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Mu-Tron III help, no sweep

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  • Mu-Tron III help, no sweep

    I have a vintage Mu-Tron III that has no sweep. All the controls have an effect on the sound, it just doesn't sweep. This one is the version with the power switch as part of the footswitch, if that makes any difference.

    I am not familiar with this circuit, nor with the six legged 0802 thingy, whatever it is. Any suggestions on where to start?

    Page 14.
    It's weird, because it WAS working fine.....

  • #2
    According to your schematic, page 13 maybe, your 6 leg thingie is a vactrol. The two polarized legs are the LED and I'd assume two photocells inside for the other four.

    I don't know the unit, but what sweeping are you expecting from it? It doesn't have a speed control I can see, as far as I can tell it is an envelope following effect, not a phaser.
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      The pdf seems to open but only seeing empty pages.
      - Own Opinions Only -


      • #4
        Looks like it sweeps with the envelope only, no LFO. Check the switch contacts on the "External Control Signal" jack page 13

        edit: just noticed the page 14 schematic doesn't have that jack. Maybe C8 is open.
        Last edited by dmeek; 06-13-2021, 08:56 PM.


        • #5
          The Mu-Tron III is selectable envelope filter (Auto Wah).
          Check Led drive O805 (Red dot on sch), R23 (value depends on charactistics O805) and IC A5, A6.
          It's All Over Now


          • #6
            Not a phaser of course, hence no LFO sweep. I should have known this, I built a clone of this pedal from a pedal forum years ago. I just yesterday worked on a Mu-tron II from the same fellow, and that one is a phaser, Oh well.

            This one doesn't have the external jack, nor the separate power switch. Aside from that, how would one go about shooting this? If it turns out to be the vactrol 0805, then what?
            It's weird, because it WAS working fine.....


            • #7
              About vactrol

              vactrol 0805 = led driver and 2 photo resistors similar to the optoisolator in Fender amps tremolo
              Usually the LED driver crashes. With the ohmmeter being tested quickly. If driver is a defect, open it carefully and replace it.

              DIY vactrol

              It's All Over Now


              • #8
                A little ditty that I found on the opto.

                Attached Files


                • #9
                  What I have done so far, swapped out the three dual op amps one at a time with a 4558, no change. The LED side of the vactrol looks good in circuit, open in one direction, 1.8v in the other. I do get maybe 30% increase in resistance (while playing a chord) between A2 pin 7 and A3 pin 6, as well as A3 pin 7 and A4 pin 2. So does this mean the vactrol is operating, and the op amps are good?
                  Last edited by Randall; 06-15-2021, 04:29 AM.
                  It's weird, because it WAS working fine.....


                  • #10

                    Yes this mean the vactrol is operating.
                    How the sound is manifested when change the "mode switch" (O502). Does the selects HP, BP or LP filter.
                    Mu-Tron is advanced (selectable) version of the EH Doctor Q envelope filter.
                    If you have only clean sound (no sound change when change O502) check that the "foot switch" (O505) correct or connected correctly.
                    The depth of the effect depends on the values of R23. 1)
                    Last edited by vintagekiki; 06-15-2021, 01:49 PM. Reason: 1)
                    It's All Over Now


                    • #11
                      Just for future reference for anyone requiring info re the Vactrol optocoupler the Musitronics part number 0805A is listed in the parts as a P873-13 made by the Hamamatsu Corporation.
                      I was under the impression it was no longer made but Mesa Boogie have one for $12.25 U.S. ! Mesa's part number SKU 516512 listed as a LDR - Dual Cell P873-13 !


                      • #12
                        Click image for larger version  Name:	moddedMutron3.gif Views:	0 Size:	31.1 KB ID:	934719 Click image for larger version  Name:	20141010210840.png Views:	0 Size:	388.9 KB ID:	934720

                        MU-TRON III operating instructions 1)
                        Last edited by vintagekiki; 06-15-2021, 11:16 PM. Reason: 1)
                        It's All Over Now


                        • #13
                          Got it fixed, even though I still don't really understand how it works. After verifying that the op amps and vactrol were functioning, there really wasn't much else left to this circuit. There are two PS filter caps and three small signal capsThe pos/neg supplies looked good, so I set about looking at the electrolytic signal caps. Sure enough, the 3.3uF cap was bad. Replaced it, and it tried to work, but it wasn't quite there, so I replaced the other two as well, a 2.2uF and a 4/7uf for good measure.

                          And that did the trick, although along the way it developed a nasty hum. This actually took longer than repairing the box. I determined there was no ground between the board and chassis that I could find. I don't know why it was quiet before, but once I installed a ground wire from the board ground bus to the AC safety ground lug, all was well. I know the chassis needs to be grounded to the board because the in/out jacks get their grounds from the chassis. I added a couple of toothed washers there as well.
                          It's weird, because it WAS working fine.....