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Marshall VS100R hum problem

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  • #16
    Originally posted by nickb View Post
    Please go ahead and measure the ACV on IC3 pins 4 & 8.

    PS: Measure the DCV at the same time ( don't use the series cap)
    OK: pins of IC3 to ground NO 0.1uF cap

    Pin 4: ACV: jumps a little bit then 0.000 after a couple seconds of turning it up. DCV:-12.07

    Pin 8: ACV: 0.000 DCV: 12.05

    I did read those voltages with the cap in series but the values were the same

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    • #17
      Well, it's not coming on the power supply. We have reached an apparent dead end as the pulling CON1 says it's before power amp, the shorting R77/R49 test says it at or before IC3's power supply, the ACV test says it not the IC3 power supply yet earlier when the 12AX7 was pulled that didn't help either. The 12AX7 is right before IC3. I therefore wonder if the reduction of hum is less than when pulling CON1 so misleading us.

      Measure the ACV on the speaker with R77/R48 shorted and then again with the 12AX7 pulled. If the hum reduction is much less then the 0.2VAC we got when pulling CON1 then we need to start looking at the daughter PCB. Ir would not do any harm to apply some switch cleaner to all the multipin connectors and plug/unplug a few times. You might get lucky and save some time.
      Experience is something you get, just after you really needed it.

      Comment


      • #18
        Adding a suspect: it might be not some hummy stage (or it could be *one* part but not the full story) but a grounding problem.
        Pulling Con1 not only removes preamp signal but also disconnects power amp gtround from preamp one.

        So I would pull Con1 but solder a wire copying what Con 1 ground contact + wire do.
        Or IF you have the means (single wire + proper connectors + crimping tool or long nose pliers) build a new "Con 1 wire" which just connects grounds as it does.

        Important: join same pads at the connectors base as would be joined by Con 1 , we want to reproduce the exact same path, in grounding problems grounding " here" or "just 1 inch away" is NOT the same.
        Juan Manuel Fahey

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        • #19
          Originally posted by nickb View Post
          Well, it's not coming on the power supply. We have reached an apparent dead end as the pulling CON1 says it's before power amp, the shorting R77/R49 test says it at or before IC3's power supply, the ACV test says it not the IC3 power supply yet earlier when the 12AX7 was pulled that didn't help either. The 12AX7 is right before IC3. I therefore wonder if the reduction of hum is less than when pulling CON1 so misleading us.

          Measure the ACV on the speaker with R77/R48 shorted and then again with the 12AX7 pulled. If the hum reduction is much less then the 0.2VAC we got when pulling CON1 then we need to start looking at the daughter PCB. Ir would not do any harm to apply some switch cleaner to all the multipin connectors and plug/unplug a few times. You might get lucky and save some time.
          OK. I cleaned the switches with deox and ran the tests:

          - Overdrive channel:
          ACV across the speaker with R48 shorted, without the tube: 0.002
          ACV across the speaker with R48 shorted, with the tube: it starts on zero and increases slowly, after a couple minutes reaches up to 0.213 or so.
          Same test on the Clean Channel is 0.000 to 0.002v

          ACV across the speaker with R77 shorted, without the tube: 0.000
          ACV across the speaker with R77 shorted, with the tube: 0.000
          Same test on the clean channel i got without the tube: 0.001 and with the tube increases slowly up to 0.25-0.27

          Now that you mentioned the daughter pbc, I found the hum increases (like adding a higher octave) when I push the power dimension button.

          I did this test: Overdrive channel, with the tube, no shorts, power dimension button OFF, ac voltage across the speaker = increases slowly up to 0.2 or so
          Same test but with power dimension button ON: 0.106 or so but the hum increases like I said, louder with a higher octave

          I repeated it without the tube: Power dimension button OFF: 0.002
          Power dimension button ON: around 0.050 and the hum gets louder and the pitch slightly changes. Its more like two octaves sounding at the same time, the lower octave is always there and the higher octave sometimes cutts off.

          Let me know of this could help.

          Comment


          • #20
            Originally posted by J M Fahey View Post
            Adding a suspect: it might be not some hummy stage (or it could be *one* part but not the full story) but a grounding problem.
            Pulling Con1 not only removes preamp signal but also disconnects power amp gtround from preamp one.

            So I would pull Con1 but solder a wire copying what Con 1 ground contact + wire do.
            Or IF you have the means (single wire + proper connectors + crimping tool or long nose pliers) build a new "Con 1 wire" which just connects grounds as it does.

            Important: join same pads at the connectors base as would be joined by Con 1 , we want to reproduce the exact same path, in grounding problems grounding " here" or "just 1 inch away" is NOT the same.
            All right. I looked the schematics and I saw the ground cable between the two PCBs goes from pin 1 of the CON1 of the preamp PCB to the pin 1 of the CON1 the power PCB so I placed a cable with alligator clips between those pins, I covered the next pins with tape to avoid the alligator clip to touch the other pins just in case.

            With this done, I read the AC voltage across the speaker with the tube in place= 0.003 on the overdrive channel and reads the same on the clean channel. Also the hum remains the same either if is connected this cable or not.

            I left the ground cable connected between the PCBs and disconnect the main ground wire which is connected from power PCB to the chassis through a green wire. I turned the amp on and touched this cable end on different parts of the chassis. The hum doesn't change if I touch the chassis here or 1 inch away. Let me know if I did this right LOL

            Thanks

            Comment


            • #21
              Well that wasn't quite what I intended you to do but it's an interesting result nevertheless. It would seem that, since the signal path is shorted out, that some part of the current in the tube is apparently flowing along the ground which has resistance and leads to the hum. I assume that the hum is increasing as the tube heats up.

              The dimension control will increase the bass and therefore the hum. I'd consider than normal.

              Hmmm... how did 'clean the multipin connectors' get translated into 'clean the switches'? The reason for asking you so do that is because of the grounds in each connector. A bad contact means high resistance and that it turn means a bigger signal from and hum currents.

              There might be bad ground solder joint on the preamp board that is associated with tube currents.

              You should probably check the ACV on the supply to tube at the junction of R22 to R19 just to eliminate that although I don't really think it's the cause.

              I just read your result on power amp ground test. The eliminates that ground.
              Experience is something you get, just after you really needed it.

              Comment


              • #22
                Quote:" With this done, I read the AC voltage across the speaker with the tube in place= 0.003"

                Just my 2 cents worth: 3 mv's ac on the speaker to me is outstanding!

                It's not hum, it's normal. It's a tube amp.

                I don't fret until I see 9 mv's or so.

                Comment


                • #23
                  Originally posted by Jazz P Bass View Post
                  Quote:" With this done, I read the AC voltage across the speaker with the tube in place= 0.003"

                  Just my 2 cents worth: 3 mv's ac on the speaker to me is outstanding!

                  It's not hum, it's normal. It's a tube amp.

                  I don't fret until I see 9 mv's or so.
                  Except he's got 250mV
                  Experience is something you get, just after you really needed it.

                  Comment


                  • #24
                    OOPS.

                    That is cause for concern.
                    Yup.

                    Comment


                    • #25
                      Originally posted by nickb View Post
                      Well that wasn't quite what I intended you to do but it's an interesting result nevertheless. It would seem that, since the signal path is shorted out, that some part of the current in the tube is apparently flowing along the ground which has resistance and leads to the hum. I assume that the hum is increasing as the tube heats up.

                      The dimension control will increase the bass and therefore the hum. I'd consider than normal.

                      Hmmm... how did 'clean the multipin connectors' get translated into 'clean the switches'? The reason for asking you so do that is because of the grounds in each connector. A bad contact means high resistance and that it turn means a bigger signal from and hum currents.

                      There might be bad ground solder joint on the preamp board that is associated with tube currents.

                      You should probably check the ACV on the supply to tube at the junction of R22 to R19 just to eliminate that although I don't really think it's the cause.

                      I just read your result on power amp ground test. The eliminates that ground.
                      Sorry, you mean to clean the tube socket? I will do it when I get home and check all the ground connections around the tube

                      Comment


                      • #26
                        Originally posted by avoverdu View Post
                        Sorry, you mean to clean the tube socket? I will do it when I get home and check all the ground connections around the tube
                        No,not the tube socket (but it wont do any harm) The single row multipin connectors like CON1 etc.
                        Experience is something you get, just after you really needed it.

                        Comment


                        • #27
                          Originally posted by nickb View Post
                          No,not the tube socket (but it wont do any harm) The single row multipin connectors like CON1 etc.
                          Got it, I will!!

                          Comment


                          • #28
                            Originally posted by avoverdu View Post
                            Sorry, you mean to clean the tube socket? I will do it when I get home and check all the ground connections around the tube
                            AC voltage on that R22 to ground= 000

                            AC voltaje between the leads of the filament heaters supply = 13.64

                            I also cleaned the tube socket, all the multipin connectors and re-solder all the components around the tube that are connected to ground. Hum is still there of course

                            Comment


                            • #29
                              It was worth a try. Have you checked C21 and C19? These to caps are critical in establishing the signal ground AC level. The easy was is just to tack a new cap across the cap to be tested.
                              Experience is something you get, just after you really needed it.

                              Comment


                              • #30
                                Originally posted by nickb View Post
                                It was worth a try. Have you checked C21 and C19? These to caps are critical in establishing the signal ground AC level. The easy was is just to tack a new cap across the cap to be tested.
                                C21 and C19 on the preamp PCB?? I checked those days ago with the multimeter and they seemed fine

                                Comment

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