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Marshall VS100R hum problem

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  • #31
    I just want to point out something that could really screw up the testing and troubleshooting here, in case anyone is unaware.
    The muting circuit in this amp is a real strange one.
    For any kind of noise tracking, you should probably use a dummy plug in the input jack (and short tip to sleeve). Otherwise the grid of V1A is grounded via LK12 100n cap.
    Even the power amp input will not work by itself without a dummy plug in the gtr input jack. The point above V1A labelled "FXLPOUT" is coming from the Fx return.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #32
      Originally posted by g1 View Post
      I just want to point out something that could really screw up the testing and troubleshooting here, in case anyone is unaware.
      The muting circuit in this amp is a real strange one.
      For any kind of noise tracking, you should probably use a dummy plug in the input jack (and short tip to sleeve). Otherwise the grid of V1A is grounded via LK12 100n cap.
      Even the power amp input will not work by itself without a dummy plug in the gtr input jack. The point above V1A labelled "FXLPOUT" is coming from the Fx return.
      I noticed that too. I just did a test, I disconnected CON1 and connected the tip of my guitar jack cable to R1 of the power PCB and the sleeve to ground and it sounds with almost no noise. My common sense tells me that the power amp is full blast and is normal to have some noise. What else do you suggest???

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      • #33
        Originally posted by avoverdu View Post
        C21 and C19 on the preamp PCB?? I checked those days ago with the multimeter and they seemed fine
        A multimeter is of very limited value in testing caps. Tack a known good one on there.

        G1: Thx- I was aware of the input muting. The problem in this case is well after that, although I have to admit I am just about out of ideas on how to narrow it down further.
        Last edited by nickb; 12-04-2017, 08:21 PM.
        Experience is something you get, just after you really needed it.

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        • #34
          Originally posted by nickb View Post
          A multimeter is of very limited value in testing caps. Tack a known good one on there.

          G1: Thx- I was aware of the input muting. The problem in this case is well after that, although I have to admit I just about out of ideas on how to narrow it down further.
          Ill get a couple of those tomorrow and put them in just to be sure. Please let me know if you have any ideas, I appreciate it mate

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          • #35
            I have an update:

            On the preamp board I changed C21 and C19 on the supply and also changed C26, C27. On the rest of the preamp board I changed C45, and C2.

            This are the results: Everything on zero, no guitar plugged in, no dummy jack

            a) Clean channel,: when I turn it on is silent for a second, then a tiny pop and hum appears and increases. It takes around 30 seconds to get to its maximum volume. If I crank the volume the hum increases a little bit. If I crank the treble, as soon as I move it away from zero the higher octave of the hum disappears.

            b) Overdrive channel: silent, then I tiny pop and hum appears. The volume is about the same as the clean channel.

            NOW EVERYTHING THE SAME BUT WITH THE GUITAR PLUGGED IN:

            a) Clean Channel: everything on zero and the hum is louder compared to the previous test. sometimes gets a little louder and comes back to the original. If I crank the volume the hum increases a little bit when I put my hand closer to the chassis or if I move around the guitar it gets the volume of the hum changes. If i set the volume like 9 o´clock the hum is as loud as the guitar signal but if i crank the treble or the mids the hum is reduced as soon as im not on zero.

            b) Overdrive: pretty much the same but when I turn up the volume more than zero the hum disappears almost completely. The treble pot make noises the you turn it more than zero but it doesn't affect the hum.

            Any ideas or theories will be appreciated. Thank you guys

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            • #36
              ***sigh*** I know it was a long shot.

              The slight pop is the power on/off muting circuit dropping out I would guess. That sounds about normal.

              For now, I'm out of good ideas.

              The 30 second delay has to be a clue. Also, we know the tube is involved. Let's try to eliminate the heater current. Disconnect the transformer heater winding (blue & brown I think) and feed the board heater supply 12.6V or so from a power supply or batteries. Turn the amp on then connect the heater supply and observe what happens to the hum over a minute or so. Leave the pots down and the guitar unplugged as before. That's just confusing us with more noise. Switch off and disconnect the heater supply.
              Experience is something you get, just after you really needed it.

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              • #37
                Originally posted by nickb View Post
                ***sigh*** I know it was a long shot.

                The slight pop is the power on/off muting circuit dropping out I would guess. That sounds about normal.

                For now, I'm out of good ideas.

                The 30 second delay has to be a clue. Also, we know the tube is involved. Let's try to eliminate the heater current. Disconnect the transformer heater winding (blue & brown I think) and feed the board heater supply 12.6V or so from a power supply or batteries. Turn the amp on then connect the heater supply and observe what happens to the hum over a minute or so. Leave the pots down and the guitar unplugged as before. That's just confusing us with more noise. Switch off and disconnect the heater supply.

                Ok. I have a laptop power supply that has 12 and 14 volts DC. The filament winding of the transformer pushes 13.99v AC so Im going to use the 14VDC on the power supply.

                1) No guitar plugged in, clean channel, all pots on zero, heater filament circuit disconnected from transformer and connected to 14VDC power supply: I turned on the amp, and the hum is there; increases just a tiny bit in time. If I disconnect the power supply the hum doesn't change at all. The overdrive channel same behavior.

                Comparing the loudness of the hum between the one produced with the transformer and the one produced with the power supply, the one with the transformer is louder

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                • #38
                  Can you please repeat but measure the VAC across the speaker so we have some numbers to work with.

                  You are really good at following directions It makes it so much easier
                  Experience is something you get, just after you really needed it.

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                  • #39
                    Originally posted by nickb View Post
                    Can you please repeat but measure the VAC across the speaker so we have some numbers to work with.

                    You are really good at following directions It makes it so much easier
                    Here we go:
                    Everything on zero, no guitar plugged in, filament circuit connected to transformer:
                    Clean Channel, AC Voltage across the speaker= 0.25 (silent for about 5 seconds, then hum increases in time; it takes about 3 minutes to reach 0.22v but varies up to 0.26 even 0.27v after 5 minutes
                    After this I switch to overdrive channel and voltage across speaker= 0.205v

                    Everything on zero, no guitar plugged in, filament circuit connected to 14vdc power supply:
                    Clean Channel, AC Voltage across the speaker= 0.015v (similar behavior but jumps in values around 0.015v, 0.010,0.012 it doesn't take that long to get those values)

                    Same situation, overdrive channel, ac voltage across speaker= 0.001v stable!

                    I hope this helps. Thanks a lot

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                    • #40
                      It sure does.
                      Clearly hum is getting in through the tube,jumping from filament to cathode.

                      Didn´t even look at the schematic yet, but you need to turn that 12VAC filament supply you curently have into a 12V DC clean one ... no big deal
                      Juan Manuel Fahey

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                      • #41
                        Originally posted by avoverdu View Post
                        Here we go:
                        Everything on zero, no guitar plugged in, filament circuit connected to transformer:
                        Clean Channel, AC Voltage across the speaker= 0.25 (silent for about 5 seconds, then hum increases in time; it takes about 3 minutes to reach 0.22v but varies up to 0.26 even 0.27v after 5 minutes
                        After this I switch to overdrive channel and voltage across speaker= 0.205v

                        Everything on zero, no guitar plugged in, filament circuit connected to 14vdc power supply:
                        Clean Channel, AC Voltage across the speaker= 0.015v (similar behavior but jumps in values around 0.015v, 0.010,0.012 it doesn't take that long to get those values)

                        Same situation, overdrive channel, ac voltage across speaker= 0.001v stable!

                        I hope this helps. Thanks a lot
                        Well that proves it's coming from the heater. Have you acutally tried a new tube? I thought you had.
                        Experience is something you get, just after you really needed it.

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                        • #42
                          I´m somewhat worried that on schematic kindly supplied in post #6 , filament wires leaving btransformer (page 4) are both Blue, while those reaching the tube on page 1 , one of them through fuse F2, are one Blue and one Brown, yet I see no point showing the colour change.
                          Maybe younger sharper eyes can find that point.

                          I was about to suggest a bridge + 2 capacitors and 1 resistor simple and clean DC supply but hesitate to do so until the doubt is cleared.
                          Juan Manuel Fahey

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                          • #43
                            Originally posted by J M Fahey View Post
                            I´m somewhat worried that on schematic kindly supplied in post #6 , filament wires leaving btransformer (page 4) are both Blue, while those reaching the tube on page 1 , one of them through fuse F2, are one Blue and one Brown, yet I see no point showing the colour change.
                            Maybe younger sharper eyes can find that point.

                            I was about to suggest a bridge + 2 capacitors and 1 resistor simple and clean DC supply but hesitate to do so until the doubt is cleared.
                            The heater supply cables are brown and measure 13.9 vAC between the two terminals. They go across a fuse (F2) and to the tube socket. I checked all the connections there and they are fine.

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                            • #44
                              Originally posted by nickb View Post
                              Well that proves it's coming from the heater. Have you acutally tried a new tube? I thought you had.
                              I have tried several tubes: Marshall original ones, EH, Chinese and nothing.
                              Last thing I did I placed the best tube I have (sovtek 12ax7 LPS which I have on V1 on another amp), clean channel and the AC voltage across the speaker = around 1.8 vAC. and overdrive channel = around 1v. Clearly the hum is louder with this tube. Why?? why is it always louder on the clean channel when you compare it with the overdrive channel?

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                              • #45
                                Originally posted by avoverdu View Post
                                I have tried several tubes: Marshall original ones, EH, Chinese and nothing.
                                Last thing I did I placed the best tube I have (sovtek 12ax7 LPS which I have on V1 on another amp), clean channel and the AC voltage across the speaker = around 1.8 vAC. and overdrive channel = around 1v. Clearly the hum is louder with this tube. Why?? why is it always louder on the clean channel when you compare it with the overdrive channel?
                                OK one more test. Disconnect your DC supply and measure the DC resistance between pin 9 of the tube socket and ground.
                                Experience is something you get, just after you really needed it.

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