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1985 Mesa Studio .22

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  • 1985 Mesa Studio .22

    Getting 13 watts output. Does anyone recognize these modifications that are written as per Mesa?

  • #2
    Can't say for certain, but judging from the values listed, perhaps they are screen grid resistors and grid stoppers, plus a plate resistor.

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    • #3
      Not sure if this one is a 22 or a 22+.
      Both schematics attached. R131 is in the 'footswitch supply' area. R272 is in the -16V supply area. Without knowing what the 5K6 is connected to there is no way to guess.
      The 22 schem. shows the change to R131, and a few other additions in the same area.
      The 22+ schematics describes them on pg.3
      Attached Files
      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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      • #4
        Good find, g1!

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        • #5
          Originally posted by g1 View Post
          Not sure if this one is a 22 or a 22+.
          Both schematics attached. R131 is in the 'footswitch supply' area. R272 is in the -16V supply area. Without knowing what the 5K6 is connected to there is no way to guess.
          The 22 schem. shows the change to R131, and a few other additions in the same area.
          The 22+ schematics describes them on pg.3
          Mesa verified by serial # that it is a Studio .22. it has an eq but is not a +. Waiting on a middle pot from Mesa parts to install, then will check output again. Middle pot was bad causing intermittant output signal.

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          • #6
            Soldered in new middle pot and still getting intermittant volume when turning bass or middle pot now. Pretty sure bass pot wasn't causing issue before.

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            • #7
              Originally posted by g1 View Post
              Not sure if this one is a 22 or a 22+.
              Both schematics attached. R131 is in the 'footswitch supply' area. R272 is in the -16V supply area. Without knowing what the 5K6 is connected to there is no way to guess.
              The 22 schem. shows the change to R131, and a few other additions in the same area.
              The 22+ schematics describes them on pg.3
              All pots read o.oo@0 and rise normally to highest number @10, except for volume pot which reads 0.00@0 &10, but 198k @6.

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              • #8
                Originally posted by ca7922303 View Post

                ...except for volume pot which reads 0.00@0 &10, but 198k @6.
                Looks like you had the bass middle and treble pots set to 0.
                Amp will have no output at that setting.

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                • #9
                  Originally posted by Dave H View Post

                  Looks like you had the bass middle and treble pots set to 0.
                  Amp will have no output at that setting.
                  I was talking about the resistance of each pot with amp not on.

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                  • #10
                    All pots read normal now except volume pot. It reads (4 ohms @ 0 volume), (290k @ 7 volume), and then (4 ohms @ 10 volume) resistance power off. With power on, now feeds back with Reverb tank plugged in and volume knob above 2. No other control knob has any effect on feedback except volume knob. No feedback at any volume without Reverb tank connected.

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                    • #11
                      The volume pot reading will depend on the settings of the tone pots. Vol. at 10 will read different with all tone pots up compared to all tone pots down.
                      Is the intermittent fixed? Try poking around with an insulated tool like a chopstick or plastic prod to see if you can create the intermittent fault around the tone controls.

                      Reverb howl can be caused by bad connections to tank, or even positioning of tank.
                      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                      • #12
                        Always measure pots (and resistors) disconnected from circuit (i.e. when you can't predict how circuit and components interact).
                        Last edited by Helmholtz; Today, 06:58 PM.
                        - Own Opinions Only -

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                        • #13
                          Originally posted by g1 View Post
                          The volume pot reading will depend on the settings of the tone pots. Vol. at 10 will read different with all tone pots up compared to all tone pots down.
                          Is the intermittent fixed? Try poking around with an insulated tool like a chopstick or plastic prod to see if you can create the intermittent fault around the tone controls.

                          Reverb howl can be caused by bad connections to tank, or even positioning of tank.
                          Intermittent is fixed after replacing middle pot. Reverb howl is new, checked connections and location is as it was before howl started. Poking around didn't turn up anything. Somehow tone controls are not having any effect on resistance readings I'm getting on volume pot while all are in circuit. Also, with a signal thru input, 1000hz@.30v sine, with master volume and volume pot both on 8, presence control @ 0(2 3vac), presence control @ 10(12vac). All other tone controls have little to no effect on readings.

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