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Thread: JCM 600 troubles

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    Question JCM 600 troubles

    While I would like to say that I am a completely proud owner of the Marshal JCM 600 tube amp, a technical problem is preventing me.

    One day while I was practicing, I switched my amp from Clean to Overdrive, played on that channel for a bit, and then proceeded to switch back to Clean. There was--and is--absolutely no output whatsoever. The channel simply seems dead. I can switch back to the Overdrive channel and play on that one without any issues, but the Clean simply outputs nothing--not even radio interference. My EL34 power tubes are relatively new, the preamp tubes appear to have no visible flaws (cracks, etc.), and none of the major fuses are blown. Does anyone have any ideas?

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    Sounds like you have a sticky relais or a bad channel switch. Does the failure also occur when you use the footswitch instead of the push button on the facet? If the footswitch works the problem is related to the button resp. vice versa but if the amp sticks in the OD1 Channel then probably RL1 is bad....

    cu
    bluesfreak

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    Quote Originally Posted by bluesfreak View Post
    Sounds like you have a sticky relais or a bad channel switch. Does the failure also occur when you use the footswitch instead of the push button on the facet? If the footswitch works the problem is related to the button resp. vice versa but if the amp sticks in the OD1 Channel then probably RL1 is bad....

    cu
    bluesfreak
    What do you mean by 'button resp.' and RL1? Relay 1?

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    Have you tried to see if switching fails only with the button on the front panel or also when using the footswitch? If the footswitch works and the front panel button doesn't then the problem is the siwtch on the PCB. If both do not work the problem is most likely relay #1

    cu
    bluesfreak

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    I just checked to ensure that both the footswitch and front panel channel switch button properly switch between the clean and overdrive channels. Both work flawlessly. The clean channel simply does not output anything. I swapped around both the preamp and power tubes, but to no avail. I am suspicious that the power tubes are to blame, but that would still surprise me given how new the tubes are. Are there any visual defects that I should look for when inspecting the power tubes? They are EL34s

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    Quote Originally Posted by Rockworth View Post
    ...I am suspicious that the power tubes are to blame,...
    Logically this makes no sense. The power tubes amplify the signal from both sections of the pre-amp, clean and distortion.

    If they work for the distortion signal, why would you think that they would not work for the clean channel?

    As bluesfreak has been trying to tell you, the problem is in the switching circuit or the clean channel circuit.

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    Lifetime Member Enzo's Avatar
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    Your channel indicating lights may switch, but that doesn;t guarantee the relays themselves are activating. And if they are activating, that is no guarantee the contacts with in them are functioning properly. or it may be that the relays are fine, and the circuit for the dead channel has a problem. IN that case, I would go through that channel, first looking at each tube that is involved to verify it has B+ on its plate and something on the cathode to indicate current is flowing.

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    Quote Originally Posted by Enzo View Post
    Your channel indicating lights may switch, but that doesn;t guarantee the relays themselves are activating. And if they are activating, that is no guarantee the contacts with in them are functioning properly. or it may be that the relays are fine, and the circuit for the dead channel has a problem. IN that case, I would go through that channel, first looking at each tube that is involved to verify it has B+ on its plate and something on the cathode to indicate current is flowing.
    I just printed off the electrical schematics for the amp. What are you referring to when you say "it has a B+ on its plate"? Am I correct in assuming that I need to connect an ampmeter to determine if current is flowing through the cathode? (My apologies for my newbie questions, but I am seeking to understand exactly how my amp works.) You had mentioned the relays. Where might those be on the circuit schematics? The preamp PCB or power PCB diagrams?

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    Lifetime Member Enzo's Avatar
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    Oh my, if you are that unfamiliar with the electronics involved, it may be best to get someone locally to help you. The voltages in these amps can KILL YOU.

    What I am looking for is high voltage on the plate connection of each tube. We refer to that as B+, for historic reasons we can discuss elsewhere. I want to see 150-250VDC on each plate most times. The tube cathodes are all separated from ground by a resistor of some value. If current flows through teh tube, Ohm's Law tells us a voltage will develop across that resistor. SO I am measuring the voltage at each cathode. if zero, then the tube is not conducting current. If we find a volt or two or whatever, then we know the tube is conducting.

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    I swear, this amp has a demon. I just took the amp to a repair tech; he plugged in a microphone into the input jack, had the amp on clean, and the channel worked. I took the amp home, plugged in my guitar on the clean channel, and it worked--at least while it was on the clean channel. I switched the amp to overdrive, played some, and then returned to clean. Once again I was treated to complete silence. Something strange is going on with this amp. I had mentioned to the repair tech that Relay 1 might be at fault, but he was not convinced.

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    This indicates that there is a fault with the channel switching. However, why did the tech not switch channels ?. I personally still suspect a sticky relais, it probably got loose during transportation and is now stuck again when you tested. If you can use the amp with the chassis removed use a non conductive stick and whack a bit on the relais to check if sound comes back or not.

    cu
    bluesfreak

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