinum Subscriber Platinum Subscriber QRZ Page

    It is working ok now mostly thanks to those who clued me in on the signal ground versus the chassis ground.

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