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Blues Jr mod disaster - help needed! :P

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  • Blues Jr mod disaster - help needed! :P

    Hey guys,
    I know Iím just a new member, but would appreciate any help for a damn old fool!

    I recently purchased a Fromel Complete mod kit for my BJ.
    I have a Blue Junior III 2010 (REVISION D I assume) and decided to install a Fromel kit, but have stuffed something!

    It powers on fine, light comes on, but I canít get anything from the guitar.
    There is a slight stable hum that is a bit louder than before, which ramps up with the volume (gain), not the master.
    And also changes pitch with the treble, which it did do at high volumes before also, if you paid attention while not playing.
    There is also a pitch change when you touch the guitar input, or put the jack in, which used to be silent.

    In fact the whole amp was very quiet until I threw a spanner in the works.

    The tubes fire up and everything, but just no signal.
    No smoke from anything.

    The original large capacitor always read 6 volts after it self-discharged, and the new one read exactly the same.
    Itís the only thing I put in that is polarized (apart from the jack).

    I did accidentally lift the pad on the PCB when installing the new WIMA cap on the bass pot (C6), but I scratched back the lead and it seemed to solder on okay, thatís only thing I can think of that didnít go as planned (or so I thought).
    To be fair, Iíve never seen a pad lift so fast.

    Iím no tech, but I live out in the sticks in Australia, so there was no real option but for me to do it myself.
    I did start an electronics degree when I was young and both my brother and father are electronic engineers, so I thought I would still retain some of the skills I had, but maybe not!
    First time Iíd even used a multimeter for a long while.

    How can I problem solve this thing to track down the problem?

    I know itís hard without the amp in front of you, but if you could give me some easy rookie tips, that would be great.

    The only vague instructions were with the input jack, so maybe I screwed that, but not sure how.

    I did the Fromel Complete for MIM (made in Mexico) set, which is:
    Replaced C25 main filter electrolytic cap with 100uF/450v.
    Replaced C5 with a 250pf Silver Mica
    Replaced C6 + C7 with .1uF + .015uF WIMA caps respectively.
    Put a jumper on the mid pot.
    Replaced bias resistor with a 33k at R52
    Installed new input jack, not mounted to the board.
    I disconnected the trans leads and dressed them better and replaced them.
    This is the kit:

    Note: The white resistor is just to discharge caps.
    And I didnít put the extra little cap in (disable Ďsparkle modí)

    Hereís the schematic:

    Anyway, thanks for your time...

  • #2
    The nice thing about modern Fender schematics is that they have test points throughout. It'd be best, at this point, to inject a signal and follow it through the amp to see where it's getting lost. If you have a scope, use that. If not, use ACV on a DMM. (schematic attached)

    "Yeah, well, you know, that's just, like, your opinion, man."


    • #3
      Those two black plastic rings appear to isolate the new input jack. How did you install them?
      When the going gets weird... The weird turn pro!


      • #4
        Originally posted by DrGonz78 View Post
        Those two black plastic rings appear to isolate the new input jack. How did you install them?
        I only used one black ring, on the outside under the nut.
        Would it be grounding it under the chasis or something??


        • #5
          If you look at the schematic, it is supposed to ground to the chassis, so that wouldn't be your problem- unless you have it miswired and are grounding out the tip/signal.
          "Yeah, well, you know, that's just, like, your opinion, man."


          • #6
            It's gotta be that input, i didnt even want to put it in, but I thought I better not waste it :P


            • #7
              If you suspect a jack issue, plug in a guitar cord. Measure continuity tip to sleeve on the end that goes to the guitar. Is it shorted grounding signal? Measure plug tip to the junction of R1 and R2. Is there continuity there and your signal path is good? Measure plug sleeve to chassis. Is it being grounded as it should?
              "Yeah, well, you know, that's just, like, your opinion, man."


              • #8
                I believe the mod removes the chassis ground.

                Just found from the website (WISH IT WAS IN THE INSTRUCTIONS!):

                "Switchcraft Input Jack - Your input jack is soldered directly to the circuit board, apply some tension when the amp is plugged in and you could cause major damage to the circuit board in the amp. This mod gets rid of the wimpy plastic input jack and replaces it with a high quality metal switchcraft jack that is panel mounted and NOT directly soldered to the circuit board. Included is a shoulder washer and isolation washer so the input jack does not ground directly to the chassis and prevents ground loops. The jack comes pre-wired with high quality, Teflon Coated, Silver Plated Solid Copper Core wire. The solder I use is Silver bearing. This is the same quality you will find in high end boutique amplifiers costing thousands of dollars."

                Even so, the chassis ground have added hum, but wouldn't necessarily eliminate the signal entirely?


                • #9
                  You need to mount the new jack with both fiber washers. The intent is to not let the metal jack threads come in contact with the metal chassis, and instead make your ground from the jack via a wire directly to the board, This is an attempt to lower ground loop noise, but if this connection is not made correctly, it would cause a loss of input signal, and possibly even the noise and pitch changes you describe. Try plugging your guitar in with your cable, and then attaching a clip lead, or whatever you have, from the guitar bridge to the amp chassis, assuming the guitar is properly grounded.
                  It's weird, because it WAS working fine.....


                  • #10
                    got that sommabitch!
                    Last edited by Cobraclutch; 11-21-2019, 07:01 AM.


                    • #11
                      Does that last post mean you got it working? I read the post before edit and you said after isolating the jack it was quiet with no guitar plugged. But it was loud as hell with cable plugged in, noise from amp not guitar.
                      When the going gets weird... The weird turn pro!


                      • #12
                        Yes fixed it, didnít have time to explain haha.

                        The input socket ground was in the wrong hole as it had changed - oops!
                        And the washers.

                        Back to working and quiet!

                        Thankyou, youíre all handsome and powerful men...


                        • #13
                          That was going to be the next thing I was going to suggest to look at too. Nice 👍 Glad you got it all working.
                          When the going gets weird... The weird turn pro!


                          • #14
                            Thanks mate, youíre the one that got it solved really.
                            Because when i measured it with the washers out there was still ground. But as soon as i put that washer on i was getting nothing from the input back to ground, so bingo, has to be the soldered ground.


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