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Thread: Marshall Valvestate 8100 no sound at all

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    Marshall Valvestate 8100 no sound at all

    Greetings,

    A have a Marshall Valvestate 8100 and it is producing no sound at all and the tube does not glare at all. I assume there is an issue in the power supply.
    (Because, the heater for the tube takes power before the rectifier and the zener diodes, is that correct? )

    Furthermore, I attached a multimeter to measure AC voltage at the Secondary of the power transformer and I got 0V. I attached the ground of the multimeter
    to the 2nd White wire and the RED at the 1st White wire. It must show me about 30V AC, is that correct? Instead it shows me 0V.

    When I attach the multimeter at the Primary wires it shows me 220VAC, that is correct.

    Any response will be highly appreciated.


    Thanks in advance

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    Supporting Member Jazz P Bass's Avatar
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    Did you disconnect the white & red secondary wires when you ran the test?
    You should, as something may be "loading" it down.
    If there is not any measurable voltage on the disconnected secondaries, then it sounds like the transformer is toast.
    As a final check, with the power disconnected from the transformer, measure the secondary coil resistance.
    White to red. Red to red.
    If they show open, then that is what they are.
    I am assuming that the fuse is good & the on/off switch is good.
    A cute way to test that is to measure the resistance at the power plug.
    If the fuse is good & the switch is good, you will be reading the resistance of the primary.
    Attached Files Attached Files

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    Bent Member Chuck H's Avatar
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    This may seem pedestrian, but since you went right to checking the secondaries... Did you check the mains fuse? Some other failure in the amp may have caused too much current draw and blown the fuse.

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    Thank you for your answers,

    Finally, the transformer seems to be OK. I bypassed the switch and the fuse and connected its Primary wires directly to Mains and it works OK.
    From 220V Mains I get ~58V White-WHite (+/-) and ~28V any_white-Ground. I suppose this is OK.

    According to the schematic I measured the voltage at Zener diodes ZD1 and ZD2 in respect to ground. ZD1 shows me -15V at its back side as it should be,
    but ZD2 shows me about 10V instead of +15V as it should be. Is that a problem? The cause of this wrong measure is the ZD2 or something else before it, like a resistor or a cap?

    Additionally, I measured the voltages at the Collectors of the Power Transistors NPN BDV65 and PNP BDV64. BDV65 collector has about 25V and BDV64 collector has -38.5V with respect to ground.

    Is something wrong with these values?

    Thanks a lot!!!!
    Attached Thumbnails Attached Thumbnails valvestate80_80w_8080_8100_8412.pdf  

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  5. #5
    Supporting Member Jazz P Bass's Avatar
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    "Something" is loading down your plus supply.
    It's hard to tell from here.
    I am not that well versed in this model amp.
    (ie: I do not remember.)
    The VE+ is down & the +15 volts is down.
    First thing is removal of the tube.
    Next, somehow you need to disconnect the VE+ connection to the power amp.
    The 15 volt zener can be disconnected by removing R102.
    If VE+ pops up to normal you have to find what is pulling it down.
    Power transistors usually short at failure, and this does not sound like a full short.
    Still, you will have to check everything on the VE+ side of the power amp.
    When you have full VE+, reconnect R102 & see if the +15 volt rail is back up.
    The 15 volt zener can be checked with "diode check" on a DVM.
    If the +15 stays low then you may also have an issue downstream from it.
    You could try disconnecting the +15v rail from the downstream circuit.
    If it is still low I would replace the zener diode.
    Sounds like a fun project to me

    Edit: disconnect the speaker until you get the power supply stabilized.

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    Last edited by Jazz P Bass; 12-24-2011 at 09:07 PM.

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    Old Timer Gtr_tech's Avatar
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    If the main rails are OK, then you have a shorted op amp loading the 15v rail that is low. Check for any op amp that is hot....and gets hot quickly. That'll be the bad one.

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    I hope you realize that bypassing the fuse is a big no-no. If there had been a serious fault you would have probably blown the amp beyond being repairable, or started a fire!
    Be sure to check that all 4 diodes of the rectifier BR102 are good.

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