Marshall Valvestate(VS65R) no speaker output

Discussion in 'General Electronics Chat' started by wolf1419, Sep 8, 2009.

  1. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    I have good signal at the line out, FX out and headphone out...so all input section is fine, but nothing coming out at the speaker wires...the headphone jack is a switching output, but cleaning and working it does nothing to even get a crackle out of the speaker wires....(the speaker has been tested independently)....is it true that the whole power amp stage and output transistors, etc. are all fine if the headphone has power, which should be shown in the enclosed attachment(schematic).....:confused:
     
  2. S_lannan

    Active Member

    Jun 20, 2007
    247
    2
    by the looks of it, if the headphones work the power amp should be fine.

    switching sockets for pre-out power-amp in and such are pretty notorious for going faulty, so i wouldn't be surprised if it's just a faulty switching socket.

    try connecting a speaker to the junction between r21 and r24, and have the other speaker lead connected to the circuit's ground.

    Be careful not to short out anything, it would be ideal to test that with a oscilloscope probe.
     
  3. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    That's how I'm reading it also(total amatuer here..just trying to learn).....Just a question in the meantime....it appears by schematic that it's the sleeve(gnd) that gets "switched", so I removed the chassis and checked continuity throughout the jack - full continuity with no headphone and "open" when headphone is inserted as it should be(checked from speaker neg to far side of jack/switch) ...... I will try your other test as soon as I get enough confidence to get in there and do this very carefully(unsure if boards need to be removed, etc)......meanwhile, are there any other simple continuity/value checks from the topside of board, after verifying that the jack/switch appears to operate properly...?...I have continuity(closed) from common sides of R21,R24, D12, D13, C8 to speaker out......what is the logic and impact behind D12, D13, C8, etc on this circuit and speaker output and headphone circuit......I can follow a simple series circuit, but am having a terrible time understanding the concepts of more complex circuits and how they relate to the whole.......Thanks!
     
    Last edited: Sep 9, 2009
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    How does the phones sound?
    Are they at a usual volume or much lower?
    Do you have a multi meter?
    If so measure the voltage at the gate of T16 to ground.
    This FET is part of the speaker protection.

    Greetings,
    Bertus
     
  5. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
    117
    0


    Hi Bertus,
    You gave me some teaching and success with a Fender amp a long while back...I'm still pretty green, but before I got your post I was curious of the same thing about actual headphone volume. I have another VS65R to compare, and I found the ailing amp headphone to be much, much lower...distorted and weak...some crackling and volume abnormalities while adjusting the clean volume, but never at full power like the good one.
    Just as I finished I saw your thread and was happy to see you again, and also heading for the same point......I've found T16 in the schematic and on the board....and the middle contact is what I think you want to test...is that correct...? With amp powered up and checking voltage from that point to any gnd point on the amp..is that correct also..before I proceed, so that we are on the same page...and any input, or all counterclockwise controls.
    I see T16 connection between 1&2, which are the preamp out, but don't know enough to explain how this transistor and gate affects the rest of the circuit and how you are able to deduce that this is the "key" component that we need to check.....please explain if it can be done to a novice.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    The FET T16 gets a signal of the circuit build round T13, T14 and T15.
    This circuit checks if the powersupply is OK.
    If there is a failure the circuit gives T16 a signal to turn down the volume to protect the speaker.
    The the circuit you see a triange with R41 ( 4.7 Ohms 1/2 Watt ), this is the input of the security circuit.
    You could check if this resistor is OK.

    Greetings,
    Bertus
     
  7. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
    117
    0
    I see the circuit building around T13, T14, T15, and R41 measures 4.7 Ohms, so that's good......THEN, while
    getting ready to test voltage, I had taken chassis out and must have had something move, twist, and speaker wire must have touched the chassis...started smoking...luckily stopped it real quick before damage, moved speaker wires clear and with headphones heard glorious sound coming out....slightly moving board about and got cutting out, weak, etc...used a matchstick to try to find a loose joint, etc......everything seemed to make it go out until I hit R21/R29 and BINGO...
    definite looseness there, lodged matchstick between the 2 to make good temporary contact, while moving other sections of the board and all seemed fine....hooked up speakers and "glorious sound there also.....letting it run to see if anything else shows up.......if all is well I'll remove the board and resolder the lugs for R21/R29 and should be fine...a little nervous about the short "smoking event, but no harm found"....LUCKY, LUCKY!!!! Thanks

    Reflowed solder at R21 & R29, tapped all over the board for problems(none found), reassembled it all and it's all working fine....Sounds great!! Banged it around a little to test for potential problems....Solid as a rock!!!!
     
    Last edited: Sep 10, 2009
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