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  • Koch Power Transformer

    Hello!

    I have repair a Koch Jupiter 45 a month ago. The amp came with the transformer HT tap with only 10VAC ( it should be 250VAC ).

    This amp has two 12ax7 on the pre amp section and two mosfets on the power section.

    I have send an email requesting a quote for the transformer and since they didnt bother answer me i send it to a shop for a rewinding.

    After one month playing with it, the same thing as happen again. this time 50VAC on the HT so i send it again for repair.

    My question is;
    Do you think the circuit has a fault that is pulling a large amont of current of the transformer?
    Or it was just a bad winding?

    Thanks

  • #2
    i guess there is a fault in the power section and there is not an adequate protection via fuse or other.
    When powering an amp you have to apply voltage gradually and monitor current.

    Comment


    • #3
      The voltage was applied gradually and everything was fine during a month playing.

      Comment


      • #4
        two mosfets on the power section
        I mean, on the output section

        Comment


        • #5
          I really need some help on this.

          I send the transformer once again for a rewinding ( same shop ).

          I put a smaller fuse on the HT tap, connected the amp to a load while monitoring the output and the voltage on the B+.
          I let the amp about 1 hour on the load, everything looked ok, so i was about to turn off the amp, immediately after i pull the jack with the signal the B+ drop to 70v.

          It happen again,

          What may be causing this?

          Comment


          • #6
            Can you post a schematic?
            Are the mosfets running off the same HT?
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #7
              I can't find the schematic.
              The mosfets are running on other transformer tap.

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              • #8
                Does that (other) tap remain working when the HT tap goes bad?
                Last edited by g1; 01-28-2020, 10:29 PM.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #9
                  Yes.
                  Only the HT stops working.

                  Comment


                  • #10
                    Maybe bad diodes or filter caps in HT supply?
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    Comment


                    • #11
                      Before connecting the transformer i replaced the diodes ( just to be sure, they were ok ), check solder joints and the resistors around the tubes.

                      The capacitors seem ok to me. I thought replacing it but i didnt do it.

                      While testing, i replaced the fuse ( 160ma to a 100ma ). I guess if the circuit was pulling to much current, the fuse will blow??

                      Comment


                      • #12
                        The fuse should be designed to blow to protect the winding, but I'm not sure of the value. You would not need a 100 or 160mA winding to run a preamp tube or 2. So it's possible the winding would be over-stressed before the fuse blew.
                        I would replace those HT filter caps anyway.
                        But I also think you should try a phone call to them directly in Netherlands. They should be able to tell you whether there have been any problems with those transformers. http://www.koch-amps.com/koch-amps-contact.html
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          I will try to contact them and replace the caps.
                          Hope they will answer me

                          Comment


                          • #14
                            I have talk to them ( koch ) on the phone. They said its very strange. Its not a usual thing like this to happen

                            Comment


                            • #15
                              Maybe the guy who did the rewind could tell you the current capacity of that winding?
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment

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