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TH30 Blowing HT Fuse with tubes out

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  • TH30 Blowing HT Fuse with tubes out

    Hi, trying to fix an Orange TH30 for a friend but it's got me stumped.
    It's blowing the HT fuse even with all tubes removed.
    I've checked both the OT and PT for shorts and they seem to be fine, I've replaced the bridge rectifier, just in case it was failing. I've gone over the PCB multiple times checking for any little shorts or breaks and can't find anything.

    The only thing I can think of is a leaky filter cap, but wanted to get a second opinion before ordering $30 worth of caps and spending a couple of hours fitting them.

    Any ideas?

    cheers

  • #2
    Might be a good time to build a 'lamp limiter' to keep from stressing the amp.

    There can be no second opinion as you have the amp.

    Try your best to narrow it down.

    Power Tranny: flying leads.
    Remove all tubes and disconnect the OT.
    Remove any dropping resistors between power rail stages.

    You know the drill.

    Orange_th-30-twin-channel-amplifier.zip

    Comment


    • #3
      Your meter can only test things at a volt or two. A output transformer can be shorted in a way that only appears with a few hundred volts. Your meter cannot detect that.


      Don't throw parts at it. If you guess filter caps might be at fault, don't just order a set and replace them, try simply disconnecting them one at a time. Remember, at this point you are looking for a short on the power supply, not tone.

      Tube sockets and circuit boards can arc, another thing that only happens at high voltage, which your ohm meter will never detect. So try this: set up the amp and turn off ALL the room lights. Now when you power on, do you see any tiny blue sparks anywhere on the sockets or board?

      Especially on a problem like this, don't decide it has to be a bad part, in many cases it is something like an arc, and you could replace every part on the board and still not cure an arc.
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #4
        Fully agree and add: divide and conquer:

        1) as Nevetslab just said, "never ignore the obvious"; start by fully disconnecting PT from mains cable - power switch - fuse holder ... still blowing fuse?

        2) if fine, reconnect transformer primaries but disconnect *all* secondaries, this is a complex multi-secondary transformer.
        So remove F2 , F3 , F4 and only then reconnect PT to mains.
        **Always through an around 60W lamp bulb limiter**

        3) if fine, reconnect fuses one by one, in inverse order: F4 > F3 > F2 , each time turning amp ON to check for a gross short (limiter lamp glowing full bright).

        4) my main suspect is the HV line, I expect a short after reconnecting F2, but please test and post results first.
        Juan Manuel Fahey

        Comment


        • #5
          And if you must suspect the output transformer, just disconnect the center tap of its primary from the B+.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Thanks for all the advice everyone. I went and picked up the supplies to make a lightbulb limiter.

            The amp seemed to be behaving with the limiter in use, a quick flash off the bulb as the current rushed in, but then settled down, same results with tubes out(slightly lower glow, but I'm guessing that the filaments) and the fuse wasn't blowing.

            The HT fuse was only blowing when connected directly to the mains power and instantly when you hit the standby switch. So I started removing the dropping resistors in the power supply as recommended by Jazz P and narrowed it down to the first filter cap, after removing that cap the fuses stopped blowing.
            I'm guessing that means that there was a tiny leak in the cap??? enough to blow the HT fuse on startup, but not enough to light up the bulb. Does this sound feasible to you guys?

            Comment


            • #7
              Noooooooo! just found a couple of 100uf/450v caps in a drawer, so paralleled them to check it before I ordered a replacement cap and it's back to blowing fuses, so I guess that cap was fine.

              Should add, other things I tried earlier, disconnected PT secondaries, fuse blows with the 6.3v secondaries disconnected, the HT-fuse is connected between the HV secondaries and the Bridge rectifier, which was already replaced.

              The fuse blows with the OT disconnected, all tubes removed.

              Fuse doesn't blow if First Filter (C2) is removed, but does if replaced by another.

              Fuse still blows if first dropping resistor is removed (R2) and OT centre tap removed.

              I also tried removing the cathode bypass caps, just in case, with them removed, the fuse doesn't blow, but then replaced with something I had lying around and fuse blows again.

              I've had the lights off and had a good look around for any arcing, but can't see anything.
              Checked the leads and connectors from the PT

              I'm really running out of ideas on what could be wrong, with all these things in mind the only thing I can think of is that the PT is failing at high voltage but reading fine with the multimeter, but then shouldn't the lamp be glowing brighter when using the limiter if that was the case?

              thanks again for the help, it's much appreciated

              Comment


              • #8
                Originally posted by jop120 View Post
                Noooooooo! just found a couple of 100uf/450v caps in a drawer, so paralleled them to check it before I ordered a replacement cap and it's back to blowing fuses, so I guess that cap was fine.

                Should add, other things I tried earlier, disconnected PT secondaries, fuse blows with the 6.3v secondaries disconnected, the HT-fuse is connected between the HV secondaries and the Bridge rectifier, which was already replaced.

                The fuse blows with the OT disconnected, all tubes removed.

                Fuse doesn't blow if First Filter (C2) is removed, but does if replaced by another.

                Fuse still blows if first dropping resistor is removed (R2) and OT centre tap removed.

                I also tried removing the cathode bypass caps, just in case, with them removed, the fuse doesn't blow, but then replaced with something I had lying around and fuse blows again.

                I've had the lights off and had a good look around for any arcing, but can't see anything.
                Checked the leads and connectors from the PT

                I'm really running out of ideas on what could be wrong, with all these things in mind the only thing I can think of is that the PT is failing at high voltage but reading fine with the multimeter, but then shouldn't the lamp be glowing brighter when using the limiter if that was the case?

                thanks again for the help, it's much appreciated
                Have you removed C1 10nF/1kV across the AC input terminals of the bridge BD1? Does it matter whether you switch into Full (250VAC) or Half (180VAC) when switching out of Standby? With R2 lifted out of circuit, and C2 removed, do you measure any resistance between the Bridge Output (to O/T C/T) and Gnd? You appear to have zoomed in on where the problem is: Not the O/T primary, Not following R2 (remaining power supply distribution), and, with the O/T Primary C/T removed, that eliminates the four power tube sockets. Is R1 still 220k, or now something much lower?
                Logic is an organized way of going wrong with confidence

                Comment


                • #9
                  You are using slow-blow fuses?
                  Originally posted by Enzo
                  I have a sign in my shop that says, "Never think up reasons not to check something."


                  Comment


                  • #10
                    Originally posted by jop120 View Post
                    Thanks for all the advice everyone. I went and picked up the supplies to make a lightbulb limiter.

                    The amp seemed to be behaving with the limiter in use, a quick flash off the bulb as the current rushed in, but then settled down, same results with tubes out(slightly lower glow, but I'm guessing that the filaments) and the fuse wasn't blowing.

                    The HT fuse was only blowing when connected directly to the mains power and instantly when you hit the standby switch. So I started removing the dropping resistors in the power supply as recommended by Jazz P and narrowed it down to the first filter cap, after removing that cap the fuses stopped blowing.
                    Then that cap is shorted, period.
                    Or was disconnectd and reconnected the wrong way. (+ to - and - to +).
                    I'm guessing that means that there was a tiny leak in the cap???
                    I wouldnīt call that tiny.
                    enough to blow the HT fuse on startup, but not enough to light up the bulb.
                    No, whatever blows a proper rated fuse, specially the HV winding, the most powerful of them all, MUST make the bulb shine quite brighter.

                    Noooooooo! just found a couple of 100uf/450v caps in a drawer, so paralleled them to check it before I ordered a replacement cap and it's back to blowing fuses, so I guess that cap was fine.
                    Quite the opposite: a short in parallel with a cap or *anything* , stays a short, soyour test means nothing.

                    That cap is a strong short suspect (or wrong connection).

                    So far strong suspects still are main filter cap and bridge rectifier ... IF disconnecting OT does not cure thye short.

                    So disconnct first filter cap and second diode bridge and youīll probably find the culprit.

                    Asd mentioned above, C1 is also suspect.
                    Juan Manuel Fahey

                    Comment


                    • #11
                      You are using slow-blow fuses?
                      yep

                      Have you removed C1 10nF/1kV across the AC input terminals of the bridge BD1?
                      Just tried but no difference

                      Does it matter whether you switch into Full (250VAC) or Half (180VAC) when switching out of Standby?
                      does it on both

                      With R2 lifted out of circuit, and C2 removed, do you measure any resistance between the Bridge Output (to O/T C/T) and Gnd?
                      No resistance between any of these points.

                      Is R1 still 220k, or now something much lower?
                      its a 220K and have tried removing, with no effect

                      Quite the opposite: a short in parallel with a cap or *anything* , stays a short, soyour test means nothing.
                      Sorry, lazy typing, I meant I removed the 220uf cap, paralleled the two 100uf's to make a 200uf and attached it with lead-wires

                      So far strong suspects still are main filter cap and bridge rectifier ... IF disconnecting OT does not cure they short.
                      agreed, those two 100uf's i used to test it with aren't new and have been sat in a drawer for years, so I wouldn't bet on them being good. Can't think of anything else, so have ordered a new cap to try, have a spare bridge rectifier as well so will change again just in case (they're pretty cheap after all). Will update once i've got the cap and installed it.

                      cheers again everyone.

                      Comment


                      • #12
                        Just want to double check, removing the cathode bypass cap made enough difference to stop the fuse from blowing?
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


                        Comment


                        • #13
                          Cool

                          Just in case check wiring with a loupe or high power reading glasses (I use +4 ones for that) and under good light.

                          Often itīs not a "parts" problem but "something else".

                          You might have a solder drop touching what it shouldnīt, a nut/washer/whatever pinching wire insulation (that burnt me many times) , insulation paper/film crushed and shorting what it should insulate, etc.

                          Thatīs why shotgunning *everything* may still not solve a problem
                          Juan Manuel Fahey

                          Comment


                          • #14
                            I suggest rigging up something like this for when you have a fuse eating amp. Just clip in across the fuse holder and save yourself the cost of 10 or so fuses while you troubleshoot
                            .Click image for larger version

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                            It's weird, because it WAS working fine.....

                            Comment


                            • #15
                              The new cap arrived today and after installation the amp seems to be behaving again and its sounding right.

                              Randall - great idea, reckon I burnt through about $10 worth of fuses, that would have been a much better idea.

                              G1 - yeh, removing the cathode bypass caps did stop the fuse blowing, i'm not totally sure why, but now that it's working I don't fancy rocking the boat.

                              Thanks again everyone, massively appreciated.

                              Comment

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