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Thread: MESA problem

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    MESA problem

    I have a Triple recto here with an odd problem . All 3 of the GAIN pots ( green ch. isnt near as bad) are super noisy and cut out the signal . Also have a low freq hum on all channels . I did replace the orange and Red ch pots with new ones from Mesa.

    Ive swapped out all 12ax7s

    ill post a video of the noise . The amp has much less overdrive than it should--these are very high gain amps as you know .

    I did find that there is 8.2 VDC on the orange and red gain pots ,,,but not on the green ch. pot .

    i dont have a scope - just a meter


    https://vimeo.com/399527974
    Attached Files Attached Files

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    Last edited by Valvehead; 03-23-2020 at 12:53 AM.

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    Supporting Member Randall's Avatar
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    The schematic link doesn't work on my end, but I'd say you have one or more leaky coupling caps. Likely where the scratchy and DC on the pots is from.

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    It's weird, because it WAS working fine.....

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    Have you tried spraying the pots with DeOxIt D5?

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    Lifetime Member Enzo's Avatar
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    Also most times I go into a recent era Mesa I find the pots are loose. meaning the little tabs that hold them together are partly sprung and the pot isn't stable. The rear cover has to be tight for the innards to stay aligned. A little pliers action and they are tight again.

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    Yep-- i was just thinking leaky caps---thats what causes the dc on the pots correct ?

    i tried cleaning the old pots and it didnt help---the pots in there now are brand new from Mesa ...this was before i found dc at the gain pots . The video has the new ones installed .

    ill try another scem link =

    http://diy-fever.com/wordpress/wp-co..._rectifier.pdf

    if you look at the scem Preamp PT1...im thinking it might be a cap before the ch2 / ch3 Gain pots ? Maybe that .02 right after RLY1a ?

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    Last edited by Valvehead; 03-22-2020 at 06:43 AM.

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    Member Jon Snell's Avatar
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    I usually start from the output side and work backwards. That eliminates possible errors. A good place to start is RY4b centre contact. Check for DC on there and then you will have an educated idea where it originates from.

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    Senior Member Pedro Vecino's Avatar
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    Post

    Quote Originally Posted by Valvehead View Post
    im thinking it might be a cap before the ch2 / ch3 Gain pots ? Maybe that .02 right after RLY1a ?
    I would start by disconnecting a pin from that capacitor. If nothing is achieved I would change the relay.

    If you were to finally replace the relay (I have seen more relays failing than capacitors in those amps) you could use this method to avoid disassembling the board completely.
    It is necessary to disintegrate the relay itself (dremel, cutting pliers, etc.), remove the eight pins and install two contact strips (with the unused pins cut off). Finally install the new relay on them. The latter is the easiest

    Click image for larger version. 

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    Last edited by Pedro Vecino; 03-22-2020 at 06:49 PM.

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    Member Jon Snell's Avatar
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    If you monitor the common pin of the relay, that is the point depending when you get DC on there, that will tell you where the DC is coming from.
    The relay is not at blame!

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    Id like to thank Mesa for labeling the pcb UNDER the parts...............

    I had a few minutes to check the amp , heres what ive got =

    at the .02 just after V1a and Rly1A , ive got 218v on 1 end and as i switch thru the channels i get
    green 0.5mv
    Orange 4.7v
    Red 1.2v

    shouldnt that stay at 0.5mv ?


    At the .002 / 680k just after that .02 i get the same readings on each end =
    green 0.5 mv
    Orange 6.2v
    Red 1.5v

    so how is the voltage increasing to 8.2v at Gain pots 2 and 3 (orange / red) ??


    ill have to study the scem so i can label which relay is which

    EDIT- i was able to pull that .02 cap leg out of circuit --where it meets the 82p cap . I measure about 1mv there

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    Last edited by Valvehead; 03-23-2020 at 03:13 AM.

  10. #10
    Member Jon Snell's Avatar
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    Whatever you do remember there are lethal voltages where you are probing!
    I am an engineer and work on these amplifiers. I use component numbering, not colours of wires etc. Sorry I have no idea what your measurements mean except there is anode voltage on one end of each decoupling capacitor and there should be almost zero at the other. If not, disconnect the coupling capacitor and measure again.
    Gain pot 1 should be zero and if not I would check pin 2 of V2a. If the valve is a Sovtek, replace it and check again, if not suspect the J176 on its grid feed, it may be leaky.
    Otherwise, if Gains 2 & 3 show DC on them, regardless of the position of RY1b then your issue is with the .02 decoupling capacitor from RL1a normally open CH2/CH3 connection.
    Those J176 FETS are known to give issues especially after valve failure.
    Good luck and keep a clear, organised head.

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    Quote Originally Posted by Jon Snell View Post
    Whatever you do remember there are lethal voltages where you are probing!
    I am an engineer and work on these amplifiers. I use component numbering, not colours of wires etc. Sorry I have no idea what your measurements mean except there is anode voltage on one end of each decoupling capacitor and there should be almost zero at the other. If not, disconnect the coupling capacitor and measure again.
    Gain pot 1 should be zero and if not I would check pin 2 of V2a. If the valve is a Sovtek, replace it and check again, if not suspect the J176 on its grid feed, it may be leaky.
    Otherwise, if Gains 2 & 3 show DC on them, regardless of the position of RY1b then your issue is with the .02 decoupling capacitor from RL1a normally open CH2/CH3 connection.
    Those J176 FETS are known to give issues especially after valve failure.
    Good luck and keep a clear, organised head.
    Thanks for your help
    Im aware of being shocked by tube amps........I learned the hard way .

    The .02 just after V1a = i pulled 1 leg off the pcb , i have 217vdc on the tube side , and the leg in the air measures 1mv

    V2 pin 2 is
    ch1 0.6v
    ch2 7.2v
    ch 3 1.8v

    the j176 has a +15 vdc supply

    i measure anywhere from 0.6 up to 7.2 vdc there depending on which channel selected

    the ch1 gain pot has 0.6vdc on it

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  12. #12
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    "Id like to thank Mesa for labeling the pcb UNDER the parts..............."

    One of the reasons I despise working these amps.

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    It's weird, because it WAS working fine.....

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    I've considered making layout drawings of Mesa amps but it would take about 10X longer than the repair itself. I don't see enough Mesas to justify it.
    However if someone else had the time to do it, it would be a much appreciated public service.

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  14. #14
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    And the winner is...................Jon Snell !

    I couldnt find any leaky caps .

    it was that J175 near V2 .

    new one in and the amp works perfect .-----THANKS !!!

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  15. #15
    Member Jon Snell's Avatar
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    My pleasure.

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