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how to repair using a schematic and DMM?

Discussion in 'Electronic Basics' started by Kaycho, Aug 11, 2005.

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  1. Kaycho

    Kaycho Guest

    I would like to know how to troubleshoot circuits using a schematic
    (with voltages) and a DMM/VOM. Specifically I'm working on transistors
    in a TV. Should i take measurements with the tv plugged in? Can I
    make in-circuit tests or do i have to desolder things?
  2. Tom Biasi

    Tom Biasi Guest

    You need more than a schematic and a DMM.
    You need some basic training. Don't be in a hurry to jump in and fix
    Before you can fix something you need to know how it works and what is
    happening to make it not work. The fixing part is easy most of the time.
    When you troubleshoot you are looking for a reason the unit doesn't perform
    as it is supposed to.
    TV's are very complicated and require more test equipment than a schematic
    and a DMM. They are also very dangerous.
  3. Alan Adrian

    Alan Adrian Guest

    Hehe... if this is not a troll post.... you are about to get a bit of a

    I'll try to put this mildly...... TV's have very dangerous voltages in them
    and are not the place to learn how to troubleshoot. People die from messing
    about inside TV's...

  4. Heh.

    Go to

    and select the icon for the table of contents...then start reading.

  5. I grew up in my dad's tv repair shop.
    Most tv repairmen are starving these days.
    Reason: Television sets are disposable.
    It takes more time to repair one than it takes to earn enough money to buy
    one by raking leaves.

    The warning about high voltages can't be stressed enough.

    You can even be shocked months after the set had been unpluged if you get on
    the wrong spot.
  6. Rich Grise

    Rich Grise Guest

    This is true _EVEN THOUGH YOU'VE DISCHARGED IT!_ The tube is a big
    Leyden jar. I've discharged them (fat clip lead to ground, small
    screwdriver under the cap), and then later, drew another arc - the
    glass itself holds a charge.

    And I also concur with the others - unless you already know how
    a TV works, and how to troubleshoot, it's not worth it, unless you
    want to call it "home schooling". :)

    Good Luck!
  7. Kaycho

    Kaycho Guest

    I generally know how tv works; I've read a few books. I've also read
    Sam Goldwasser's site.

    Thanks for the caution.

    The problems I'm facing are with defective components that i think can
    be tested with only a DMM. I just need some pointers on
    troubleshhoting them. I suppose I can simply replace the component I
    suspect is at fault based on deductive reasoning of how the tv works
    and which component failure will cause a certain tv problem, but I'd
    rather have some test to back up/confirm this suspicion.

    I'm curious what type of circuits did you guys learn on?

    Thanks so much for the replies.
  8. John Fields

    John Fields Guest

  9. Rich Grise

    Rich Grise Guest

    Simple ones. :)

  10. Kitchen Man

    Kitchen Man Guest

    My experience with television is that there is no point trying to use
    deductive reasoning - the damn things work on magic. Better luck
    might be had finding an old tape player or stereo receiver that's
    crapped out. I tracked down a bad part in a home stereo power amp
    once, with only a dmm and no schematic, and I once found a bad chip in
    a tuner by using freeze-spray. But at the time I did that, I'd had
    quite a bit of job experience with troubleshooting circuitry.
    Well, I learned electronics on the AN/APG-109 RADAR set. Ten months
    of training and a couple years of OJT and I was ready to go. Thirty
    years later I'm still learning.
  11. redbelly

    redbelly Guest

    Simple AND SAFE ones!

  12. John Fields

    John Fields Guest


  13. Good for you! I built a radio from a kit at 8, and was working part
    time in a TV shop at 13. This shows that not everyone was stupid or
    lazy as kids, when we were kids.

    Link to my "Computers for disabled Veterans" project website deleted
    after threats were telephoned to my church.

    Michael A. Terrell
    Central Florida
  14. I built a foxhole radio (razor blade, pencil lead, safety pin, etc)
    when I was 9, and again when I was 13. The second one I used until my
    parents bought me a multi-band radio for a high school graduation

    I was always interested in electronics. It wasn't until my teens,
    though, that my late 50's Fender Super guitar amplifier taught me
    about how a filter capacitor can retain a charge, when I touched one
    of the connectors on the Standby switch. :)

  15. redbelly

    redbelly Guest

    John, let me know if I'm reading too much into this: is
    "finding out" about AC mains a euphemism for getting


    p.s. pardon my ignorance ... what is B+ ? TIA.
  16. John Fields

    John Fields Guest

  17. redbelly

    redbelly Guest

    Ouch. I'd guess a lot of us have had that happen.

    Thank you.

  18. ehsjr

    ehsjr Guest

    I feel your pain! :)

  19. clf

    clf Guest

    Some of my own "electrofying" adventures....

    1. When I was old enough to be able to unscrew a lightbulb from a nearby
    table lamp, and understand that the "knob" had to be turned to get it to
    come on - I did just that - only I replaced the bulb with my finger.
    Ahhhhh - the rush. I've been shocked many times since. Usually it is still
    about 1x a month. Some not so bad, others - OUCH! The worst ones were as bad
    as some "RF" burns I've gotten. Those can tend to hurt too!

    2. I was working on a Heathkit Transmitter. I needed to take a voltage
    reading inside and I just couldn't get the probe in place due to a small
    wire that kept getting in the way. I'm thinking the ole familiar electronics
    safety rule - "KEEP ONE HAND OUT"..... but dummy me, I ignored it and as I
    was putting my hand in to hold the wire, kept thinking "you're going to get
    lit up" and damned if I didn't. That sucker hurt. I didn't get the voltage
    reading either, so the pain was for nothing. I turned the thing off til my
    arm stopped paining - went back - rigged something up inside to the point I
    needed and then placed the probe on. I waited til the set warmed up to
    stabilize the reading to make sure it was correct.

    3. Was working on an old Tube Type CB once, sitting on a "Cement" porch -
    bare foot. Set hadn't been plugged in for a while. I still got a tingle.
    Though I was a bit shocked (pardon the pun) - I reasoned that there must
    have been enough voltage stored yet to do that. So, I guess I wasn't too
    awfully surprised considering it all.

    So, I always tell people I have an "electrofying" personality!

    Just some of my adventures in "diverted" electricity. I try NOT to get
    shocked, but we all know - S*&t happens.

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