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Valvestate VS265 Hum, RF, Voltages

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  • #16
    Originally posted by Jon Snell View Post
    Don't allow yourself to get confused between DC and Hum. DC is not hum!

    By the way, just to be clear, when G1 suggested checking for DC on opamp outputs I don't think it was in the context of tracking down hum, but rather trying to account for the discrepancy in voltages at the voltage regulator inputs as initially described thusly:

    "On the input of REG1 and REG2 I would have expected an approximate mirror image as well but in fact REG1 is +21V and REG2 is -28V. Meanwhile on the other side of R21 and R20 I have +40V and -40V, respectively."

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    • #17
      Try and deal with one issue at a time. It will save confusin.
      The hum issue should be first then the noise problem.
      Either way they could both be related and turn out to be something really simple ... it normally does!
      Keep a straight head and take frequent breaks to allow you time to digest what you are finding, stage by stage, piece by piece and you'll get there.
      Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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      • #18
        Originally posted by Jon Snell View Post
        Try and deal with one issue at a time. It will save confusin.
        The hum issue should be first then the noise problem.
        There is an issue with the low voltage power supply. My feeling is to solve that first as it could also solve the hum.
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #19
          Originally posted by g1 View Post
          There is an issue with the low voltage power supply. My feeling is to solve that first as it could also solve the hum.
          As it happens I isolated the power/power amp board and that is where the background hum originates. I swapped in new filter caps but no difference and the original caps measure up fine on the ohmeter. I also determined that most of that hum is coming from right power amp and the left one is relatively quiet (presumably normal/expected amount of hum). (I know, I should have checked that before swapping the caps, but I had it in my head that I had already determined that the problem was in both sides. In hindsight it was likely the preamp noise issue where I made that determination.) Comparing the two amps should help me narrow down the problem.

          As for the preamp problem, I'll revise my description slightly. I talked about how turning down the volume to zero introduced a loud hum and lots of the local radio station. I suggested that if the volume was not on zero there was no noise unless the reverb pot was turned up. After plugging in a guitar cable again I noticed that that wasn't 100% accurate. The same problem exists even with the volume pot turned higher than zero i.e. the cable is acting like an antenna and moving it around affects the amount of hum and RF, but it is the case that turning the volume pot to zero increases that noise greatly. Do you think it is possible that this is related to the low voltage power supply?

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          • #20
            OK, I think I'm done with the power amp, as in, I'm not even sure it's worth troubleshooting beyond what I've done. Neither amp (L or R) on their own has a particularly loud hum, it's just that the left amp has a hum that's roughly twice as loud as the other side, and when they are combined it's noticeable enough to sound like it might be a problem. But is it more than usual for this model? I really don't know. I'd rather leave well enough alone, at least for now, and tackle the preamp.

            I was hoping one of you might be able to comment on this paragraph that I wrote in post #19?

            As for the preamp problem, I'll revise my description slightly. I talked about how turning down the volume to zero introduced a loud hum and lots of the local radio station. I suggested that if the volume was not on zero there was no noise unless the reverb pot was turned up. After plugging in a guitar cable again I noticed that that wasn't 100% accurate. The same problem exists even with the volume pot turned higher than zero i.e. the cable is acting like an antenna and moving it around affects the amount of hum and RF, but it is the case that turning the volume pot to zero increases that noise greatly. Do you think it is possible that this is related to the low voltage power supply?

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            • #21
              We are talking about the clean channel volume control? Which VR number is that on the schematic (they are not labelled)? This is part of the problem with Marshall schematics and why not many people get involved with these threads. You need the schematic but you also need the unit in your hands to figure out the schematic.
              So it takes a lot more legwork to communicate and that is before we even get to the issue of their connectors. Sorry to rant but I think a lot of people wonder why they don't get many comments in these marshall threads and they should know it is not their fault.

              Anyway, I do think that any fault in the preamp is probably responsible for the imbalance in the voltage feeding the +/-12V regulators. One technique is to desolder the supply pins on the IC's in suspect areas and see if the supply issues straighten out. In this case I would be looking at IC's around the clean ch. input and volume control.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

              Comment


              • #22
                Originally posted by g1 View Post
                We are talking about the clean channel volume control? Which VR number is that on the schematic (they are not labelled)? This is part of the problem with Marshall schematics and why not many people get involved with these threads. You need the schematic but you also need the unit in your hands to figure out the schematic.
                So it takes a lot more legwork to communicate and that is before we even get to the issue of their connectors. Sorry to rant but I think a lot of people wonder why they don't get many comments in these marshall threads and they should know it is not their fault.

                Anyway, I do think that any fault in the preamp is probably responsible for the imbalance in the voltage feeding the +/-12V regulators. One technique is to desolder the supply pins on the IC's in suspect areas and see if the supply issues straighten out. In this case I would be looking at IC's around the clean ch. input and volume control.
                Yes, it's most noticeable with the clean channel volume control. The same thing happens on the drive channels but to a lesser degree. I've been focusing on the clean channel.

                The pots are:

                Preamp board
                -------------------
                VR1 -> clean vol
                VR2 -> bass
                VR3 -> mid
                VR4 -> treble
                VR5 -> OD1 gain
                VR6 -> OD1 vol
                VR7 -> OD2 gain
                VR8 -> contour
                VR9 -> OD2 vol
                VR10 -> bass
                VR11 -> mid
                VR12 -> treble

                Daughter board
                ---------------------
                VR1 -> FX Mix
                VR2 -> clean reverb
                VR3 -> OD reverb
                VR4 -> Speed
                VR5 -> Depth

                With both IC2 and IC4 pulled the voltage/current anomaly still remains and the RF is still present in the reverb circuit (RCA cable and reverb tank are out of the equation). As for the volume control, if the input cable is positioned just so it's like there is no issue there at all, i.e. there is no noise even when turning the control to zero, which otherwise introduces a loud hum. It may be that this amp is particularly susceptible to RF and that I'm in a hotspot. When I get a chance in a day or tow I may try it at a different location. In the case of IC2 and IC4 I pulled them out and put in sockets for ease of troubleshooting. When you say "desolder the supply pins on the IC's" are you making a distinction between disconnecting only the supply pins and removing the whole chip as I have done, or does it amount to the same thing for what you are suggesting? I would think yes, same thing, but just checking. Also, since there is still a current imbalance with IC2 and IC4 out of circuit, what do you reckon the next one to try would be? IC3?
                Last edited by bobloblaws; Yesterday, 07:39 AM.

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                • #23
                  Well, I went and pulled all the IC's (including IC1 and IC5) from the preamp board, one at a time, and in each case there was still roughly a 28mA difference in voltage drop across R20 and R21 (daughter board disconnected). I also swapped around C9 and C10 and R20 and R21 for fun. I did notice that with the 2 OD switches pushed in the difference is only 10mA. Are we chasing a phantom here?

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