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Pilot light on - everything else dead. Blackstar HT-60

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  • Pilot light on - everything else dead. Blackstar HT-60

    So, when I tested my Blackstar HT60 soloist rebuild I blew the HT fuse (630mA). It had been almost working for ten minutes- sound but quiet.

    I pushed in the OD selector and loud pop, a little smoke from under pcb (no signs of scorching tho) and it blew the HT fuse.

    So I started testing again and saw that the neutral on the power socket was live and vice versa. Can’t believe I did something so stupid. As I am building this as a head, everything is the other way up, so I must’ve forgotten to invert the power socket. I call this stupidity tax. Every once in a while I do something so stupid, that I have to pay stupidity tax. I am thinking the PT might have blown - but need some guidance of what to test first

    My question is what damage am I likely to have done? Now with everything wired correctly (and using a lamp limiter) the pilot light is on, but that’s about it. There’s about 100vdc at the plates (should be 570)

    all suggestions gratefully received.

    cheers

    steve

  • #2
    I would put the LBL between the wall (or variac) and the amp, pull all the secondary transformer leads from the board, and see if the transformer is drawing current (lighting bulb) even with everything disconnected. If it is then it is almost certainly shot. You could monitor current with a meter but I find LBLs to be easier if I don't need to know actual current value.

    If it is drawing current you can then pull the transformer primaries from the switch, fuse, etc, and repeat and see if it is still drawing current from mains in case there is some failure before the transformer.

    Doesn't seem like reversing hot and neutral would have caused the failure you described,

    If the transformer is not drawing current with secondaries unplugged then measure the secondary AC voltages and see if they are what you would expect. You can do this with LBL in place and it won't affect the measurements much if is not drawing current.
    Last edited by glebert; 01-09-2021, 03:21 PM.

    Comment


    • #3
      Originally posted by glebert View Post
      I would put the LBL between the wall (or variac) and the amp, pull all the secondary transformer leads from the board, and see if the transformer is drawing current (lighting bulb) even with everything disconnected. If it is then it is almost certainly shot. You could monitor current with a meter but I find LBLs to be easier if I don't need to know actual current value.

      If it is drawing current you can then pull the transformer primaries from the switch, fuse, etc, and repeat and see if it is still drawing current from mains in case there is some failure before the transformer.

      Doesn't seem like reversing hot and neutral would have caused the failure you described,

      If the transformer is not drawing current with secondaries unplugged then measure the secondary AC voltages and see if they are what you would expect. You can do this with LBL in place and it won't affect the measurements much if is not drawing current.
      Ok, so this is what happened:

      light on only when I connected the blue green blue from the secondary. They were reading blue to CT 13.8v on both sides and 28v blue to blue.

      other measurement on the secondary are: black 12v, red 370v
      Suggests the PT is ok.

      so it looks like I have a short somewhere south of the fuses F1, & F2 . If the culprit is the bridge rectifier would the led still come on? If any of the caps have failed, would it cause a short?

      there’s nothing obvious that I can see. I did replace the S/b switch, but that seems fine.

      what should I look at next?

      thanks in advance

      steve
      Attached Files

      Comment


      • #4
        Are all the tubes pulled?
        nosaj
        Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

        Comment


        • #5
          Originally posted by Steve Blackdog View Post
          about 100vdc at the plates (should be 570)
          Did this re build involve replacement of hi voltage power supply filter caps? If so, are you absolutely sure all of them are connected with correct polarity? The symptom you mention ^^^ points my attention towards perhaps a reverse-installed filter cap. Worth a check, shouldn't take long.

          Enjoy. Every. Sandwich.

          Comment


          • #6
            Originally posted by nosaj View Post
            Are all the tubes pulled?
            nosaj
            Yep

            Comment


            • #7
              Originally posted by Leo_Gnardo View Post

              Did this re build involve replacement of hi voltage power supply filter caps? If so, are you absolutely sure all of them are connected with correct polarity? The symptom you mention ^^^ points my attention towards perhaps a reverse-installed filter cap. Worth a check, shouldn't take long.
              Filter caps are original. I will check them all again, though.

              thx

              Comment


              • #8
                BTW, PTs all draw some current with all secondaries disconnected, about 5-10 watts--this is normal. But not enough current to make a 40w light bulb stay bright.
                --
                I build and repair guitar amps
                http://amps.monkeymatic.com

                Comment


                • #9
                  Originally posted by Steve Blackdog View Post

                  Filter caps are original.
                  So much for the polarity reversal theory. OTOH there still may be a cap failure. Leaky caps heat up as you try to charge them. My 2 cents worth. If no fault found, carry on.
                  Enjoy. Every. Sandwich.

                  Comment


                  • #10
                    I assume you have the schematic. If not, I'm attaching what I think is your amp.

                    With the OT disconnected, but the PT connected and using an LBL, does the bulb glow bright? If not, then what voltage do you have at SP4 (OT center tap) with its lead disconnected? I see in your first post you indicated 100V, but what about with the OT disconnected as well?
                    Attached Files

                    Comment


                    • #11
                      Originally posted by Delta362 View Post
                      I assume you have the schematic. If not, I'm attaching what I think is your amp.

                      With the OT disconnected, but the PT connected and using an LBL, does the bulb glow bright? If not, then what voltage do you have at SP4 (OT center tap) with its lead disconnected? I see in your first post you indicated 100V, but what about with the OT disconnected as well?
                      thanks Delta

                      next step then to try with ot disconnected. I have the schematic thanks

                      cheers

                      Comment


                      • #12
                        Originally posted by Steve Blackdog View Post

                        Ok, so this is what happened:

                        light on only when I connected the blue green blue from the secondary. They were reading blue to CT 13.8v on both sides and 28v blue to blue.

                        other measurement on the secondary are: black 12v, red 370v
                        Suggests the PT is ok.

                        so it looks like I have a short somewhere south of the fuses F1, & F2 . If the culprit is the bridge rectifier would the led still come on? If any of the caps have failed, would it cause a short?
                        Sounds like the problem is nowhere near the OT since those fuses go to the +/-12V and 3.3V converters.

                        Comment


                        • #13
                          I see no way wiring the mains plug backwards would affect the amp. Otherwise a zillion amps from the two-wire power cord era would blow up every time you flipped the ground switch.

                          This amp appears to have a bunch of silicooties. And what looks like ribbon cables on the drawing. ANy chance a ribbon cable is on the pins off one to the side or off a whole row? Just assuming you had it all apart, perhaps a misconnection was made.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment


                          • #14
                            I don't know but if the pilot light comes on then it's not a low voltage issue.
                            It would be nice if you could provide the connector & pin #'s as opposed to the wiring colors (they are not on the schematic).
                            I have a feeling that you have a high voltage issue.

                            Comment


                            • #15
                              Originally posted by Jazz P Bass View Post
                              I don't know but if the pilot light comes on then it's not a low voltage issue.
                              It would be nice if you could provide the connector & pin #'s as opposed to the wiring colors (they are not on the schematic).
                              I have a feeling that you have a high voltage issue.
                              Especially since the HT fuse has already blown once.
                              nosaj
                              Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

                              Comment

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