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Harman Kardon AVR 3700 blown standby power

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    Harman Kardon AVR 3700 blown standby power

    Hello,

    This is a new iteration of the same issue with the standby power being defective. Based on the past users experience and the AVR 370 schematic I was able to finally found all the bad parts and slowly going to put this back together. As a short pro-log, the initial state of the fuse (8A) led me to take a cautious approach to this.
    Is never good to to replace the exact fuse and power back on, this is the worst approach.
    Instead, put a small 1A fuse and use Variac to slowly increase the voltage. I did this and the 1A fuse blow quietly at very low voltage, with no bang and whistles. Then I remove the board and to see the previous damage.
    I attached a picture of all the parts that have been damaged because of the 8A short. It's like the pins of the OB2358 have been all put to 260V DC, therefore the diodes and resistors got all blown up. The replacement IC will not work without the correct feedback from the optocoupler, there is a zenner there also that went bad. There is no need to put the exact zenner values, the working voltage is far less than the diode value.

    Finding the chip is tricky, I fast found one in an IKEA Led lamp power supply following a thread on internet about that lamp. It's PHY368DA and as you can see, it works just fine, it's a sort of a similar/clone of the OB2358.
    I also ordered OB2358 from Ali, but it will take time. I will use the IKEA lamp power board as a test jig to check the OB2358 I will receive, to check if they are real or if the working voltage is right. I will put a follow up wit more details on this thread.
    I am still waiting for the small SMD resistors and diodes to come so I tested with thru hole variants, it works. Also used the rebuild transformer, since I had no idea if I will be able to get one exactly the same.

    Cheers
    Attached Files

    #2
    Re: Harman Kardon AVR 3700 standby transformer

    OK, so I did the transformer re-winding because I really wanted to see if it works, after I got the IKEA chip in place. I checked the transformer I just build with the universal meter, it measures quite good.
    The burned TF91 had a short in the primary winding so it was measuring just 1.2 ohm, no mH reading.
    Measuring the rebuild TF91 is a lot different. I believe this is easier than the "ringing" of the transformer others mentioned, much cheaper for sure.

    Cheers
    Attached Files

    Comment


      #3
      Re: Harman Kardon AVR 3700 blown standby power

      Once, I had to buy a similar transformer(part# CLT9Z076ZE) for an AVR1700, and it was very cheap from Harman directly. It was $6.72. Email: parts@harman.com, (800) 336-4525

      Comment


        #4
        Re: Harman Kardon AVR 3700 blown standby power

        Originally posted by tibimakai View Post
        Once, I had to buy a similar transformer(part# CLT9Z076ZE) for an AVR1700, and it was very cheap from Harman directly. It was $6.72. Email: parts@harman.com, (800) 336-4525
        Yes, thank you for your suggestion, I tried first HK US and I was redirected to the Canadian side, for an "info@eriksonconsumer.com" support email and then to a company called Jam industries... quite complicated.
        But I was surprised to receive an email from their rep and they actually got the transformer! I bought the last 2 they had, just in case... I might use-it or not, but I felt that for the price it was well worth-it. See the picture!
        I also asked for a remote ( original), they have-it in stock but for a... 110$!
        No way I will pay that much for a remote, even original.
        I still wait for the parts from Newark, some are slow from UK, the 10 ohm resistors I believe they come from there. I will put a 1A mini fuse in the supply of the TF91, just to prevent such an avalanche of damaged parts..

        Cheers!
        Attached Files

        Comment


          #5
          Re: Harman Kardon AVR 3700 blown standby power

          Finally It's working.
          I had waited for a long time for the Newark parts to come, it was on hold for no reason.
          It works with the rebuild transformer, I just leave-it and not put the brand new replacement I bought, just curious. It does not warm up at all, nice job, ME!

          To prevent a future failure of the transformer / driver chip and burn the main fuse, I added on the PCB an extra fuse 1A and a small cap 6uF at 400V I recovered from a little power adapter.. It was easy to drill 4 small holes and clean the green film to be able to solder.
          I attached the pictures of the working unit below.
          I never had this HK model before, to use-it I have to find a compatible remote.
          I found-it dumped in recycling and since it was not very scratched I decided to bring-it back to live. This will replace one of my older units I have in the basement and see how it goes.

          BTW, it looks so clean inside because I washed with dish soap and warm water. I hate the dust these things can have inside. While wet I blow the water out of the BGA chips, not to leave calcium deposit. After dried in warm air flow and it's done. Like new, at least inside!
          Cheers
          Attached Files
          Last edited by dvd4me; 04-12-2021, 04:36 PM.

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