Rewiring vintage TV, trying not to die. Very simple question

Discussion in 'The Projects Forum' started by bigalthethird, Feb 10, 2010.

  1. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
    9
    0
    What's up guys? I'm new here, and I just have a very simple question that I simply can't get the answer to.

    I have 1950 television, and I'm redoing it just slightly. I'll leave the details out, but in short I redid the power supply a little bit.

    Now, I left the original wiring mostly intact, and soldered the new power wires exactly where the old ones were. But (stupidly), I never took voltage measurements of the case with the old wiring. Basically, I'm paranoid of touching the case while it's on (which is necessary to adjust any of the picture settings - the knobs are all metallic), as I don't want to get fried. With the old wiring, I never got any sort of shock. And the new wiring is identical, at least I'm almost sure it is. But here's what's confusing me:

    When I hook up the red wire of my multimeter to a wire I know is hot and the black one to the other wire for the power supply, it reads 120V (normal). When I hook up the red to hot, and the black wire to any part of the casing, it reads somewhere in the 30 volt region (but it fluctuates wildly). Why is this!? What should it read when I hook the red to hot and the black to the case/knobs?

    I've heard differing opinions. Some people say that if the case is grounded, it should give a voltage reading. Others say that if this happens, you'll get shocked if you simply touch the case. What's the right answer?

    Thanks guys
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    Welcome to AAC

    I'm glad you are still alive

    Many pieces of equipment from older years were constructed with a live (non grounded) chassis.

    TV's in particular had lethal voltages inside, earlier ones often had poor if any shrouding. You need special probes (and multimeters) to measure these. So be very careful what you connect to.

    You need to provide some more information.

    It would help to know where you are in the world as standards differed?

    What is the set make/model?

    Is your meter set to AC or DC?

    Does the set mains lead have two wires or three connecting. Is there an earth lead?

    If there is an earth wire what is the resistance between it and chassis when the TV is disconnected from the mains?

    You may find better help here, these guys specialise in your type of question.

    http://www.vintage-radio.net/forum/index.php
     
  3. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
    9
    0
    Hey dude, sorry my reply is late. And thank you for helping!

    I live in the US. I'm not next to the tv now, but it's a 1950 Hoffman, and I don't remember the model. It only has a two prong plug.

    My tester is set to AC and it rates properly. When put into the wall, it reads 120 volts. When you put one of its leads on a hot wire, and another on the ground wire, it reads 120. When you put one on hot, and the other on the case, it averages about 30V. And when you put both leads only on the case, it reads 0 all the time.

    So what the hell is going on!? Will this damn thing shock me if I just touch the case with one hand? I don't think so, but I dunno. If I touch with both hands? I doubt it, but not sure.

    Thank you very much!
     
  4. 3ldon

    Active Member

    Jan 9, 2010
    82
    3
    Chances are you can ground the case, it should *not* be connected to either side of the 120Vac input by any significant circuitry. (bad/leaking caps?)
    When you measure from the case to the "hot" line, you are really just measuring the insulation resistance of the transformers.
    Take a 400-600K resistor and connect it across the meter, this will reduce the 1 or 10Mohm input resistance to ~~500K, and this will allow you to measure the "real" voltage.
    If you get more than 10 volts, you should look into the matter.
     
  5. bigalthethird

    Thread Starter New Member

    Feb 10, 2010
    9
    0
    First off, thank you so much for the help!

    Can you explain in slightly greater detail how you would remeasure it with that 400-600K resistor? Where would the leads go, and where would the resistor go?

    And last thing, seems to me that if you connect both meter leads to the case, and there is absolutely no voltage reading, that when you touch the case with your hands (same as the meter leads in this case), you won't get voltage (amperage) coursing through your body. Right?

    Thank you
     
  6. legac

    Well-Known Member

    May 4, 2005
    54
    0
    First of all you shoud have an isolation transformer. If this old rig does not have a big transformer ( parallel heater) then it has a "live" grounding. It means that one of two AC wires is tied to the chassis.
     
  7. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Like he said.

    Follow the power cord to the on switch or fuse.
    Follow from there to see if the power then goes to a big transformer. If not you may certainly have a hot chassis. If so, the product was enclosed in a cabinet that was not conductive and had control knobs that also were not conductive of electricity.
    When you pull the chassis and start poking around you put yourself at serious risk.

    Even with a power transformer you have to stay away from the high voltage cage and the side of the CRT (picture tube) where the big wire goes.
     
  8. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    If he can't give you the information you need and pictures,
    don't kill him with information he don't under stand.How do you
    know that he has a clue about your advise. Hey dude tells how
    much he will listen,don't be a part of this.
     
  9. flat5

    Active Member

    Nov 13, 2008
    403
    17
    I simply told him to follow the power cord.
    That's not too technical. If he does not know what a "big transformer" is and can't find out, yes he should walk away from any further fooling around with this equipment. Probably should anyway. Find an old TV guy to fix it.
     
  10. alim

    Senior Member

    Dec 27, 2005
    113
    1
    A SM will be useful , but you will have to heve the model #.
     
  11. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Pardon me, but what is an "SM"?
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Sadomasochist. ;)
     
  13. alim

    Senior Member

    Dec 27, 2005
    113
    1
    Sorry for the shorthand SM means Service Manual.
     
  14. flat5

    Active Member

    Nov 13, 2008
    403
    17
    "SM means Service Manual"

    "It's all clear now" (Humphrey Bogart)
     
  15. rockymt_stromster

    New Member

    Feb 15, 2010
    2
    0
    Concerning adjustment

    If recollection serves me the adjustments on these old tube sets was done with thin plastic tools with a flat head that was inserted through a brown back board that kept people safe. So besides being non-conductive the tool did not pose any magnetic influence either, these sets being very susceptible to magnetic influence - read inductance and making adjustment pretty funky.

    I know I got bit more than once touching and old tube set. These things had low heater voltages as well as higher voltages for cathode and anode. Man is this bringing back memories.

    As a safety measure use one hand so you don't get between the power source and a ground or return. Do not go anywhere near the flyback transformer (that which drives the Cathode Ray Tube - AKA Picture tube) hi voltage very low current but still could kill you.

    Cheers
     
  16. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Those are diddle sticks.
     
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