6100 Lm Gain Channel Issues

Discussion in 'Marshall Amps' started by Phony iommi, Mar 16, 2019.

  1. Phony iommi

    Phony iommi Member

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    First timer here.
    I have a 6100LM and the 3rd channel (high gain) is starting to go out. It sounds as if you are using a distortion pedal with a dead battery in it. Does anyone have any insight as to what the issue may be? The clean and crunch channels both work fine. I’ve never serviced an amp before but I’m sure I can fix it if it’s only a parts swap issue. Thanks in advance for any insight anyone may have.
     
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  2. Gunner64

    Gunner64 Well-Known Member

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    Be patient, I supect Anitoli will be along shortly..our resident expert on these amps..he knows . And welcome here!...Phony Iommi.... lol..awesome
     
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  3. anitoli

    anitoli Well-Known Member

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    When you say starting to go out do you mean fading in and out or is the gain loss constant?
     
  4. Phony iommi

    Phony iommi Member

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    So when I play through channel 3 the volume dampens, the gain is still there but the sound is broken up and highly distorted. It sounds like the level wants to come back up when the guitar is not being played, but as soon as I strum it the volume goes down significantly (50% perhaps) it’s almost as if it can’t output sound and is dampening as a result, like a foot pedal with a bad battery in it.
    I hope this helps somewhat
     
  5. Jethro Rocker

    Jethro Rocker Well-Known Member

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    I would guess initially one of the preamp tubes for channel 3 since the other channels work. Thses would be V3, 4 amd 5. Could be any one of them. Also try cleaning the contacts on the volume pot for channel 3.
     
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  6. Phony iommi

    Phony iommi Member

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    Thanks much for the welcome and glad you like the handle. I used to play in a sabbath tribute band under that name.
     
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  7. Gunner64

    Gunner64 Well-Known Member

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    Even awesomer
     
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  8. Phony iommi

    Phony iommi Member

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    okay excellent, I’ll try cleaning the volume pot contacts and see what goes. Not sure how to check preamp tubes but I wouldn’t be surprised if one went out. I read they eventually go and I’ve owned this head since new and nothing has ever been done to it. Thank you.
     
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    1. It could be a preamp tube; buy one new preamp tube and swap out the tubes one at a time. Retest the audio channel 3 every time you swap one out.
    2. The Voltage regulators can go bad. You can measure the regulators w a DC volt meter if you have one.
    3. If not, try removing the FET TR101 temporarily from the circuit, and then retest the audio channel 3.

    6100 red channel.png

    6100 regs.png
     
  10. anitoli

    anitoli Well-Known Member

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    From experience with TR101, if that FET shorts you will still have plenty of volume but the gain will be lower with it dimed and the boost on than channel 2, you will notice it. If the issue is volume check/clean tubes V2-3-4-5, remove the jumper cable CON101-203 and clean and reseat, what is it doing now?
     
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  11. Phony iommi

    Phony iommi Member

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    Okay, I cleaned the volume pot to no avail problem still persists. On the preamp tube issue I removed the back cover and there are 5 tubes labeled “ecc 83”. Are these the tubes in question? If so would I be able to remove one that is known to be good and test it on one that is suspected to be bad or does the amp need all of them in place at once to work? I won’t have any more time until Monday to pull it apart and get to the jumper cable as advised on this thread but I will give that a shot as well.
    Lastly one nugget that may help or not, today I noticed that when the amp is first turned on the issue isn’t as noticeable as when it is warmed up. In other words the more the amp warms up the worse it gets.
    Thanks everyone
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    :welcome: Phony

    Turn amplifier OFF when removing and loading tubes.
    There are 7 12AX7/ECC38 in there I believe. Mark them 1 through 7. While you are at it clean the tube's pins and socket prongs with a decent lubricating electronics contact cleaner.
    Swap tubes, like #1 with #2. Make notes of any changes. Place #2 back and then swap #1 with #3 and so on. Keep notes for each setup.
    As mentioned V3, V4 and V5 are critical Channel 3 preamp tubes.
    If nothing much shows up then it is not the preamp tubes.
    Maybe something will show up.
     
    Last edited: Mar 16, 2019
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  13. Jethro Rocker

    Jethro Rocker Well-Known Member

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    Yes since the other channels work I would concentrate on those ones.
     
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    I be able to remove one that is known to be good and test it on one that is suspected to be bad or does the amp need all of them in place at once to work?

    Many amp malfunctions are a bad preamp tube.
    Buy 1 new 12AX7. For the channel 3 to function I think all the preamp tubes need to be installed.
    Swap that new tube with the old ones one at a time, and re-test each time you swap one out.
    By process of elimination you will find a bad tube if there is one.

    After that you can measure DC power supplies plus 15 volt, minus 15 volt, and 5 volt supplies.
     
  15. anitoli

    anitoli Well-Known Member

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    Channel 3 uses V2A-3-4-5. That's it. The only tube that is shared is V2, the first triode feeds channel 2 and three.
     
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  16. anitoli

    anitoli Well-Known Member

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    There is also a high probability of a bad solder joint that breaks open under heat expansion. Remember these amps were soldered with lead free shit and it will cause issues.
     
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  17. HOT TUBES 70

    HOT TUBES 70 Well-Known Member

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  18. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    Welcome to the forum new MF’r, love the handle as well! Cheers Mitch
     
  19. Phony iommi

    Phony iommi Member

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    Okay here’s the current state of affairs I may have stumbled upon something.
    I went to guitar center and purchased a new tube to swap and test. I went through every tube, cleaned the ports with dioxit and nothing changed. Same results. However! When I closed up the back and started messing around with switching the power output, even changing the speaker output just to see if anything would appear I deduced the issue is coming from the “low volume compensation” button on the front above the input jack.
    So, the problem is now this: When the low volume compensation button is pushed IN the problem goes away.
    When the low volume compensation button is NOT pressed in the sound chokes out and sounds like crap.
    This situation occurs ONLY on channel 3, channels 1 and 2 are not affected at all by this. If anyone out there has any ideas of how to approach the situation I’d appreciate it. Bear in mind I’m a repair novice and I’m seriously considering just living with the problem. It really doesn’t bother me or affect my use of the amp at this point. Only thing I tried is spraying dioxit into the button and that didn’t do anything. Thanks again to everyone on his thread!
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Not exactly, the LVC drops signal for all three channels. When it is not engaged all channels are being feed more signal but it allows the issue of Channel 3 to be pronounced .
     
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