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PI-190DTLB 5v earthing out

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    #1

    PI-190DTLB 5v earthing out

    Hi have this board which appears to be earthing out on the 5v rail .

    All caps have been changed ( FCs and low esr hitano).

    Both shottky diodes renewed ( D205 and D206).

    power mosfets are new ,bridge rectifier ok ,main cap showing 339v.

    14v rail not earthing so ok not sure where to check next .
    Originaly the outputs showed 22v on the 14v and 8v on the 5v, after i had replaced all the parts it blew the R217 resistor (0.33 2watt)
    so have now changed that too.

    I dont want to power it up when i am getting continuity on the 5v so unless
    i am wrong and it should be that way .

    Any input would be great
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    #2
    Re: PI-190DTLB 5v earthing out

    The Source Resitor R217 burning out, would make me immediately look at the Mosfet again(even if it was new),also as you have changed lots of parts, then solder bridges may have inadvertantly occured,also all the caps the right way around ?? I have messed that up before.

    Comment

      #3
      Re: PI-190DTLB 5v earthing out

      All caps are right and have checked for bridges these were the first things i did (messed up too before )
      I will retest the mosfets but am sure these are ok

      Comment

        #4
        Re: PI-190DTLB 5v earthing out

        Does the replacement for D206 have a metal mounting tab? If yes, then is it properly insulated from the heatsink?

        Comment

          #5
          Re: PI-190DTLB 5v earthing out

          Both shottky have plastic casings

          Comment

            #6
            Re: PI-190DTLB 5v earthing out

            Something may be confusing your meter. I say plug it in and see how it goes. At worst, it will simply refuse to power up.
            Originally posted by PeteS in CA
            Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
            A working TV? How boring!

            Comment

              #7
              Re: PI-190DTLB 5v earthing out

              Well i suppose worst that can happen is it may blow the resistor again

              Comment

                #8
                Re: PI-190DTLB 5v earthing out

                Was the Mosfet faulty initially ??/ which is maybe why you replaced it ??
                If it had gone short cct, then certainly all resistors AND the surrounding diodes may also be damaged.

                Comment

                  #9
                  Re: PI-190DTLB 5v earthing out

                  Ok breif repair history

                  Board came in with no power most caps bulging so all replaced .
                  Mosfets tested ok but had spares and there were heat marks so changed those for new , transformers tested ok so resoldered .
                  Shottky tested 10100 ok 1060 faulty so changed both (both new).

                  On now tracing the 5v rail i have noticed on the schematic that ZD203 bridges between earth and 5v so i have tested this .

                  Result with analogue meter
                  X1 = open
                  X10 = open
                  X100 = open
                  both ways

                  DMM on diode setting
                  reading both ways down to zero

                  So i am assuming that this is where the problem lies .
                  I am going to remove it and check the 5v pin but the only replacement i can get (6v2) are axial ones ,can get smd type if i want to buy 5oo and wait 2 to 3 weeks
                  Axial should be ok or should i wait for surface ones and have a serious excessany thoughts ?

                  Comment

                    #10
                    Re: PI-190DTLB 5v earthing out

                    Just use a regular axial zener diode. Makes sense that the zener shorted if something on that rail went terribly wrong. It's just that they insist not using them anymore (of course, so that more expensive components blow when the power supply acts up), so i didn't think of that.
                    Originally posted by PeteS in CA
                    Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                    A working TV? How boring!

                    Comment

                      #11
                      Re: PI-190DTLB 5v earthing out

                      Just looked at the scheme and it does not say the wattage

                      1.3 or 0.5 which would be best ?

                      Comment

                        #12
                        Re: PI-190DTLB 5v earthing out

                        The zener is there to protect the rest of the parts if the power supply goes awry. It does not do anything during normal operation. Since it's job is to short out and shut down the power supply, use a 1.3W diode. A half watt one may simply melt and allow the power supply to do further damage.
                        Originally posted by PeteS in CA
                        Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
                        A working TV? How boring!

                        Comment

                          #13
                          Re: PI-190DTLB 5v earthing out

                          Ok thanks Guys will get them and see what happens

                          Comment

                            #14
                            Re: PI-190DTLB 5v earthing out

                            First, determine if you really do have a zero ohm short to ground on the 5v, I would have a look at ZD203 - desolder one end. These things often short, because they are only a volt or two higher than the rail. All your secondary voltages are high, but that might be due to the 5v short. Have a look at the feedback circuit D201/C206 also. In very troublesome circuits, you can usually apply a lower voltage (via variac) that will prevent parts blowing up while you do more troubleshooting. This supply should start up at 150v - 180v (don't bring it up slowly). IC201 is the drive oscillator and receives a startup voltage from R233 and R234. Operating B+ is derived from D201. Post results!
                            Is it plugged in?

                            Comment

                              #15
                              Re: PI-190DTLB

                              Please help me.... what is it this part >> ZD204 D202 D203 ??? ...

                              Comment

                                #16
                                Re: PI-190DTLB

                                Originally posted by user1111 View Post
                                Please help me.... what is it this part >> ZD204 D202 D203 ??? ...
                                please help

                                Comment

                                  #17
                                  Re: PI-190DTLB 5v earthing out

                                  Hi,

                                  I have the same board from a Lifetec 19" LCD.

                                  But it's a different problem, I have no power on the screen (no LED, nothing at all).
                                  I've taken it apart and when I plug in the power cord, I hear a ticking noise every second coming from the little transformer component between the AC-in and the primary capacitor, I'm guessing it's for the filtering right ? (my board has the same reference but a slightly different layout).

                                  Could it be the white glue that has become conductive ?

                                  Caps are all crappy HEC but they're apparently OK (no bulging, so at the moment I'll leave them as is).

                                  All three fuses are good.

                                  Comment

                                    #18
                                    Re: PI-190DTLB 5v earthing out

                                    Originally posted by SuperDuty View Post
                                    I hear a ticking noise every second coming from the little transformer component between the AC-in and the primary capacitor,
                                    Ticking suggests

                                    1) the startup cap is bad
                                    2) the SMPS IC is bad
                                    3) something is shorted causing the SMPS to restart over and over (like a diode)
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                                    Comment

                                      #19
                                      Re: PI-190DTLB 5v earthing out

                                      The ticking sound stopped when I screwed the board in place, so I guess it was just a grounding problem.

                                      Now there is still no power, the main switch is good (not the one in front of the screen, but the power supply switch which is next to the AC-in plug), I'm not getting voltage around the AC-in, or very low voltages, the voltages at the varistor are constantly changing from negative to positive, but still very low (I'm not sure how to test this one), I noticed the primary cap gets warm.

                                      Comment

                                        #20
                                        Re: PI-190DTLB 5v earthing out

                                        On the schematic in post #1 of this thread, there's a zener diode (ZD203) across the 5V output. Try checking it for a short.
                                        Last edited by jetadm123; 02-04-2013, 03:09 PM.

                                        Comment

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