fx Loop on Marshall JCM800 not working?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JohnnyL, Jul 19, 2006.


  1. JohnnyL

    JohnnyL Member

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    Looking for information on how to check or diagnose the loop on my 4210. How should I test it? Thanks guys!

    Johnny
     
  2. John Phillips

    John Phillips Member

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    Connect a cable directly from the send to the return.

    It's a very simple series loop, and the most likely cause of trouble is a failure of the switch contact in the return jack, which will stop the amp working or make it erratic. If running a cord from the send to the return cures it, you just need to clean the contact, which involves pulling the chassis to do it properly.
     
  3. JohnnyL

    JohnnyL Member

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    Thanks for the reply Mr. Phillips. I was hoping you would comment. I should have stated in the first post that I have done the patch cable from send to return and do not get a signal-the amp is quiet. Otherwise the amp works and sounds great to me. I will go ahead and pull the chassis this weekend and check it out and post my findings. I was really curious whether there was a way to check it without pulling the chassis! I'm not lazy but am always anxious to learn of other ways to do things from the experts here!

    Thanks Again.

    Johnny
     
  4. John Phillips

    John Phillips Member

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    Ah - I understand what you mean by 'not working' now :).

    If it's like that, there is really only one explanation, which is that the send jack must be damaged or the connection to it broken. The signal path simply goes to the send jack and to the switch contact on the return jack together; then from the plug contact on the return jack onwards - so putting a cord in the return jack interrupts the signal path. If the amp works with nothing in the return jack, the only possible problem can be with the send jack. You should find that either the jack contact is bent so it's not touching the plug, or the wire between the two jacks has broken.
     
  5. JohnnyL

    JohnnyL Member

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    Thanks John. It sounds like I'm might get off easy on this one. I'll know once I pull the chassis.

    Thanks for your help.:BEER

    John
     

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