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Boogie Mark IV B with bad optocoupler on lead channel help!

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  • Boogie Mark IV B with bad optocoupler on lead channel help!

    Hi! Iīm pretty sure that one of the optocoupler inside my Mark IV associated with the lead channel is bad or dead.
    When I switch from rythm 1 or rythm 2 to Lead it has almost no gain present only a clean sound and the gain red led indicator just flashes a sec and then shut off. Thereīs a big pop sound when switching from rythm 2 to Lead.
    Any body can help about which LDR could be the bad one? also can someone explain how to test for a bad Optocoupler with a digital VOM?
    Iīm handy with a multimeter and Iīm well aware of the risk of a functional amp. I appreciate any help.

  • #2
    LDR - light dependent resistor

    Inside the thing is a LED and a photocel - a photoresistor. The photocell has a high resistance until you shine a light on it, then the resistance goes down. SO if we run our signal into one end of the photocell, and out the other, the signal will not pass through when the LED is dark, but when the LED is lit, the low resistance allows the signal to flow.

    SO we can test the two halves of it. The LED side, when it is to be ON, check and see if you have a volt or two voltage drop across it, just as if it were a plain old LED sitting there. The diode test on your meter probably will check it too. Once you know your circuit is turning the LED off and on, you can measure the resistance of the photocell side and see if it goes from high to low resistance when the LED is lit.

    You could have a bad optocoupler - another name for the LDR - or it could be working, but the circuit it is in is not right. Popping when switching channels often means there is unwanted DC voltage on the signal path.

    Looking at the Mark IV drawing I have, I see another thing. The photocell side of the LDR is really a resistor that changes value, but it is being used as a switch: high resistance means OFF and low resistance means ON. SO Mesa just drew it as a switch.
    Education is what you're left with after you have forgotten what you have learned.


    • #3
      Thanks Enzo! The popping occurs only when switching from Rythm 2 to Lead and not from Rythm 1 to Rythm 2. I donīt have the footswitch controller. Iīm only switching channels from the back of the amp.


      • #4
        Here's some more info;

        Because there's a relatively gradual transition when the optocouplers (vactrols) are switched there's an absence of popping with Mesas. A failed vactrol won't switch, so it won't pop in its own right. They usually fail so that they go permanently high. There's a possibility that a failed vactrol is resulting in a charged cap popping somewhere, but I've never had this happen. A high degree of isolation exists between the LED and LDR - something like 2-3kv, so it's unlikely that one has failed and is leaking its control voltage into the audio path.

        When the amp is switched to 'lead' do the tone and volume controls associated with that channel work to any degree? Even though the sound is low level and clean? Mesas switch a lot of circuitry and it may give a clue to what's going on.

        BTW, all of the switching can be checked with the amp in standby - safer that way.


        • #5
          Thanks Mick!

          "When the amp is switched to 'lead' do the tone and volume controls associated with that channel work to any degree? Even though the sound is low level and clean? Mesas switch a lot of circuitry and it may give a clue to what's going on."

          Yes. It work to some degree.
          Last edited by Harrysong; 11-05-2013, 11:15 AM.


          • #6
            Just to clarify something that I should have asked before - when the amp is switched to lead, do the rhythm channel controls affect the lead sound? Check the rhythm gain controls too

            The reason I ask is that sometimes an LDR can fail and not switch between the two tone stacks, or leaves two gain controls connected and both will interact but attenuate the signal.
            Last edited by Mick Bailey; 11-05-2013, 09:41 AM.


            • #7
              Yes! the rhythm channels controls both affect the lead channel volume. Even more than the Lead control itself!.also the lead treble, Lead Bass and Lead Middle donīt work. Weird.
              I also identify that the loud Pop happens when I switch from Lead to Rythm 2 ad not the other way round.

              Hereīs a video that may show more or less whatīs going on


              • #8
                Ok, that's progress.

                Channel switching is a bit of an illusion with Mesa amps - there's only one channel on the Mk IV and various component groups are switched in/out to give the impression of channel switching. To do this, more than one Vactrol (or to use the schematic reference from now on - LDR) is operated at any one time, that is, more than one LDR is associated with channel or clean/crunch switching.

                Very often with these amps a single LDR can fail and not have too much effect - perhaps a little less gain - but everything works fairly normally. With your amp it would appear that;

                a) LDR4 is not switching the lead tone stack in when the lead channel is selected
                b) LDR10 and LDR6 are not switching the clean/crunch tone stacks out when the lead channel is selected
                c) Either LDR8, LDR11, LDR13, LDR14 (or all) are not operating correctly.

                The LED status for your channel switching looks like it could be the clue - I don't think you have an LDR problem as there would have to be too many that had failed simultaneously and this is unlikely. I think the problem is with the switching matrix. A good start would be to check the DC voltages as per schematic. I'll take a further look to see if I can visualise what's wrong.
                Last edited by Mick Bailey; 11-05-2013, 03:06 PM.


                • #9
                  Thank you very much Bill!


                  • #10
                    Do you have the schematic?

                    See the rotary Mode Select switch? It connects one side of the LEDs (which in turn switch the LDRs) associated with the lead channel to ground. If you look at the numbers on the LEDs these correspond to the LDRs being switched to 'ON'.

                    This looks like it isn't happening, but at the same time I don't think LDR6 and LDR10 are releasing - Q8 could be permanently on or shorted, the Mode switch could be faulty, or the 12v may not be correct. I'd also check the 5v to the ICs.


                    • #11
                      Here is a Mk4, not the B version, but it ought to be similar.

                      Education is what you're left with after you have forgotten what you have learned.


                      • #12
                        Some difference in the LDR references with the B version. This is the only one I could find that shows the matrix;



                        • #13
                          Thanks Gentlemen! Iīll check the voltages when back from work. The last time I pull out the chassis about a week ago, I spray a little bit of contact cleaner in the Mode switch but doesnīt improve the faulty Lead channel.
                          Last edited by Harrysong; 11-05-2013, 06:00 PM.


                          • #14
                            When looking at the switching matrix it helps to work on it if you think of it just having high or low logic states. High would be close to 12v, low would be close to ground - 0v.

                            If you look at part 2 of the matrix schematic you can see that the LEDs associated with each optocoupler are connected to three 'rails'. Each one gets switched low to turn on the group of LEDs attached to it, but the others need to be high.

                            The transistors attached to the end of each rail are used with the foot switch, as are the two ICs.

                            A simple check can be made to see if things are healthy with the switching. You first need to make sure that R122 is good and that it has its 12v supply. Now, off the back of the Mode Select switch at each position check the connection voltages with respect to ground and post back these voltages. The amp can be in standby for this, but turn the FX loop off. The conditions need to be;

                            Position R1 - connection R1 low, R2 high, LD high
                            Position R2 - connection R1 high, R2 low, LD high
                            Position LD - connection R1 high, R2 high, LD, low


                            • #15
                              Well I finally measure some items.

                              Position R1 - connection R1: 2.06V, R2: 10.22V, LD: 8.13V

                              Position R2 - connection R1: 10.19V, R2: 2.10V, LD: 8.12V

                              Position LD - connection R1: 10.38V, R2: 10.38V, LD: 1.78V

                              Also 5 V to ICīs are ok.
                              I really didnīt find a place to measure the 12 V.