Power Supply board - burnt out resistors

Discussion in 'Technical Repair' started by sunomo, Dec 23, 2018.

  1. sunomo

    Thread Starter New Member

    Dec 23, 2018
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    Hi everyone, I'm trying to repair a Sony PVM 2730QM monitor. The picture flickers and shakes very much and I hear a strange electric noise.

    The source of the noise is the Power Supply board. I dismantled the board and through an eye inspection, there is an external burn between the C621 C682 capacitor block and the R679 resistor. I think that the resistor burned first and those on the capacitor protection are just residual.

    I can easily replace these components, but I need to understand the reason of the fault, otherwise it's going to happen again. I tested D609 diode and Q606 transistor off board with multimeter and they should be ok. Any suggestion to start testing or repairing?

    All the schematics attached. Thanks in advance!

    Full Service Manual: https://archive.org/download/sony_PVM-2730QM_Service_Manual/PVM-2730QM_Service_Manual.pdf
     
  2. AlbertHall

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    Jun 4, 2014
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    The black mark on the resistor doesn't look like it was caused by overheating - that generally shows worst in the middle and getting less towards the ends and evenly around the circumference. That resistor is blackest at one end and the blackness does not spread around the circumference. The insulating sleeve (which I guess was around the resistor) has a very localised black circular mark which again doesn't look like it was caused by the resistor overheating.

    I think this was caused by arcing from the resistor through the sleeve to whatever was next to the sleeve. Part of the strange noise may have been the sound of that arcing. Was one of the capacitors next to that burnt mark on the sleeve? If not what was?

    Measure the values of all the components connected to the collector of Q602 - four resistors, two capacitors and the diode.
     
  3. oz93666

    Distinguished Member

    Sep 7, 2010
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    That might be true for carbon resistors , but I think the one pictured is wire wound , in which case a hotspot could have developed leading to failure consistent with picture evidence .

    Many KV would be required to produce arcing... Where could this come from?

    The fault may have occurred because the two components were in contact , make sure there is space all around the resistor . Not sure of the reason for the fiber glass sleeve this will only cause overheating.
     
    Last edited: Dec 24, 2018
  4. sunomo

    Thread Starter New Member

    Dec 23, 2018
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    Thanks for your messages. Those 2 capacitors were inside that sleeve, not the burnt resistor. The black spot on the resistor was in front of the black dot of the sleeve. Maybe the noise was an arcing between these two blocks, but why? Also I suspect that it started from the resistor because the capacitor sleeve inside is completely white. I've tested off circuit the components near Q602, four resistors and the diode, and they are ok. I've replaced the 2 capacitors and the burnt resistor. Now should I plug in the electricity?
     
  5. AlbertHall

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    Jun 4, 2014
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    Yes, I think so. I can't think of anything else you could usefully do. Just ensure you have as much space between the components as possible.
    There will be quite high voltage peaks there because of the back emf from the transformer. Those components are intended to absorb the energy from those peaks and so reduce the voltage. So faulty components could increase the voltage.
     
  6. sunomo

    Thread Starter New Member

    Dec 23, 2018
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    Hi, I've turned it on, I still hear that sound, but far less than before. Anyway now on the +15V live I have +13.5V and on the +140V line I have +135V. Unfortunately I have nothing on the +8V line and the monitor does not switch on. What else should I check?
     
  7. AlbertHall

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    Check R677, D656, C659, and C655.
     
  8. sunomo

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    Dec 23, 2018
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    C659, R677 and D656 are ok. On R677 I have just 0.2V but it's very hot. About C655 I don't have anything to test a 2200 capacitor. So I swapped it with the equivalent C654. The result is that now I don't have anything on both 8V and 15V lines. Tomorrow I buy 2 replacement capacitors, but is that the key or the fault is still somewhere around?
     
  9. AlbertHall

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    It does sound like C655 is faulty, but I don't think it is the only fault. Let us know what happens with the new capacitors.
     
  10. sunomo

    Thread Starter New Member

    Dec 23, 2018
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    Thanks for you message. Ok, I've replaced those 2 capacitors, and now I've around 8V 15V 140V on the respective lines, the monitor degauss but the front panel and the tube don't switch on. Maybe it is due to the fact I've no voltage on FA8 line (the one in the lower right corner in the scheme)? What should I expect there? Btw I still hear a sound from the supply board, but now it's very similar to a cricket sound...
     
  11. AlbertHall

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    Then I think the problem is in the rest of it, not in the PSU :(
    Do you have a schematic of the rest?
     
  12. AlbertHall

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    There should be 0V on that line. If there was a voltage, it would shut down the supply.
     
  13. sunomo

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    Dec 23, 2018
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  14. sunomo

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    Dec 23, 2018
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    I've another PVM2730QM 100% working, so I've tried to put into this faulty one a working PSU. The monitor degauss and the tube turns on. However the control panel with its leds are still disabled and the screen is light blue. Since I also hear a constant hiss, I think that also the flyback needs to be replaced. Btw no cricket sound with this one. Anyway to start understanding what's wrong with the faulty PSU I could try to compare the out voltages between the twos, but what else?
     
  15. sunomo

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    Dec 23, 2018
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    Here is the voltage value comparison between the faulty PSU with a working PSU.
    Woking PSU:
    8V line 12.3V
    15V line 15.7V
    140V line 141.8V
    +18V line 21.9V
    -18V line -21.6V
    Faulty PSU:
    8V line 10.5V
    15V line 15.6V
    140V line 129.7V
    +18V line 23.2V
    -18V line -22.5V
    Are these values acceptable? This deviation could be the reason why the tube does not switch on? If so what else should I check on the faulty PSU?
    Thanks
     
  16. AlbertHall

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    Is this with both supply boards in one monitor?
    If so is this the working or non-working monitor?
     
  17. sunomo

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    Dec 23, 2018
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    I've put in the same non-working monitor a working PSU taken from another working twin monitor and then I've put back the original faulty one. I got these values for 2 different PSU into the same non-working monitor. The Working PSU is able at least to switch the tube one, the faulty original PSU is even unable to do that.
     
  18. sunomo

    Thread Starter New Member

    Dec 23, 2018
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    Maybe it's due to that fact 129.7V on the 140V line are not enough to let the tube start?
     
  19. sunomo

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    Dec 23, 2018
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    The problem is that the 140V line is not stable and it rages from 123V to 138V. Why is that? Anyone?
     
  20. JoeJester

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    Apr 26, 2005
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    Have you checked C652? Specifically the ESR? 160V rating seems low to me leaving not much overhead.
     
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