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Jcm 2000 Dsl 100 Head Cuts Out

Discussion in 'The Workbench' started by Jock68, Oct 19, 2018.

  1. Jock68

    Jock68 Member

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    Recently this amp has been cutting out, sometimes it is after 5 mins, sometimes it is after 60 mins. No sound at all just goes dead. Always works for a few mins after a few days break.
    I have re biased and replaced some pre amp valves, without success..problem is still there.
    Is this a trip to an engineer or something that is obvious to someone?
    Any advice would be appreciated, cheers.
     
  2. Mavrim

    Mavrim Member

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    Could be a bad solder connection on the board! I had an intermittent low volume problem that would go into a crackling sizzle noise when shut off. Found a plate resistor going to phase invertor that barely had solder around the resistor lead. This was on my JVM1, but still could be a the cause of your intermittent problem as amp is cool it works, then as it warms up and flexes it fails.
     
  3. Alabama Thunderpussy

    Alabama Thunderpussy Well-Known Member

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    Tech time.
     
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  4. Zed 67

    Zed 67 New Member

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    It would be a good idea to go over the solder joints like Mavrim suggested. It could also be a heat related problem and I'd check all the electrolytic capacitors. You can use a multimeter if it has a capacitance range on it but the ideal instrument to use is an ESR meter.
    Sometimes capacitors will have physical signs they are on their way out. Look out for the cans of the capacitors bulging, this is caused by the electrolyte in the capacitor heating up and vaporising or for signs of leakage from the can.
    Obviously this work should be done by someone who knows the dangers.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Several things you need to check, one is:
    1. Take the output tubes out
    2. Turn amp ON but leave stand by turned OFF. (switch is in warm-up position)
    3.Measure pin 5 of output tube sockets (should be negative DC voltage)
    4. Write down readings of each socket pin 5.
    5. Turn standby ON. (operate position)
    6. Re- measure pin 5 of each socket, write down readings here in the thread.

    Leave the standby ON for 1 hour.
    After 1 hour, re-measure pin 5 voltage of each socket and write down the readings here in the forum.

    * this is an insulation breakdown test.
    This will determine if the insulation of the board is good.
    If the insulation is failing, the bias voltage will change (a lot) when you turn the standby to the operate position.
    Also after 1 hour (standby in operate position) re-check the DC bias voltage on pin 5 of each output tube socket.

    If the bias voltage remains steady : then the board passes.
    If the bias voltage changes a lot over 1 hour : then the insulation is failing and the amp should not be operated. (till the problem is repaired)
     
  6. NewReligion

    NewReligion Well-Known Member VIP Member

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    Of course follow all excellent offered advice listed above.

    You may also think to simplify your trouble shooting as I do by.

    1. Switch out everything you can from guitar, cords to cab,keep pedals out of the loop. Remove foot switch. Try all channels.

    2. Plug your guitar straight into the return of the FX loop. If it stays on the issue is preamp. If it fails it is in the Poweramp side.

    It is from here we can explore further or at least give a Tech some usable data.

    Disclaimer...On Heavy Cold Meds lol

    Best...David ♫
     
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  7. Jock68

    Jock68 Member

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    Looks like the Preamp
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Plug a guitar cable between effects send and effects return. Then Re-test.

    Plug a guitar into effects return, are you getting sound that doesn't cut out?
     
  9. Jock68

    Jock68 Member

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    Cleaned up the pre amp and reset the tubes, did not replace the cover and spring on #1 and it works. Somehow the spring inside the cover pressing down on #1 seems to be causing the problem. Many thanks to you all for helping me out, much appreciated.
    Currently running with cover but no spring in #1
     
  10. mickeydg5

    mickeydg5 Well-Known Member

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    If that is the case press down on it with something while playing. See if you can cause the issue. Fun.
    Truly being the problem, it is either the tube or the socket.
    Retension the prongs in the socket so that all are secure on the pins of the tube. Try again, pressing down.
    If the problem persist then it may be a defect in the tube or at the socket.
     

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