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Vox AC4 blowing main fuse. done stuff, need more help

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  • Vox AC4 blowing main fuse. done stuff, need more help

    2 fuses in this one. not shown on schematic. one for heater, one for main. Heater fuse stays intact on power up, therefore tube heaters and LED pilot glows.
    Little 125mA fuse blows instantly. it is connected between PT CT and ground.

    checked the power transformer, checks out good.

    Blows fuse with the tubes removed.

    Changed the rectifier diodes because I read where that was an issue at times (common problem?).

    Checked the "big" resistors (220 & 470 ohm) all good.

    Checked some caps, the undesignated 100uf/350V and 33uf/350Vin bottom right corner of sch. OK

    Checked C17, a 10uf/350V, because I could get to the yellow glue to cut the cap loose and remove from board. OK

    I haven't yet pulled C7 & C8, because of where they are and the yellow glue holding them in.

    Can I test for fuse blowing with the OT unhooked from the circuit? trying to narrow down the fault but don't understand if that's ok or not. I have been powering up with a 16 ohm dummy load.

    I fixed a Fender Champ recently and hoped this "easy" circuit would also be an easy fix, but not yet.

    help please, thx
    Attached Files

  • #2
    Yes, you can safely power up with OT disconnected. Also, you can run without a dummy load when the power tubes are pulled--the load is for the power tubes.

    You'll save fuses and a lot of stress if you use a bulb limiter while you're testing for this fault condition!

    Any areas on the PCB look burned or discolored by heat?
    --
    I build and repair guitar amps
    http://amps.monkeymatic.com

    Comment


    • #3
      I went ahead and unplugged the OT, which also takes out the attenuator circuit, still a problem.

      Originally posted by xtian View Post
      You'll save fuses and a lot of stress if you use a bulb limiter while you're testing for this fault condition!
      what a dunce i am. I've blown several fuses, and i built a light bulb limiter last year. I forgot to use it.

      Originally posted by xtian View Post
      Any areas on the PCB look burned or discolored by heat?
      everything looks fine.

      Comment


      • #4
        Unless you have a cap tester that tests them at high voltage, they are still suspects.
        They often collapse only when the voltage gets fairly high, and will still read ok when tested with a regular cap meter such as found on a DMM.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #5
          I guess I need to invest in one of those.

          Comment


          • #6
            If you still have a short with all of the tubes pulled and the diodes known good and in proper orientation, just float the caps. You only need to disconnect one lead. Make sure you know how to discharge caps before you start.

            Comment


            • #7
              Originally posted by olddawg View Post
              , just float the caps. You only need to disconnect one lead.
              don't know what you mean. Can you dumb it down for me? thx

              Comment


              • #8
                Just disconnect the positive side of the cap and let it "float" out of circuit.
                When the going gets weird... The weird turn pro!

                Comment


                • #9
                  Ah, i see. Don't think i can really do that as these are radial caps sitting right down on the PCB with about 1/4" leads. Unless someone can explain a better method, these just about have to come off the board anyway.

                  Comment


                  • #10
                    Originally posted by Kenrod View Post
                    Ah, i see. Don't think i can really do that as these are radial caps sitting right down on the PCB with about 1/4" leads. Unless someone can explain a better method, these just about have to come off the board anyway.
                    You just use a solder sucker or desoldering braid to free one lead from one pad. The hole is lightly larger than the size of the cap lead. With the solder removed it should be an open circuit.

                    Comment


                    • #11
                      I'm back at it this morning. Still no good at freeing one leg of a radially mounted cap. i pulled both C7 & C8 and checked them, found C8 to be shorted.
                      Now to find that replacement cap and get it back together.

                      if anyone has noticed my sporadic time frame on my post updates, this is a sporadic hobby (as i have too many) and I have very understanding friends. lol

                      Comment


                      • #12
                        I replaced the 220uf 63V cap that tested bad. Still blowing a fuse. Something is still shorting to ground, just haven't found it yet.
                        Per a previous comment, I'll see if anyone here at work has a tester that will help properly test the higher voltage caps.

                        It's a simple enough circuit, so I'm staying the course for now.

                        If there are other suggestions, i'm listening. TIA

                        Comment


                        • #13
                          Where did that attachment go from post #1, was it ever there?
                          Can you repost the schematic you are using?
                          I sure hope we are not losing attachments again.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #14
                            It was there originally, but here it is again.


                            edit: well thats odd i dont see my own attachment

                            edit again: I see my attachment while editing. do you guys see an attachment?
                            Attached Files
                            Last edited by Kenrod; 09-14-2016, 12:47 PM.

                            Comment


                            • #15
                              Nope.
                              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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