JCM900 4101 blowing fuses

Discussion in 'The Workbench' started by Paul Saunders, Feb 21, 2021.

  1. Paul Saunders

    Paul Saunders New Member

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    I have Marshall JCM900 4101 combo that is blowing fuses. During a recent service I was told I needed a new output transformer so I had one fitted. The amp was working fine when I got it back . I use the amp running through a Two Notes Captor X most of the time. It has just started blowing fuses not long after switch on at low volume.


    Has any one got any suggestions on what I can investigate to try and locate what is causing this to happen?


    Regards


    Paul
     
  2. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    What fuse is blowing? Disconnect the secondaries of the power transformer and power on the amp. If it still blows the fuse your PT is bad. If it doesn’t, check your power tubes as bad tubes can blow fuses.
     
  3. South Park

    South Park Well-Known Member

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    Your load box might not be matched with the amp . Unhook it
     
  4. Paul Saunders

    Paul Saunders New Member

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    It still has the fault without the load box and ran for a month after purchasing it after the fuses went I went back to basics and ran the amp on its . Without wanting to appear to to much of a noob are he secondaries on the power transformer the output connections (I have the circuit diagrams and did a degree in electronics a long time ago 1984!)

    Thanks for the help will try your suggestions later

    Paul
     
  5. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The secondaries on the PT is the high voltage.
     
  6. XTRXTR

    XTRXTR Active Member

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    Wall VAC input is Primary, HT Fuse is on the secondary B+ High voltage. You pull all tubes to keep the current draw to zero. You should be able to pull all your tubes out replace correct fuses then power on and see if HT blows, if it does you have likely got PT problems. Its better if you have Variac to slowly raise the wall VAC input. Or a light Bulb current limiter, if it lights up bright you have PT problems. You could have a short, B+ touching ground near the tubes wires or board.
     
    Last edited: Feb 23, 2021
  7. Paul Saunders

    Paul Saunders New Member

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    Pulled all the power tubes and it stopped blowing fuses so new power tubes (any recommendations) has gat to be my next try

    Thanks all

    Paul
     
  8. XTRXTR

    XTRXTR Active Member

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    Check your bias voltage, schematic says -37 to -40 VDC, I would lower to max cold what ever that is for that amp and then get a bargain value Quad Matched set run at 50% of max dissipation, if your using a load device then there is no sense to me to run them any hotter than they have to operate. That way you're saving your tube life and not driving the amp too hot. If biased cold they will provide decent power tube distortion at a lower volume. Volume is the reason you are using that device... I don't think those devices are good for the OT either so running cooler will save the life expectancy of the amp overall.
     
  9. Paul Saunders

    Paul Saunders New Member

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    Thanks all for replies and advice it was bad power tubes replacements arrived today and all is good. I really appreciate the help and it is great to be back using the Marshall I am so glad I bought it every time I hear it.

    Thanks again for taking the time to give me advice a new matched quad has definitely done the business

    Regards

    Paul
     
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  10. neikeel

    neikeel Well-Known Member

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    Glad you got it sorted. Of course running a big amp hard with the master up into an attenuator will trash power tubes.
     
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  11. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I trust that you also checked the bias on your new tube set. You wouldn’t want to wear them out prematurely.
     
  12. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    Glad you fixed it.
     
  13. Paul Saunders

    Paul Saunders New Member

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    Yes did check all looks good . I am running the two notes Captor X not just to keep the volume down whilst running (something I am not really doing) I run at a bit higher than without it but this unit has some really useful features in the form of cab simulation, EQ, twin track, Di and a few others I have found it really useful so far and would recommend having a look at this unit.
     
  14. Paul Saunders

    Paul Saunders New Member

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    Thanks for this not something I would have checked otherwise. It is good for Me to learn my way around the amp circuitry and I feel much better knowing that the amp is running as it should. Can you recommend where I can get more parameters for my amp (a JCM900 4101) as I would like to be able to keep an eye on how it is running from now on
     
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  15. XTRXTR

    XTRXTR Active Member

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    Here is a schematic set. 3rd and 4th pages show bias voltage per design at the bias point and also some notes bottom left corner of 3rd page makes some statements about bias.

    In the old days Marshall provided reference voltages at several test points within the circuit schematic at idle - no signal. The voltage from the wall can vary during peak electric grid times - breakfast, dinner, weather. If I were you I would make my own voltage chart of the amp. I write mine down on the schematic itself. Make sure you measure the voltage from the wall as well.
    115 - 125 VAC Varying wall voltages make the voltages inside the amp change with amp at idle - no signal.

    In the future you can use these voltages you measured as a reference to see if the amp is still operating the same when you last changed tubes or whatever reason you open it up again. Always test around same time of day or check the wall voltage write down time of day test was performed etc.
     
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  16. Paul Saunders

    Paul Saunders New Member

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    Thanks for this. I am UK based so amp was built when we had 240v but now we have 230v so will be a good idea to keep an eye on various voltages had a quick look at the schematic really nice that the bias check point is clearly marked with voltages listed as well will go through the amp with it tonight after peak time and do a 1st set of readings.
     
    Last edited: Mar 2, 2021
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  17. Paul Saunders

    Paul Saunders New Member

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    I also spoke with Marshall at their Milton Keynes place and they do a full range of spares for my amp which is more good news. Got some prices from them for 50 + 50 capacitors and a pre amp tube cover and they are really good
     
    Last edited: Mar 2, 2021

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