G3 issue

    Hello!

    I have built Madamp G3 about two month ago. It worked well, sounded really nice, was a lot fun playing.

    Yesterday while I was playing it muted. One of the fuses was blown. I have checked the resistors, they were ok. I have disconnected connection B and C, removed the tubes, replaced the fuse, to check the voltages. After I have turned on, the same fuse was blown again.

    The initial measurements were:

    No.CheckpointShould beIsComment
    1Wall plug socket*230 VAC243Power Supply input voltage at start of measurement
    2V3-4/56,5 VAC6,6Heater voltage at tube socket V3 - between Pin 4 und 5
    3BR1- ~/~208 VAC224Input of bridge rectifier BR1 - between the two "~" lugs
    4(A) lug 21 t **260 VDC279Supply voltage to V3
    5(B) lug 18 t237 VDC253Supply voltage to V2
    6(C) lug 18 b225 VDC239Supply voltage to V1
    7V1-1225 VDC239At plug socket
    8V1-381 VDC79,1At plug socket
    9V1-6105 VDC103,7At plug socket
    10V1-71,2 VDC1,34At plug socket
    11V1-8110 VDC115,1At plug socket
    12V2-1185 VDC197,5At plug socket
    13V2-350 VDC53,3At plug socket
    14V2-6166 VDC172,6At plug socket
    15V2-83,0 VDC3,52At plug socket
    16V3-1251 VDC288At plug socket
    17V3-6254 VDC287At plug socket
    18V3-3+810,5 VDC11,26At plug socket, please measure at both lugs!
    19(R13) lug 10 b47 VDC49,6an R13 unten
    20Wall plug socket*230 VAC236Power Supply input voltage at end of measurement

    Before connecting B and C:

    B: 329 VDC
    C: 327 VDC


    Please help me find the solution.

    Thank you!

    2 Mal editiert, zuletzt von tori (30. Juli 2017 um 11:58)

    Hi tori and welcome to the forum,
    if the fuse is blown with tubes removed and B & C disconnected at the power supply you should check (or just replace because they are cheap) D1 and D2. If these are OK it might be the output transformer (less likely), the wiring having a short to ground (more likely) or something in the power supply (more likely). You can also disconnect C from the power supply and give it one more try (if you have more fuses at hand ;)
    Regards,
    Martin

    Hi Martin,

    thank you for your help. I have checked D1 and D2 with multimeter, they are ok. I have checked the wiring and have not found any problems so far. I have disconnected "A" from power supply and another fuse was blown. :)

    What can I do next?

    If the fuse blows w/o anything connected to A/B/C the power supply itself is the problem. It's just 10 parts and R22,R23,R24 (if wired correctly!) are out of the game. So I would disconnect the left leg of R23 and give it another try, if it still blows the fuse, it's the PT, BR1, C12 or C13.
    Regards,
    Martin

    Hi Martin,

    thanks a lot for your help. It turned out the PT is wrong. So I should just order a new one from musikding and replace it? Or is it likely that there is another problem which caused damage to the PT?

    Regards,
    Tori

    OK, so it's definitely the PT :(
    I don't think this is caused by a broken part of the power supply, but you could measure them (bridge rectifier, capacitors and resistors to ground). And you can measure the resistance vs. ground at the HV and heater "inputs" of the power supply to be sure there is no short to ground.

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