Anyone who understands about MESA MARK III, please help me please!!!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Fabio_França, Jun 4, 2005.


  1. I have a Mesa Mark III , one I posted in another topic, today I had a problem with it that I didn't figger out. One of the EL 34 broke... right after it happened I changed to a JJ E34l and this JJ lightened with a very strong color and the amp stoped working. So, I saw that the fusivel had burned, so I switched it and the amp started to work again... but the JJ e34L was overlighten... so,
    could anyone help me out?!

    And, in this picture down here is it missisng a tube socket or it´s just me?!?!?!? if you're really talking about a tube V3 (since the V3 indicates reverb and my amp doesn't have a reverb), does anyone have the schematics to solve the problem... to help me out????.... since we don't have Mesa Boogie assistence in Brazil
    The el 34 are located at the amp's edge or center???

    [​IMG]

    [​IMG]

    [​IMG]

    Thanks!!
     
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

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    The "missing tube" would be for the reverb...also you can see where the jacks would be (if you had reverb) in the second picture.

    Sounds like a tube shorted out. It may have burnt a resistor on the EL34 socket...can you take a CLEAR picture of the EL34 sockets and post them?
     
  3. John Phillips

    John Phillips Member

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    Yes, it sounds like the JJ EL34L failed. That would blow the fuse, and it may or may not have done any other damage.

    The EL34s go in the outer pair of sockets in a Simul-Class model.
     
  4. I`ll take the el34 sockets pictures and please tell me , is the V3 only used by reverb or it also works for the drive?





    today i`ve inverted the el34s with the 6l6 (el34 in, 6l6 out) and the amp worked just fine , the mark iii manual indicates that the right position, what`s your opinion?
     
  5. RussB

    RussB low rent hobbyist Silver Supporting Member

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    V3 is the Lead Channel

    V4 would be the reverb tube, if you had reverb

    Please re-read the manual. The EL34's should ONLY go into the outer sockets!
     
  6. TheAmpNerd

    TheAmpNerd Member

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    In that pic you have, Look at pin 6 of the left hand
    tube socket. It appears:

    1. it is chipped and possibly cracked.
    2. there is brown/black carbon deposit on it.

    Please confirm or deny.
     
  7. RussB i saw your pics in acp link, tomorrow i'll upload my news pics for you see. The tube that's missing is a V4 and not a V3... you i'll see tomorrow.

    Thanks!

    I don't understand, in new pics that i'll post you tell me more about this.

    Thanks!
     
  8. John Phillips

    John Phillips Member

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    The missing tube is definitely for the reverb - the two holes in the chassis next to it are for the RCA jacks that go to the spring tank. You could have reverb fitted if you want, all you need are the right parts for the board, the tank and the drive transformer, and the pot for the back panel.

    I'm not sure about any damage caused by the power tube blowing - I can't see anything that looks suspicious related to the sockets, although the screen-grid resistor has been replaced on one (with a better type actually, the green wirewound resistor on the tube nearest the FX loop jacks).

    But this

    [​IMG]

    needs fixing properly. The two resistors (the one which is sticking up at one end, and the gray one above it) have been replaced, very badly, and some damage to the board may have been done. These resistors are for the low-voltage supply for the EQ and switching, so it shouldn't affect the power tubes - although the one immediately below the cursor arrow in the pic is for the bias supply.

    Hope that is some help.
     
  9. John, you help me so much, i'll replace all of the resistors that have problem and i need other question, the board could be damage for the two resistors that you show me? :confused:

    The V4 can be prejudice other part of circuit/sound or is only for reverb?
     
  10. TheAmpNerd

    TheAmpNerd Member

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    Wow,

    The inside of that amp is notlhing like the MKIIC+.

    Many people profess to buy a MKIII "there is only a
    few resistors that are different in it".

    NO WAY...

    The boards are completely different.

    This has Pre amp and Power amp on one board
    with a separate power section.

    MkIICs, a preamp board and a power boards....

    Both will have separate Reverb, and Graphic EQ
    boards. Wtih some extra stuff for the EQ/Lead Channel.
     
  11. John Phillips

    John Phillips Member

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    The board looks badly charred at both ends of those resistors. Someone with very poor soldering skills has been working on it. To check thoroughly for potential problems, you're going to have to get the board out, which is a very major job and I wouldn't recommend it really if you're not experienced with these amps.

    On the lower one (the one that is too high at the left end) you could improve the connections by deliberately 'bridging' solder onto the trace and the diode, and the same on the left-hand end of the upper (gray) resistor, but the right-hand end of this one is a real problem because the connection is on the underside of the board. I think it runs to the end of the large electrolytic cap directly above it in the pic and could be bridged with wire, but make certain before you do anything like that. It will be a bit of a bodge no matter how you do it unfortunately.

    No, it's just for the reverb.


    Unless there's definite evidence of failure, I'd be inclined to leave the resistors on the power tube sockets alone BTW - or replace them all with a matching set of wirewounds just so everything is neat and tidy... but I'm like that ;).
     
  12. Thanks for all John, i sent the amp for my amp tech, when amp returns i tell you!!!
     

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