Valvestate 265 Hum Problem

Discussion in 'The Workbench' started by I don't play, Jul 17, 2017.

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  1. I don't play

    I don't play New Member

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    Trying to fix this amp as a graduation gift for my son who bought it second hand many years ago and put a lot of hours on it. It has developed a couple of hums...first one is a faint but noticeable hum when nothing is plugged in. Plugging in an open or shorted lead results in a much louder hum and some hiss that comes and goes.

    I have replaced the HV and LV filter caps and no change.

    I reflowed the solder joints on the input jack, even removed it to make sure the mechanical connections were OK.

    The solder connections look acceptable.

    A friend suggested plugging a lead into the FX SEND jack, which disconnects the preamp...no change when nothing is plugged into the input, but makes the hum louder when there is a shorted plug at the input.

    I found a schematic online but it is different from the actual circuit. Specifically, the schematic (V265-62-02 Iss 2) shows the ground side of the input connected to the tube grid through an unlabeled 100 nF cap when a lead is plugged in. The actual board has a 47 ohm resistor in the position labeled "LK12" (which was a jumper wire on the schematic) and a 10k resistor in the position labeled C6 (yes, a resistor in a position labeled for a cap). These do not appear to be later mods made by a tech. They appear to be original factory-installed components.

    There are a few other places where the schematic and board are different.

    I have not worked on guitar amps ever. I have an engineering degree, can read a schematic, and am reasonably proficient with a soldering iron. Instrumentation available is limited to a DMM.

    Sooo....

    1. Does anyone have an idea where I can find a schematic that matches the board?

    2. Where should I look for the broken ground connection?

    3. Any other suggestions?

    4. Should I just give up, throw it out, and find him another amp?

    Option 4 seems a shame since it is probably a simple fix (once it is found) and in its day, this was a pretty good amp.

    Thanks,

    IDP
     
  2. mickeydg5

    mickeydg5 Well-Known Member

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    Does it do the same with all GAIN and VOLUME controls at zero?
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    NEARLY EVERY TIME
    A shorted output transistor (partially shorted or leaky)
    A shorted diode (check the zener diodes which regulate the low voltage supplies*), a shorted or leaky rectifier diode
    Bad op amp (feel temperature of op amps, bad ones will get hot)

    * Look for zener diodes under the circuit board

    Clean up all ground connections to the chassis, check power supplies for excessive ripple (possible open filter caps).
     
  4. mickeydg5

    mickeydg5 Well-Known Member

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    I was thinking maybe a simple ground issue first or power supply issue next. But yes the thing has diodes up front also and op-amps galore.
     
  5. Guitarmike000

    Guitarmike000 New Member

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  6. Guitarmike000

    Guitarmike000 New Member

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    i just now ran across this post i had a bad hum in mine especially with the fx control's an reverb clean an distortion so it was so bad i couldn't stand it. so yesterday it took 2-hours start to finish checked boards for worn or burned parts caps etc. cleaned all pots push levers real good,put back and wow day an night difference dont have schematics for valvestate amps wish i did! now foot pedal needs my attention! hope you are able to fix my valvestate 2000- is apart took out input jack out when on no sound so iv'e got it on repair bench ordered a few parts should have it back very soon snows melted now i have a ton of outside projects to fix! motorhome,plow truck,stack for wood stove and dirtbikes 3-to fix. is it winter yet i already want to come back in!
     
  7. mickeydg5

    mickeydg5 Well-Known Member

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    You need to tell everyone the year and model of your amplifier. then they may be able to push a schematic your way.
     
  8. Guitarmike000

    Guitarmike000 New Member

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    yes you are exactly right.
     

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