Marshall Valvestate VS265 amp problem

Discussion in 'General Electronics Chat' started by rockjunky, Jan 18, 2011.

  1. rockjunky

    Thread Starter New Member

    Jan 18, 2011
    2
    0
    Hi all
    A friend of mine has a marshall valvestate vs265 amp that has had intermittent problems

    Basically after a certain amount of time being on, a loud hum would come in and out through the amp , he said the previous owner had to get it repaired (mentioned resoldering stuff)

    I found a schematic here, it does not appear to be 100% accurate (I will explain why soon )
    http://www.amparchives.com/Amp Arch...state/VS265 65Wx2 2x12 Combo/vs265r_2x65w.pdf

    heres what i found
    *previous preamp tube had no glow to it , replaced
    * friend had tried to resolder some of the points....whether it was him or from the previous repair some were in very bad shape , traces lifting or broken

    I had to put silastic under the REG's and resistors because the trace had lifted so no ensure no more movement

    I found REG 2 point 'com' to have had its trace completely broken, I managed to resolder it

    I found REG 2 point 'I' to only have a circuit on one side and not passing through to the other , I resoldered this
    previously R20,R21 and REG1 were getting VERY hot

    since the resolder R20,R21,REG1 and REG 2 are all still very hot
    now the amp has a loud pop when turning on and VERY loud when turning off
    it seems the hum coming in and out has gone ....I do have the amp disconected fromt he speaker box so it is not getting vibration on it which may be part of the issue if there are bad solder joints
    at the same time I don't want to leave it on for to long with the heat thinking it will fry these components

    I noticed looking at the board, the O on REG 1 and REG 2 is going to C31 & C32 which are not shown in the schematic (seems to be C20 & C18)
    I'm getting 41VDC on R21 if I remember correctly and I think -41VDC on R20...it shows 35V on the schematic so this seems quite high
    what could be the problem?

    [​IMG]
    [​IMG]
    showing 1 stamped on the revision? I think the schematic is Rev2
    [​IMG]
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    When the ground connections of the regulators where not connected
    it can be that the full 30-40 volts is directed to the opamps
    and the opamps are blown.

    Can you see any physical damage to the opamps?
    Sometimes there will be a bump or burning marks on the chips that are blown.

    I have been busy with a valvestat some time ago with an other member:
    http://forum.allaboutcircuits.com/showthread.php?t=27432

    Bertus
     
    Last edited: Jan 19, 2011
  3. rockjunky

    Thread Starter New Member

    Jan 18, 2011
    2
    0
    hi

    I did some more soldering last night on the cap's mainly and the popping is gone

    doesn't seem like the hum is there now but I am not running the amp for long and I have it removed from the speaker cabinet so there is no vibration

    I take it you are talking about op amp 7a and 1a in the schematic?

    they look ok as far as I can tell

    there is still alot of heat on resistor 20,21 and REG 1&2 ....can't touch them
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    The temperature of the resistors R20 and R21 and the regulators REG1 and REG2 depens on the drawn current.
    R20 and R21 are used to drop a part of the voltage from PSU+VE and PSU-VE to teh input voltages of the regs.
    When there are some bad capacitors that are shorting the regs, the temperature can rise.
    Also defective opamps can cause problems.

    Bertus
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    950
    You will have to do some work with this one. You might locate a bad chip by turning it on for a short period and seeing if any DIP chips get warm(if blown the may not react at all)

    Breaking the output of the regulators and inserting an amp meter would be helpful. Best if a point that can be resoldered is used.

    The fact these reg's were designed with no heat sink reveals they are not expected to 'get hot' in normal operation. The large resistors would probably get warm though.

    Remove as many of the IC's as you have to until the large current draw on the meter disappears. there you have it.

    Lot's of work - like I said.

    (in-expert removal of IC's can result in heat damage to a working device, and most of them are quite stubborn as well :) )
     
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