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power buzz

Discussion in 'Electronics & Electrics' started by warren1133725, Nov 15, 2006.

  1. warren1133725

    warren1133725 Member

    Joined:
    Nov 3, 2005
    Messages:
    71
    Location:
    South Australia
    Anyone know why 240v goods that start to fail (or are cheap) start to give off a high pitch buzz or hum?

    Currently I have a Philips LCD monitor that needs turning off as the buzzing is noticable. (this is a soft touch power button on front so I assume then it still has power to a transformer).
    Have had other devices in past either buz from new (cordless phone AC pack, useless in bedroom) or after years of service (VCR). But with more knowledge can they be fixed?

    rgds Warren
     
  2. nux

    nux Member

    Joined:
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    16,780
    Location:
    Melbourne
    Either caused by inductors or power transformers. Commonly a 100Hz hum caused by the 240VAC 50Hz power.
     
  3. wabbit

    wabbit Member

    Joined:
    Sep 9, 2003
    Messages:
    406
    Hi Warren
    There are two issues in your question.
    Firstly, the LCD monitor with the high pitched buzz indicates a problem with the switch-mode power supply. These normally operate at a frequency above hearing range but a fault condition could lower that frequency to a point where it causes inductors and transformers to emit audible sound due to vibration of their core material, as alluded to by Nuxie.
    The most common cause of this is electrolytic capacitors which dry out over time and develop a condition called high ESR. I had my own $300 digital set-top box stop working only last week due to a failed $0.65 cap. Fortunately I was in a position to diagnose and repair it myself. I no longer do service work for a living but, in work I do for family and friends, this is a common theme.
    The caps sometimes last only a short while beyond the warranty period and then fail. Quite often the device in question is judged not economically repairable and thrown away whereas anyone with the means and knowledge could probably repair it for a few dollars.
    The second issue is with plug-packs or wall-warts as the Americans like to call them. These are often poorly constructed transformers which, operating at 50 Hertz, can emit a low frequency buzz. They also seem to be not rated for continuous use and quite often fail with an open circuit primary. In many cases, even if the device has a power switch, it is on the secondary side of the transformer which means it remains energised even after the device is switched off.
    In audio equipment which uses a conventional transformer/rectifier, failure of the filter electrolytics can cause a 100 Hertz buzz to come from the speakers.
    So, to your question of can they be fixed. The answer is a qualified yes;
    if you have knowledge of electronic circuits, an ESR meter for diagnosis and take reasonable safety precautions.
    Cheers
    Rob

    PS. This is in no way meant to denigrate electronic servicemen. They have high overheads and have to charge accordingly for their services.
     
    Last edited: Nov 15, 2006
  4. Rt!

    Rt! Member

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    Location:
    Nimbin
    heat melts something and something else becomes loose.
    :)
     
  5. OP
    OP
    warren1133725

    warren1133725 Member

    Joined:
    Nov 3, 2005
    Messages:
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    Location:
    South Australia
    Thanks wabbit for your helpful post
    I have now looked into getting an ESR meter, but they seem not very common, and very expensive. The one viable option is a $70 kit at DSE (was in silicon chip 2004). I will consider this over the xmas break now ive finished exams.(along with many other unfinished projects). I will see look inside my LCD monitor and see what i can see with my cro and freq counter. Is the fault likely to be with 2 big ones? or some small/medium electrolytics? or either?.

    Thanks again Warren
     
  6. wabbit

    wabbit Member

    Joined:
    Sep 9, 2003
    Messages:
    406
    Hi Warren
    The DSE kit is the one I have (the original Electronics Australia, February 1996 version) and it has served me well both professionally and privately. It's difficult to generalise about these capacitors. In some switch-mode supplies the smaller value caps (~1uF) on the primary side can be problematic but in my STB it was a 1000 uF secondary filter cap that failed so you just have to check them all. The ESR meter kit has a chart on the front panel which shows what you should expect from a good component of various capacity and voltage. If you get a higher reading it's a good idea to replace the part. The ability to check them in circuit makes it very easy to use but you do need to be aware of possible parallel components which can give a false reading. This is where a service manual is useful but unfortunately not always available.

    Regarding your LCD monitor, I cannot stress highly enough the need for caution when working on the power supply. The entire primary side is at mains potential so don't even think of connecting your CRO there unless you have an isolating transformer.

    A good source of information for just about anything service related is:

    http://www.forums.hcsd.com.au/

    You have to register to post but it's a community of service techs and they can be very helpful plus you can also order service manuals on-line. They don't have everything but they're pretty good and the rates are reasonable. They even encourage you to make your own photocopy.
    Cheers
    Rob
     
    Last edited: Nov 17, 2006
  7. TERRA Operative

    TERRA Operative Member

    Joined:
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    6,925
    Location:
    Niraikanai
    I can vouch for the DSE ESR meter, I have built it and have found it rather useful.
    The hum can come from loose laminations in transformers. If the varnish used to hold the laminations together cracks or comes loose, the transformer will buzz at 100Hz. Fluoros and HID lighting will occasionally have this problem.
     

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