power supply board gives varying current

Discussion in 'The Projects Forum' started by keenez, May 30, 2009.

  1. keenez

    Thread Starter New Member

    May 30, 2009
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    Hello,

    I'm a newbie here. While I have some electronics knowledge, I'm not that knowledgeable so forgive me if this is a stupid question.

    I had an LCD monitor that was malfunctioning. Opened it up and saw some bulging capacitors so I replaced them. The monitor worked fine for a day but then starting to turn on and off. It seems the power supply board, which supplies 12 V DC to a logic board is varying its output from 4V to 12V. When the logic board is not plugged in, the pins seem to supply a steady 12V DC. I was thinking that the problem was in the capacitor used in the rectifier/inverter circuitry but then, wouldn't the current alternate from the output pins to the logic board even without a load? Are there any other suggestions? I was thinking of the transformer. I also hear a "hissing" sound that is in time with the monitor turning on and off.

    Thanks in advance.
     
  2. PRS

    Well-Known Member

    Aug 24, 2008
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    35
    Is the supply switching with a regular period or randomly?
     
  3. keenez

    Thread Starter New Member

    May 30, 2009
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    It's switching with a regular period. About 4 seconds or so.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Were the replacement caps rated for at least 25v?

    Are there any tantalum or electrolytic caps on the logic board? Tantalum caps usually go out with a rather loud "bang", though.

    Can you locate where the hissing is coming from? Try using some rubber tubing as a stethoscope, like the kind of tubing that's used in automotive vacuum lines or windshield washer systems.

    Look very closely for signs of things heating up, or cracked solder joints.
     
  5. keenez

    Thread Starter New Member

    May 30, 2009
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    0
    I replaced the caps with the same voltage and capacitance as the originals. They were all 25 V.

    The logic board also had 2 electrolytic capacitors that had bulged so I replaced them with the same capacitance and voltage (16 V). My only worry with those was that I could only find 85 degree replacements instead of the 105.

    I will try to find some tubing and listen for the hissing. The hissing doesn't occur when the logic board isn't connected.

    Could the issue be on the logic board instead of the power supply board?
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    You are probably having a bad primary filter capacitor, may be all the elec. caps in the primary circuit need to be changed.
    The loading is producing the hissing sound and voltage variation.
    Check the primary caps.

    Rifaa
     
  7. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    You might want to check those capacitors. You may have bought a faulty one. I've done it. It's frustrating, I know. The problem could be on either board, but I think connecting the logic board is just a matter of putting a load on a marginally-working supply so that the problem occurs then. I'd bet on the power supply board, and in particular on one of those capacitors you replaced. But you might be investigating why the capacitors blew in the first place. Could be that when you put the new ones in one of them got zapped by the original fault. I suggest you take out the capacitors again, check them one at a time with a test circuit and if you find one that's bad, check all of the components it sees when in the circuit.
     
  8. keenez

    Thread Starter New Member

    May 30, 2009
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    I don't mind trying to replace all the capacitors again that I replaced as they were inexpensive. Since I couldn't get 105 degree capacitors for the logic board, can I use capacitors with the same capacitance but a higher voltage as a replacement?

    Rifaa - I'm not exactly sure what you mean by primary filter capacitor. Are you talking about the one that is used in the AC-DC conversion?

    Thanks for all the suggestions everybody, it is greatly appreciated.
     
  9. peranders

    Well-Known Member

    May 21, 2007
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    It's only necessary to 105 deg caps if they work in a very hot environment. If you are talking about room temperature and some more it doesn't matter. Let's say there is a 70 deg environment, here you must choose 105 deg parts in order to get some lifetime.
     
  10. keenez

    Thread Starter New Member

    May 30, 2009
    6
    0
    I'm not sure what the temperature range is when the board is enclosed in its case and is at operating temperature. Ideally I'd like to put in the 105 degree capacitor, just in case, as that's what the original was set at. Since I can't find it, I'm still wondering if I can use the same capacitance but with a higher voltage rating.
     
  11. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I think you can use caps with a higher voltage rating, but that receiver was designed for the caps it has. They should work. If one of them is getting blown due to something else in the circuit you need to find that something else.

    You have a very weird symptom. The fix will be a new component somewhere. But where? That's the crux of the matter. Don't just change all the capacitors again. Odds are it will work for a little while then go kaput. You have to identify the actual problem and fix it.

    In order to isolate the problem your first need to rule out one of those boards. You might measure the current drawn by the logic board when the receiver is working right. Then substitute a resistor to mimick the load. If the unwanted switching does occur, it's the power board; if it doesn't it's the supply board. Start there.

    More later, if you want.
     
  12. keenez

    Thread Starter New Member

    May 30, 2009
    6
    0
    Hello everyone,

    So I just replaced all the capacitors as a quick test. There were only 6 and given my experience, I may have definitely done some improper soldering. However, my problem persists. I'm fairly certain it is the power supply board.

    My question now though is: Is it normal to hear clicking from a transformer?

    This is my first foray into trying to diagnose a problem. I don't have much equipment and don't have a very fancy multimeter as others I've seen.

    To diagnose my problem, I did a little more testing though.

    The voltage drop from the power supply board to the logic board only occurs when the analog svga cable is plugged in and the button is pressed to turn the monitor "on". However, when the svga cable is not plugged in, and the unit is turned "on", there is no varying voltage to the logic board but the monitor will still blink. Obviously, the problem persists.

    The capacitors "popping" is a well-documented problem for this power supply board (as are other problems). Apparently, there is a hot heat source.

    Thanks for your consideration and suggestions.
     
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