JCM 900 FX Return Broken

Discussion in 'The Workbench' started by Ryan Morris, Jan 23, 2020.

  1. Ryan Morris

    Ryan Morris New Member

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    Hi all,

    I have a JCM 900 4100 and the FX return is not working. I can get a signal out of the send just fine, but if i try to put any signal into the FX return, I get nothing. If I bridge the send and return with a patch cable, when I crank the master volume I get a very low volume, oversaturated signal coming through.

    Things I've checked:
    all tubes work fine
    jacks aren't corroded or broken
    the FX jacks and the FX pot are soldered fine
    the FX pot works (I checked with an ohmmeter)

    If I have a patch cable bridging the FX loop, there is no signal or even any voltage drop between the tip and sleeve connecters of the return jack at all.

    Does anyone have any tips for troubleshooting, or what could be the cause of the problem?
     
  2. Lance Chambers

    Lance Chambers Well-Known Member

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    Make sure the "Level" control is set to zero for pedals, middle (5) if jumping & right (8-10) for racks.

    Do you get a signal from guitar straight into FX Return?
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Check and see if IC 7 is getting hot.
    Is IC 7 in a socket?
    Check Pin 8 of IC7 to verify +15 volts DC.
    Check pin 4 of IC7 to verify minus 15 volts DC.
    The schematic is here I think...
    https://drtube.com/schematics/marshall/cd0189-iss7-0349.pdf
     
  4. Ryan Morris

    Ryan Morris New Member

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    I tried going straight into the return, no sound
     
  5. Lance Chambers

    Lance Chambers Well-Known Member

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    Oh, btw.........check all cables for continuity where applicable.

    :hmm:
     
  6. Ryan Morris

    Ryan Morris New Member

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    IC7 is fine, voltages are good
     
  7. Lance Chambers

    Lance Chambers Well-Known Member

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    Also check the pots with your Ohm meter. They should fluctuate while turning the dials.

    :hmm:
     
  8. Ryan Morris

    Ryan Morris New Member

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    I have, all the pots are good
     
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  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Oh sorry shit I was wrong. :wallbash:
    IC 7 is the reverb not effects.

    Reverb return is IC5 a TLO72 op amp.
    Is IC 5 getting hot?
    Is it in a socket? :lol:
    Pin 8 is +15 volts DC
    Pin 4 is minus 15 volts DC.
     
  10. Ryan Morris

    Ryan Morris New Member

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    Just checked all the IC's, they're all getting the right voltages. None of them are in sockets, they're all soldered on.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Replace V2 preamp tube.
    Check V2 R16 should be 2K2.
    Check V2 R3 should be 100K.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Check IC5 pin 1 for DC voltage. Should be 0 volts or close to it...
     
  13. Ryan Morris

    Ryan Morris New Member

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    R16 and R3 are fine, I'v already tried swapping V2.

    I'm currently not getting any signal at Rd, which is what sends the return signal from the FX PCB to the PCB with the valves, so I'm assuming that the problem is occurring at the return circuit BEFORE it gets sent over to the main PCB, right?
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    Is there signal on pin 1 of IC5?
    Is there DC on pin one of IC 5?
    This is the TLO72 op amp.
     
  15. Ryan Morris

    Ryan Morris New Member

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    IC5 pin 1 has about 10mV

    R2 on the FX board I'm reading as a short (it's labled as 3.3k)! Is that supposed to happen, or is that indicative of a bad resistor? I can't measure the true impedance without taking it off the board
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    Resistors can't short.
    It's the other components in the circuit that is giving a short reading.
     
  17. ampmadscientist

    ampmadscientist Well-Known Member

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    Go ahead and replace IC5. Put a socket on the board.
     
  18. Gutch220

    Gutch220 Well-Known Member

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    Old thread update.

    Did we have a conclusion to this one? This information will be useful for future users.
     
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    We do know that the ICs fail occasionally.
    The filter caps for the low voltage supplies were underrated and tend to reduce functioning with age.

    The effects loop jacks can oxidize and cause problems in the signal path.
    Check this by plugging a guitar cable from send to return.

    You can check the output of the IC for DC offset which is usually winding up as a bad thing.
    The IC spec sheet will show you where the output pins are located.
    The output of the audio ICs should be pretty close to 0 volts DC.
    If there is DC offset it can be an indication that the IC has gone bad.

    Also a bad IC with DC offset will tend to get hot (or melt)...
    This will load down the low voltage supply rails.
    If 16 volts DC is normal, the voltage can be dropped by a bad IC and will read lower than normal.
     
  20. Ryan Morris

    Ryan Morris New Member

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    I ended up selling it to a guy that didn't care about the broken FX loop... sorry that there isn't much of a conclusion to this thread.
    I will say though, that the problem was definitely NOT:
    oxidized FX jacks
    broken FX pot
    bad tube

    So most likely it was a bad IC, I just never got around to ordering any replacements. I was planning on flipping it from the start anyway.
     

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