Reason for resistor between chassis and transformer primary?

Discussion in 'General Electronics Chat' started by KMoffett, Dec 17, 2014.

  1. KMoffett

    Thread Starter AAC Fanatic!

    Dec 19, 2007
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    A web search about old radio and amplifier designs as not given me an answer, except that this is not safe.

    I was helping a friend troubleshoot a Sherwood S-5500 II audio amp. After we found the problem, a bad tube socket contact, I was disconnecting my scope ground from the chassis and got a good shock. I was between the chassis (left hand) and the scope's ground clip (right). Yeah, I "didn't" keep one hand in my pocket. :eek:

    This has a two-wire non-polarized-power-plug and a transformer-isolated power supply. After a little investigation of the schematic (attached) and the chassis, there is a 100KΩ 1/2 W resistor from one side of the transformer's primary to chassis. In other versions the resistor is replaced with a capacitor, or two capacitors tied to both side if the power line and their common connected to chassis.

    A DMM check between the chassis and line ground showed 5VAC with the power plug in one way and 115VAC with the plug reversed. Disconnection one end of the 100kΩ resistor brought the measured voltages down to 0.9VAC and 12VAC respectively.

    I know in the olden days of the "All-American 5" tube radios with a "hot chassis" this was somehow supposed the make them safer. I guess as a current limiter.

    OK, why the resistors or capacitors in an amplifier with a transformer isolated power supply?

    Ken
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    to reduce static. a high value resistor will not let enough current through to harm people, but will drain off a static charge. a lot of old radios and amps without 3 wire cords did this.
     
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  3. KMoffett

    Thread Starter AAC Fanatic!

    Dec 19, 2007
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    To drain static from the chassis to the power line? I would not consider the shock I received as harmless. Yeah, only 1.5mA, and under the industrial limit of 5mA. But, had I been in a precarious position I could have injured myself. Ones that had a capacitor instead of the resistor would do nothing for static. Not going to argue your logic, as this was from the 60's.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Is one blade of the two-wire AC-line-cord-plug wider than the other? If so, which blade is the resistor tied to?
     
  5. #12

    Expert

    Nov 30, 2010
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    The blades of the plug are equal if we are to believe KMoffett can tell the difference between a polarized plug and a non-polarized plug.
     
  6. KMoffett

    Thread Starter AAC Fanatic!

    Dec 19, 2007
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    No. It's a two wire "non-polarized" (blades identical) plug. Unless someone comes up with a good reason to leave it, the resistor will go and a two-wire polarized plug will be installed. His turntable (yeah, he's into vinyl), and tuner also have two-wire non-polarized plugs. The chassis's are all connected through the audio cable shields. There may be a "ground wire" from the turntable frame to the amp chassis too.

    Ken
     
    Last edited: Dec 17, 2014
  7. #12

    Expert

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    My opinion is to give it a polarized cord and attach about 1 meg from neutral to chassis as I see in my 1977 Phillips 'scope schematic
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
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    its for static buildup. not safety, or hum. the ones with capacitors from line cord to ground will give you a shock too. back in those days, they expected everyone to be responsable for their own safety. a three wire grounded cord would be the best for safety, but being an audio amp, it may be non polarized for hum purposes, if it hums, turn over the plug.
     
  9. MikeML

    AAC Fanatic!

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    I would put a three-wire cord on it, and connect the resistor to the green wire (not the white)
     
  10. MikeML

    AAC Fanatic!

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    If you are going to stick with only a two-blade plug, the goal would be for the turntable, the tuner, the amp, all to wind up with the resistor to the neutral side of the plug...
     
    Last edited: Dec 17, 2014
  11. alfacliff

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    Dec 13, 2013
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    do no use the resistor with a three bladed plug, the ground IS for safety. use isolation transformers if you have hum problems.
     
  12. KMoffett

    Thread Starter AAC Fanatic!

    Dec 19, 2007
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    "a three wire grounded cord would be the best for safety, but being an audio amp, it may be non polarized for hum purposes, if it hums, turn over the plug."
    So add a DPDT switch to reverse the power to the primary....or lash a Voodoo doll to the power cord with a grounding stake through its heart? :) Sorry.

    Ken
     
    Last edited: Dec 17, 2014
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