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Harman Kardon AVR 3700 blown standby power supply

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    #21
    Re: Harman Kardon AVR 3700 blown standby power supply

    I was afraid that you were going to say that! I'll see if I can manage to cobble something up to ring test it. I'll tackle it Sunday or Monday and reply back to the group. Thank you!

    Jason

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      #22
      Re: Harman Kardon AVR 3700 blown standby power supply

      I wonder ,if we have bad transformers, from where would we get new ones?

      Comment


        #23
        Re: Harman Kardon AVR 3700 blown standby power supply

        Originally posted by tibimakai View Post
        I wonder ,if we have bad transformers, from where would we get new ones?
        I'm more than a little worried about that myself...

        I still won't have time to test the transformer until tomorrow or Monday, but if the transformer has failed, I guess the first step is to call Harman international and see if they will sell a replacement. You wouldn't think that one would have much success with this, however, in car audio repair I have found many manufacturers willing to sell some parts to a hobbyist repair tech.

        I am at work today, but if things are slow I will try to see if I can find who manufactured the transformer. I searched the part number early on and google doesn't find anything, but if I can identify who the OEM is, then possibly can order from them, or get a datasheet and order something that is a close match.

        Other than that, the AVR3700 service manual says that this transformer is used in the 1700/2700/3700, so search for a receiver being sold for parts. There are a few on eBay right now that power on but the video boards are screwed (apparently another common problem with the "new" Harman brand). Looks like the going price is $100, pretty steep for a $2 transformer, but a lot cheaper than another $1000 receiver.

        *edit for another option* You may also be able to rewind it. It doesn't appear to be potted, and as long as it doesn't have a few thousand turns, it is potentially possible to rewind it. It won't be easy though. If mine is bad, I will probably start unwinding it just to see it's construction. Most of the car audio stuff I deal with uses torroidal transformers, it's still a PITA to rewind one of those, but I suspect it's easier than doing one of these little EI core(???) switching transformers.

        Good luck,
        Jason
        Last edited by jacampb2; 07-22-2017, 07:35 AM.

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          #24
          Re: Harman Kardon AVR 3700 blown standby power supply

          I mentioned earlier in this thread that I had a similar thread going on DIYaudio.

          Eva brought up something interesting. She said that it is possible that the current sense resistors are wrong, that if they were 10x's higher resistance then they should be, that the switcher would behave similarly. This has me thinking. The schematic specifies 10R and there are 5 in parallel. I had to replace them because all 5 were scorched and reading 1+ Mohm. I used 10R, I measure the resistance of the group at the CS pin because I wanted to insure no open traces or bad solder joint, and it was just a bit over 2 ohms. At the time I was thinking that 2 ohms seems kind of high for a current sense. I know that schematics are sometimes flawed and not the most recent revision. I wonder what the chances are that these were supposed to be 1R not 10R. I'm going to see if I can still find any of the pulled ones on my bench and clean it up enough to read it...

          tibimakai, you mentioned you have the AVR1700-- any chance your CS resistors are still in tact and you can verify? This would be R928, R967, R972, R970, and R971 in the attached AVR3700 schematic.

          I'm also going to double/triple check D910. I think I will pull it and test out of circuit this time just to be 100% sure.

          Thanks folks!
          Jason

          Comment


            #25
            Re: Harman Kardon AVR 3700 blown standby power supply

            Originally posted by tibimakai View Post
            I wonder ,if we have bad transformers, from where would we get new ones?
            You rewind them. Done that a couple times. It's not that difficult once you get the hang of it. But that's coming from someone who has built SMPS'es from scratch including hand-wound transformer with re-used bobbins and cores.
            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


              #26
              Re: Harman Kardon AVR 3700 blown standby power supply

              Originally posted by jacampb2 View Post
              I am at work today, but if things are slow I will try to see if I can find who manufactured the transformer. I searched the part number early on and google doesn't find anything, but if I can identify who the OEM is, then possibly can order from them, or get a datasheet and order something that is a close match.
              I think most of them are custom-made, so the part number will be unique to that receiver
              "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
              -David VanHorn

              Comment


                #27
                Re: Harman Kardon AVR 3700 blown standby power supply

                That has been my experience with the search...

                I'm really tempted to just build a new standby supply for it. I think I only need rails of ~28Vdc (for the 24V linear regulated supply) and ~7.5dc for the standby 5Vdc. Then what, something like 12V to supply the ICE switcher that does the +/-12Vdc rails...

                That's my absolute worst case plan. Hopefully Harman will sell me a transformer if it comes to it.

                Thanks,
                Jason

                Comment


                  #28
                  Re: Harman Kardon AVR 3700 blown standby power supply

                  Originally posted by rievax_60 View Post
                  It depends on available equipment. I have an audio signal generator with a low impedance output. If I apply say a 1khz square wave to the transformer primary through a 10nf capacitor, I expect to see clean looking damped oscillations at every transition of the square wave. If I then set the generator's frequency to match the frequency of the damped oscillations, the series resonance will cause a very large signal to develop across the primary.
                  The 1khz test signal from the oscilloscope can be used. Because it will be high impedance, the oscillation will be more damped. Using a lower value capacitor would help make the result more meaningful.
                  http://www.brats-qth.org/training/ad...s/scopeani.gif
                  Quick question, does the cap go across the primary, or in series?

                  Thank you!
                  Jason

                  Comment


                    #29
                    Re: Harman Kardon AVR 3700 blown standby power supply

                    Originally posted by jacampb2 View Post
                    That has been my experience with the search...

                    I'm really tempted to just build a new standby supply for it. I think I only need rails of ~28Vdc (for the 24V linear regulated supply) and ~7.5dc for the standby 5Vdc. Then what, something like 12V to supply the ICE switcher that does the +/-12Vdc rails...

                    That's my absolute worst case plan. Hopefully Harman will sell me a transformer if it comes to it.

                    Thanks,
                    Jason
                    Take it apart and carefully unwind it, counting the turns on each winding. It's not too difficult redoing that by hand. SMPS transformers have way less turns than conventional mains ones, and they're done with thicker wire which is easier to handle.

                    To take the core apart without breaking it, take all the tape off the sides first. Then place the transformer in a pot of boiling water, boil it for about 3-5 minutes to soften the glue, then tap it at the join with a sharp blade and a small hammer. Should come apart easily.

                    Even if a leg cracks off or something you can use superglue to stick it back on. It won't matter much. Done that plenty of times...
                    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


                      #30
                      Re: Harman Kardon AVR 3700 blown standby power supply

                      Thank you for the tips. I will absolutely try to rewind it if it comes to it. I'm going to try to purchase first, I would like to get this up and running sooner than later and I have so many projects going right now that I have to pick my battles!

                      Thanks,
                      Jason

                      Comment


                        #31
                        Re: Harman Kardon AVR 3700 blown standby power supply

                        Originally posted by jacampb2 View Post
                        Quick question, does the cap go across the primary, or in series?
                        He said apply the signal to the transformer, through the capacitor. That would mean in series, between the function generator output and the transformer.

                        Capacitors pass AC and not DC.
                        "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                        -David VanHorn

                        Comment


                          #32
                          Originally posted by Agent24 View Post
                          He said apply the signal to the transformer, through the capacitor. That would mean in series, between the function generator output and the transformer.

                          Capacitors pass AC and not DC.
                          Thank you, that is what I thought he meant, I just wanted to make sure. I ended up figuring it out. I thought I posted the results of the test but it looks like my reply is missing. I'll add it again in a moment.

                          Thanks,
                          Jason

                          Comment


                            #33
                            Re: Harman Kardon AVR 3700 blown standby power supply

                            Ok, I think the results are in...

                            First waveform is a known good SMPS transformer. 1kHz square wave fed through a 10nf poly cap (in series) with the scope across the same winding. When I short any other windings on this xformer, the ringing drops to 1-2 cycles and the amplitude drops dramatically.



                            Second is the Harmon Kardon transformer in the exact same setup. Shorting any other winding on the transformer changes nothing. I get a few more cycles from the ring if I check the bootstrap winding instead of the primary, but overall, it's far more dampened than the known good xformer.



                            So, am I contacting Harmon international tomorrow? I'm attaching a picture of the transformer in question, to my uneducated eye it looks like rewinding it should be fairly straightforward.

                            Thanks for all your help!

                            Sincerely,
                            Jason
                            Attached Files
                            Last edited by jacampb2; 07-23-2017, 02:47 PM.

                            Comment


                              #34
                              Re: Harman Kardon AVR 3700 blown standby power supply

                              Bummer, it does look like the transformer has gone shorted somehow
                              "Tantalum for the brave, Solid Aluminium for the wise, Wet Electrolytic for the adventurous"
                              -David VanHorn

                              Comment


                                #35
                                Re: Harman Kardon AVR 3700 blown standby power supply

                                I forgot to add above, I was using a very inexpensive "Function generator" (part of one of those ATmega based tranasistor testers) for the 1khz square wave. I tried to find the output impedance, but the spec is not published. I measured the output impedance by adding parallel resistance between the generator and the scope until I got to half of the unloaded p-p value. I read that this should give me a very close approximation of output impedance. I got 806R for the the measure output impedance.

                                Thanks,
                                Jason

                                Comment


                                  #36
                                  Re: Harman Kardon AVR 3700 blown standby power supply

                                  I called Harman Kardon tech support and they were willing to sell me the transformer. It took a bout a half hour to get it all taken care of, but in the end it was only about $15 shipped and my time is worth that much.

                                  I still might tear down the old transformer and try to rewind it when I get some of my other backlogged projects done, but for now I will try out the new factory replacement.

                                  It looks like it will be a week or so before it comes in, I'll post back when I get it installed and let you know if it was the problem.

                                  Thanks,
                                  Jason

                                  Comment


                                    #37
                                    Re: Harman Kardon AVR 3700 blown standby power supply

                                    You should do the ringing test on the replacement transformer too just to have that as a future reference.
                                    Never stop learning
                                    Basic LCD TV and Monitor troubleshooting guides.
                                    http://www.badcaps.net/forum/showthr...956#post305956

                                    Voltage Regulator (LDO) testing:
                                    http://www.badcaps.net/forum/showthr...999#post300999

                                    Inverter testing using old CFL:
                                    http://www.badcaps.net/forum/showthr...er+testing+cfl

                                    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                    http://s807.photobucket.com/user/budm/library/

                                    TV Factory reset codes listing:
                                    http://www.badcaps.net/forum/showthread.php?t=24809

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                                      #38
                                      Re: Harman Kardon AVR 3700 blown standby power supply

                                      Excellent advice, I will definitely do it and post a screen shot, maybe it will help someone else in the future.

                                      Thank you,
                                      Jason

                                      Comment


                                        #39
                                        Re: Harman Kardon AVR 3700 blown standby power supply

                                        We don't know when the transformer was originally damaged.

                                        Comment


                                          #40
                                          Re: Harman Kardon AVR 3700 blown standby power supply

                                          Originally posted by rievax_60 View Post
                                          We don't know when the transformer was originally damaged.
                                          What do you mean? Are you saying I may have damaged it by trying to test it with 12v 60hz AC? That may well be the case, but I think I had things narrowed down to the transformer before I pulled it and tried my not very well thought out test.

                                          Aside from that, the only other thing I can think of is that this blew just after some big thunderstorms went through. You would have thought the MOVs would have caught any reasonable spikes, but I've heard of lots of mysterious issues following storms.

                                          From what I could gather while talking to harman kardon today, there is a new part number for TF91, the transformer in question, I ordered the old one, the same as I had in case there were more changes with the newer revision, but I didn't ask for a new copy of the service manual to see for myself. This is pure conjecture, but maybe they did a slight revision due to a design flaw with the original part. It does seem like there are an inordinate amount of these out there that "won't power on".

                                          Thanks,
                                          Jason

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