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  • MXR M118 Signal loss

    Hi All! Does anyone know why my Green MXR Analog Delay (M118) is not passing the delayed signal through either output ( mix / delay only. Before it went out it got extremely hissy. By accidently shorting the board near the filter cap I got a brief burst of audible delayed signal.
    R18 is 47K but reads 30K
    R46 is 200K but reads 31.9K
    R48 is 9.1K but reads 5.5K
    The filter cap reads good.
    ( all readings in circuit ). I'm still tracing out the circuit, but some of the values on the schematic do not match what's on the board. Thanks Drew
    Attached Files

  • #2
    I'm still tracing out the circuit, but some of the values on the schematic do not match what's on the board.
    Does your delay have 3 SAD1024's or 1 R5101? This could be the reason for the differences?

    Check all of the voltages and let us know what you find.

    CJ

    Comment


    • #3
      There are a number of CMOS chips "in the way" of the M118 passing delay signal, and any of them could be the culprit.

      The M118 uses a clever (but somewhat fragile) tracking switched filter to give maximum allowable bandwidth at any given delay time. So, there is a master clock that drives everything (way off in the lower left hand corner of the schematic). That clock is used to drive the "transmission gates" (the CMOS switches in the 4016 chips). The same clock is divided down by a series of 4013 flip-flops to drive the delay chip. If any of the 4013 sections fries, no delay. If any of the 4016 gates in the filters fries, no delay. If the 4069 used to buffer the clock to drive the 4016s fries, no delay.

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      • #4
        OK, I have a new developement. R46 and R48 were loose from the solder points on one end. Once soldered in I got everything, delayed signal and clean back ( also got the nasty buzz I was hearing previously, back, (120hz) constant at all settings, in or out.). Now for no apparent reason ( to me ) the delayed signal is disappeared again, as well as the clean signal when the effect is engaged. Clean Signal is present in bypass mode
        Strapping another filter cap over the existing one did not affect buzz.
        I've got 11vac on the two outside taps of the tranformer with negligble voltage on the middle tap ( .003 v). Good clean ac coming in.
        R18 is good. ).
        There is one huge 22 pin IC with the ID removed. I'll be double checking all the solder joints. Since I am getting delay I don't think it's an issue with the chips.The unit has never been worked on before. Drew

        Comment


        • #5
          You have the correct layout but the wrong schematic. This is the reason I asked about the BBD chips.
          I agree, if you're hearing a delay so great news, the R5101 is working.
          Chaeck all of the jacks and wiring to and from the pc board. Make sure you have a good ground between the board and the input and output jacks.

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          • #6
            The grounds are good. I reflowed some suspicious solder joints and got the clean signal with fx engaged. Still no delayed signal at either output. Is the correct schematic available for this model?

            Comment


            • #7
              After checking around the circuit, I ended up resoldering R46. When I desoldered one end to check it, out of circuit, I noticed the other end was loose Before I had a chance to test the unit I spied the black wire from the multipin connector was separated from the delay jack ( - ). I did'nt notice it earlier, and I was getting good board to jack ground. It possibly was hanging on by the plastic shielding only. This was probably my problem from the get go. The last thing I found when putting the board back in the box was that the AC ground wire was held on to the screw eyelet by ONE strand of wire. HA I probably did the damage myself when I did a value check of the components a few months ago. Thanks for your spot on help.
              Now another question. The M118 is a bit hissy, I think by nature, but are there any components I could change out to improve the S/N ratio when the effect is engaged? Also there is a bit of AC type noise.
              IT WORKS !!!
              Drew

              Comment


              • #8
                Great... glad to hear your delay is working!

                For AC hum, replace the DC filter capacitors after the AC to DC rectifier diodes. For hiss, that's a little tricky. I've seen BBD's cause hiss, resistors cause hiss, dirty input and output jacks cause hiss, and capacitors cause hiss (although rare). If you have an oscilloscope, sometimes you can find the hiss signature on your O-scope, then start signal tracing in hopes of isolating the hiss. Other than that it is anyone's guess.

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                • #9
                  When blueprinting I usually replace all electrolytic caps, as well as any carbon resistors and out of spec ( or drifting out ) resistors. The hiss really isn't that bad, I'd go as far to say it's no worse than when I picked the unit up 20+ ya. This pedal has such a sweet sound...
                  How would blueprinting affect the vintage value? I see these going on Ebay for a couple hun all the time...not that I want to let it go!! Thanks again Drew

                  Comment


                  • #10
                    Well, a functioning one should probably sell for more than a non-functioning one. Besides, the mojo is certainly not in the caps.

                    You can probably note in any resale ad that the buyer would have had to replace the caps anyway, given the age, so yuo've saved them the trouble and expense.

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                    • #11
                      absolutely right Mark!! Thanks all! Drew

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