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VU meter adjustment (We’re nearly there!!!)

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  • VU meter adjustment (We’re nearly there!!!)

    With the support of this community I’ve nearly got an old mixer amp working. (Full story is linked if you’re interested...)

    I’ve got some leds to replace the blown incandescents on the meters, but it would be nice if the meters worked! They both respond to a multimeter connected on diode test mode, one more sensitive than the other, but there is very little deflection in normal operation on one channel and none on the other. I’d like to set the zero position and increase sensitivity.

    There are no trim pots on the board as far as I can tell.

    I assumed that the little lever at the bottom would set the zero position but it seems not to have any effect. The two screws that you can see locked with green sealant: is it worth me breaking the seal and trying either of those?
    Thank you!

    Click image for larger version

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  • #2
    Don't touch the screws. Is there a lever actuator inside the front cover of meter?
    "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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    • #3
      No, nothing on the cover at all. Just that lever inside that you can see sticking down from the centre screw (one of the ones that I’m not touching )

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      • #4
        Typically the center screw is fixed to the lever which should connect to outer end of a thin spiral spring inside. If that spring is broken or missing adjustment can't work. This retaining/recuperation spring and its "bias" determine the zero position of the needle.
        - Own Opinions Only -

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        • #5
          I just downloaded the Service Manual for the EM150ii Powered Mixer, which I believe is what we're dealing with. Regarding the mechanical Zero adjustment, it looks like you've removed the front casing of the meter, which should contain the mechanical Zero coupling adjustment. It clearly is NOT accessible from the mixer's control surface.

          If that coupling Zero Adjustment isn't on the meter cover, then trying to tweak that by manually moving the lever at the bottom end of the adjustment we see in your meter photo is very tricky. And, it would have to be done with a non-magnetic screwdriver or tool.

          The meter circuit for each channel has an input coupling caps and a resistive attenuator (series 24k R, followed by a shunt 330k R), followed by a series 27k R feeding the full wave rectifier. It's been fed from the output of the power amps. So you're correct in that there is NO meter sensitivity adjustment. I'd be looking at the input coupling caps since you're saying you get NO meter deflection from signal. I haven't read the owner's manual, but would guess the meters are calibrated for 0VU = full power. If' you're looking for 0VU (1.223V RMS), there won't be much deflection at all. You CAN drive this mixer to full power (do it without speaker attached), or with headphones connected, if not too loud)...and turn it up to full output (24.5V RMS for 75W/8 ohms).

          I'm having trouble uploading the service manual for some reason. So I extracted the schematic and just uploaded that.

          EM-150II schematic.pdf
          Logic is an organized way of going wrong with confidence

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          • #6
            Yep, nevetslab, that’s the boy. Definitely no mechanical zero adjustment on the meter cover: literally just clear plastic, not even a hole or slot. I was expecting the lever under the centre screw (the one I’ve still not touched!) to be the zero adjustment so it’s possible that the spring Helmholtz refers to is broken on both sides (I adjusted this with a bamboo skewer, not hard to move but no effect on needle position).

            Which are the coupling capacitors? Are they all of those in that part of the schematic i.e. the 4 marked 470P and the 2 marked 10/16?

            Thank you.

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            • #7
              Ok, an update. Had a closer look with good light and a magnifying glass and could see the retaining spring that Helmholtz referred to. The lever does connect to this, and does adjust to zero. It’s just before I didn’t have the confidence to move it as far as needed.

              So if I could adjust the sensitivity I’d be there...back to those caps of nevetslab....

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              • #8
                I could be wrong, but I thought that the center screw placed tension on the movement, acting sort of like a bearing. That is why I warned against adjusting the screw.
                "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

                Comment


                • #9
                  Originally posted by Rhod View Post
                  Ok, an update. Had a closer look with good light and a magnifying glass and could see the retaining spring that Helmholtz referred to. The lever does connect to this, and does adjust to zero. It’s just before I didn’t have the confidence to move it as far as needed.

                  So if I could adjust the sensitivity I’d be there...back to those caps of nevetslab....
                  Each meter has two electrolytic caps. The input cap is a 1uF/50V bipolar and the last one where the meter's (+) terminal connects is a10uF/16V polarized electrolytic. I'd be looking at the input cap (1uF/50v BP), if it IS a cap issue. The 470pF HF bypass caps shouldn't be involved. I assume they're ceramic.

                  Did you put signal in enough to get 24.5V output? I'm guessing that's what the meters are calibrated to (full power at 8 ohms = 0dB)?
                  Logic is an organized way of going wrong with confidence

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                  • #10
                    Originally posted by g1 View Post
                    I could be wrong, but I thought that the center screw placed tension on the movement, acting sort of like a bearing. That is why I warned against adjusting the screw.
                    On all meters I've seen the center screw was fixed to the lever to allow screwdriver zero adjustment

                    The spring is an essential element of the meter. Without it the needle would always tend to deflect to the scale end when current flows through the coil. The spring exerts a counter-torque, so the needle stops when equilibrium of forces is reached.
                    - Own Opinions Only -

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                    • #11
                      Originally posted by Helmholtz View Post

                      On all meters I've seen the center screw was fixed to the lever to allow screwdriver zero adjustment

                      The spring is an essential element of the meter. Without it the needle would always tend to deflect to the scale end when current flows through the coil. The spring exerts a counter-torque, so the needle stops when equilibrium of forces is reached.
                      So with those types, if you move the lever, the screw will turn along with it? I have seen (and damaged ) types where the screw is locked with thread locker and the back of the screw is a hollow where the shaft of the movement sits in. The lever on those moves independently of the screw. I may be thinking of automotive type stuff (dwell meter etc.).
                      "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

                      Comment


                      • #12
                        Originally posted by g1 View Post

                        So with those types, if you move the lever, the screw will turn along with it?
                        Yes, that's the type I'm familiar with. I would assume the screw head to look different (metal) and being secured by laquer, if the screw somehow holds the bearings.
                        Last edited by Helmholtz; 06-25-2020, 05:49 PM.
                        - Own Opinions Only -

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                        • #13
                          These Yamaha ones are definitely of the type that g1 describes: fixed centre screw which doesn’t turn with the zero adjustment lever. And yes, I would assume that the sealed screw applies tension on the bearing. In fact, I’ve seen a YouTube video where someone releases this centre screw (melts the lacquer with a soldering iron) to restore movement to a stuck needle.

                          And I think Helmholtz is right about how these meters are calibrated: I really cranked it up and did get the meters to deflect all the way, but it needed a line level source at a instrument level input setting to achieve this! Some variation on sensitivity between the two meters so there could be some drift on those caps...

                          I’m used to using meters on desks to set input levels, so these are of limited value for that!!!



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                          • #14
                            The owners manual says 0VU is 37.5W into 8 ohms, full power is +3VU (75W @ 8 ohms).
                            I suppose you could find a different spot to pick-off signal to the meter circuit and re-calibrate, but it would be a fair bit of work.
                            "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

                            Comment


                            • #15
                              Did we check the 1uF coupling caps yet? I wouldn't mess with anything until we verify there isn't a problem with the circuit.
                              "Yeah, well, you know, that's just, like, your opinion, man."

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