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    Magnavox 40me313v/f7 tv PSU

    Need help. Have a Magnovox TV, 40me313v with a bad PSU, on the hot side I believe, and I can't figure out what's going on. I've read through prior threads covering repairs of basically identical PSU's such as https://www.badcaps.net/forum/showthread.php?t=71919 and threads referenced there.I have the service manual.

    Originally the TV just stopped working. Initial examination showed the PSU fuse was fine but Q601 was shorted or otherwise not working and D653 on the cold side was bad. I replaced Q601 and D653 and that's when the problems really started. As soon as I powered the board (not installed in TV) there was a nice spark and Q601 literally broke into pieces. Fuse 602 did not blow. Well I started troubleshooting further and found Q621, Q602, R611, D609A, R609, D608, D607A, R621 were all suspect or bad. I replaced all of those and repowered the board without Q601 installed. No sparks or blown fuses. Testing voltages at Q601 pads I got ~170Vdc D-S (or D-gnd) and ~25Vdc G-S (or D-gnd). The 25v value was high compared to what the schematic shows (10.3) and the 170v is low (190v) but within specs for ratings of Q601 and I (wrongly) thought maybe without Q601 installed, these values were OK. So I installed a second new Q601. Blammo, same blow up of Q601 and it took out some of the other components I replaced and now it looks like R614 is showing somewhat low resistance. F602 is intact. It seems obvious current/voltages at Q601 are not right. Voltage at the output of the D601-D604 rectifier bridge is the same ~170V.

    What do I need to look for? At this point I'm at somewhat of a loss. Do I need to pull legs of D601-D604 to make sure they are OK? What part of the circuit drops the voltage going to the Gate of Q601? Is it D607A only or do R614, R603 and R604 (all 560k smd's) play a role? I did check capacitance of C614 and C601 and they were spot on. I don't have an ESR meter. IC601 seems ok. I broke down after the first failed repair and bought a working PSU. Non powered in-circuit comparisons of the SMD's (resistors, diodes, transistors) and other components between the two boards helped me figure out those other failed components discussed above.

    To summarize, at this point without Q601 installed I can power the board. Installing Q601 causes Q601 to blow up and take out related components as well as some components in the switching control and overvoltage control section.

    Sincerely Greg Lentz

    #2
    Re: Magnavox 40me313v/f7 tv PSU

    What is the power supply board number?

    Did you replace Q601 with the idential part number?

    Have you checked the diodes in the secondary in the event of a short circuit in one of them.
    Last edited by dick_barton; 11-22-2020, 12:15 PM.
    Willing to help but I'm no expert.

    Comment


      #3
      Re: Magnavox 40me313v/f7 tv PSU

      You should at least be using a 60~100w incandescent lamp in series with the a/c line input (or a variac) This will limit the current and may save a few components. These are hard power supplies to work on, Make sure D605 & D623 are ok, these are zener diodes so a diode test will only tell if they are shorted or open, not what voltage they zener at. Without Q601 the circuit will do nothing, and if the circuit does not oscillate, Q601 will be turned on and take 160vdc straight to hot ground.
      I suspect R611 was open and D608 was shorted originally, and when you replaced Q601 everything else went as well, Replace ic601 as well as it is likely damaged.
      When Q601 shorts it usually supplies 160vdc from D to G & S so everything connected to the gate and source will be damaged.
      Last edited by R_J; 11-22-2020, 12:24 PM.

      Comment


        #4
        Re: Magnavox 40me313v/f7 tv PSU

        Originally posted by R_J View Post
        You should at least be using a 60~100w incandescent lamp in series with the a/c line input (or a variac) This will limit the current and may save a few components. These are hard power supplies to work on, Make sure D605 & D623 are ok, these are zener diodes so a diode test will only tell if they are shorted or open, not what voltage they zener at. Without Q601 the circuit will do nothing, and if the circuit does not oscillate, Q601 will be turned on and take 160vdc straight to hot ground.
        I suspect R611 was open and D608 was shorted originally, and when you replaced Q601 everything else went as well, Replace ic601 as well as it is likely damaged.
        When Q601 shorts it usually supplies 160vdc from D to G & S so everything connected to the gate and source will be damaged.
        Thank you for the advice. Agreed that R611 and/or D608 were probably bad when I tried to repair first time. Agreed that I am only able to do standard diode check so even though zener D605/D623 do not appear to be shorted or open, they could be bad. I will replace those along with D608(again) and D607A(again). Is there any way to test IC601? Pins 1-2 showed readings indicating the "diode" was ok. I don't remember off hand what pins 3-4 showed but I know it wasn't a direct short. I do have replacement already in hand.

        Greg

        Comment


          #5
          Re: Magnavox 40me313v/f7 tv PSU

          Originally posted by dick_barton View Post
          What is the power supply board number?

          Did you replace Q601 with the idential part number?

          Have you checked the diodes in the secondary in the event of a short circuit in one of them.
          Board number is BA3AT0F0102-2. Yes Q601 was replaced with same part, TK10A50D.

          Greg

          Comment


            #6
            Re: Magnavox 40me313v/f7 tv PSU

            Originally posted by glentz View Post
            Board number is BA3AT0F0102-2. Yes Q601 was replaced with same part, TK10A50D.

            Greg
            P.S. I did check the secondary (cold side) diodes and found one bad one during my initial inspection. I've rechecked and all seem fine.

            Greg

            Comment


              #7
              Re: Magnavox 40me313v/f7 tv PSU

              Is there a way to isolate the hot side from the cold side? with the problems I'm having I'm hoping the cold side is still ok and would hate to cause more issues because of the problems I'm having on the hot side.

              Greg Lentz

              Comment


                #8
                Re: Magnavox 40me313v/f7 tv PSU

                Originally posted by glentz View Post
                Thank you for the advice. Agreed that R611 and/or D608 were probably bad when I tried to repair first time. Agreed that I am only able to do standard diode check so even though zener D605/D623 do not appear to be shorted or open, they could be bad. I will replace those along with D608(again) and D607A(again). Is there any way to test IC601? Pins 1-2 showed readings indicating the "diode" was ok. I don't remember off hand what pins 3-4 showed but I know it wasn't a direct short. I do have replacement already in hand.

                Greg
                I've heard of the light bulb trick. What is the principle/theory behind that? Is the light bulb like a very low amperage fuse or what?

                Greg

                Comment


                  #9
                  Re: Magnavox 40me313v/f7 tv PSU

                  The light bulb will limit the current, for an example: the power supply has a dead short, the lamp would light at full brightness. If the power supply is trying to work, the lamp might pulse, if the supply is working, usually the lamp will light for half a second then go dim. This is used to check the power supply under light load, once the power supply is repaired and connected to the tv the light bulb should be removed from the a/c input.
                  The cold side provides feedback via the optoisolator to control the primary, this is how the voltage is regulated.
                  Check that D654 & D652 are not shorted
                  Last edited by R_J; 11-22-2020, 08:10 PM.

                  Comment


                    #10
                    Re: Magnavox 40me313v/f7 tv PSU

                    I believe this is a similar power supply.

                    https://www.badcaps.net/forum/showthread.php?t=72916

                    Comment


                      #11
                      Re: Magnavox 40me313v/f7 tv PSU

                      Originally posted by neilc6 View Post
                      I believe this is a similar power supply.

                      https://www.badcaps.net/forum/showthread.php?t=72916
                      Yes that thread is one of the ones I've looked at. I need to figure out why Q601 continues to fry itself, taking out components in the protection and switching control circuit. While voltages at the pads for Q601 seem within spec, it's possible the current isn't being limited properly or it's not oscillating which would point to something in the Q602 part of the circuit, possibly D605.

                      Greg

                      Comment


                        #12
                        Re: Magnavox 40me313v/f7 tv PSU

                        Originally posted by R_J View Post
                        The light bulb will limit the current, for an example: the power supply has a dead short, the lamp would light at full brightness. If the power supply is trying to work, the lamp might pulse, if the supply is working, usually the lamp will light for half a second then go dim. This is used to check the power supply under light load, once the power supply is repaired and connected to the tv the light bulb should be removed from the a/c input.
                        The cold side provides feedback via the optoisolator to control the primary, this is how the voltage is regulated.
                        Check that D654 & D652 are not shorted
                        Ok thanks for the additional information. I had originally checked D654/D652 but I'll check again.

                        Re: prior post about the two zener diodes, D605 and D623. Exact parts are not available, at least not in single piece quantities. It looks like the parts are 2% tolerance and the only parts I can find are 5% at best. Is 5% tolerance OK? If not, do you have a source for suitable replacements?

                        Greg

                        Comment


                          #13
                          Re: Magnavox 40me313v/f7 tv PSU

                          I'm sure 5% tolerance will be ok for the zener's

                          Comment


                            #14
                            Re: Magnavox 40me313v/f7 tv PSU

                            Originally posted by R_J View Post
                            I'm sure 5% tolerance will be ok for the zener's

                            Ok, thanks. I did find a 2% tolerance for one of them BUT it is only 0.35W power dissipation whereas the original ZD is 0.5watt. What do you think???

                            Greg

                            Comment


                              #15
                              Re: Magnavox 40me313v/f7 tv PSU

                              Originally posted by R_J View Post
                              You should at least be using a 60~100w incandescent lamp in series with the a/c line input (or a variac) This will limit the current and may save a few components. These are hard power supplies to work on, Make sure D605 & D623 are ok, these are zener diodes so a diode test will only tell if they are shorted or open, not what voltage they zener at. Without Q601 the circuit will do nothing, and if the circuit does not oscillate, Q601 will be turned on and take 160vdc straight to hot ground.
                              I suspect R611 was open and D608 was shorted originally, and when you replaced Q601 everything else went as well, Replace ic601 as well as it is likely damaged.
                              When Q601 shorts it usually supplies 160vdc from D to G & S so everything connected to the gate and source will be damaged.
                              .

                              Question about the oscillation. Without Q601 installed, will there be oscillation? I assume you mean the 160vdc will oscillate??? Is that something I can see happen with a multimeter or do I need a scope?

                              Greg

                              Comment


                                #16
                                Re: Magnavox 40me313v/f7 tv PSU

                                Thanks to the help I received both in components to look at and theory, I think the board is repaired. Haven't reinstalled in TV yet but doesn't blow up and is producing same primary and secondary voltages as a known good board under the same conditions. After the last blow up discussed above, I decided to buy an ESR meter and one of those inexpensive transistor/diode/resistor identification and test meters. Both were useful with identifying some bad or suspect components that I missed at some point. Could have been after the initial failure in the TV or one of my attempted repairs. In summary: On the hot side - Re-replacement of Q601, Q602, Q621, D607A, D609A, D608, R611, R609 which were all bad or suspect in the case of the surface mount zener diodes (I couldn't test those). An earlier repair also included R621 and F602 which were still fine. For this last attempt on the hot side I replaced C605/C611 (high side of acceptable for ESR), R614 (weird that R603 and R604 were OK), D605, D623, and IC601. Zeners D605/D623 and IC601 were replaced just in case since I had no way to test. On the cold side D653 had been replaced in an earlier repair. In this latest and hopefully last repair I found zeners D654 and D652 were bad using my new test meter even though my standard digital multimeter in diode mode showed they were "OK". After finishing I did connect a 100watt incandescent bulb in series with AC line and crossed my fingers and flipped the AC power switch. The board didn't blow up but the light didn't come on, not even a flicker so I was a little worried. I did some testing with my known good board - no light so I then started probing both boards on both the hot and cold sides and got very similar/equivalent readings at various test points even though the values were off compared to the service manual. I assume the values listed in the service manual schematic are when board is installed and likely the TV is on. I'll report back if repair is really successful as soon as I get courage to put the board in the TV. Wish me luck.
                                By the way the troubleshooting flow chart for this board only discusses components on the hot side for a no power situation but a repair kit I bought included several diodes for the cold side. The chart does discuss these cold side components if the "power fluctuates".
                                Greg
                                Last edited by glentz; 02-02-2021, 11:16 PM. Reason: spelling/grammar

                                Comment


                                  #17
                                  Re: Magnavox 40me313v/f7 tv PSU

                                  It works!!!

                                  Comment


                                    #18
                                    Re: Magnavox 40me313v/f7 tv PSU

                                    Glad you got there in the end. That was some road to go down to finalise the repair.
                                    Willing to help but I'm no expert.

                                    Comment


                                      #19
                                      Re: Magnavox 40me313v/f7 tv PSU

                                      I fixed my 40ME313V/F7 when the transormer started making a chirping, screeching noise after being powered on. Pulled the flex cable from the Digital Main Board (BA3ATZG0401) and the power supply was happy. I removed the Digital board and supplied 21vdc from a variable power supply outputing a few amps and while using a FLIR TG267 found that C3622 had a dead short to ground. Took longer to find a replacement cap then it was to find the problem. I don't think I could have found the bad component without the use of the FLIR.

                                      Hoping that this post can help others in the future, can someone explain how power supplies configured like the 40ME313V/F7's BA3AT0F01 start up? This circuit doesn't have an IC like the FA5640, it doesn't have a connection to the hi side DC from the rectifiers. Is the switching control transistor some how jump started with the first in rush of current thru the transformer? See pic below or page 10-3 for the schematic.

                                      Thanks!
                                      Attached Files
                                      Last edited by mojorizing; 10-19-2021, 09:02 PM.

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