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Pro Jr Point-to-point hum issue

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  • #16
    If I am close (with my hand) to the pots with volume open, then it is giving much more hum.
    So please make sure that the pot cases connect to the chassis.
    - Own Opinions Only -

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    • #17
      Check heater balancing resistors (47R) and connections. Make sure that heater voltage is symmetrical wrt ground.

      It would really be helpful to know if your hum is 50Hz OR 100Hz.
      Last edited by Helmholtz; 03-30-2020, 04:57 PM.
      - Own Opinions Only -

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      • #18
        Originally posted by Helmholtz View Post
        Check heater balancing resistors (47R) and connections. Make sure that heater voltage is symmetrical wrt ground.

        It would really be helpful to know if your hum is 50Hz OR 100Hz.
        Click image for larger version

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        • #19
          Originally posted by klooon View Post
          [ATTACH=CONFIG]57604[/ATTACH]
          As the hum signal harmonics are multiples of 50Hz, your problem doesn't seem to be related to power supply ripple.


          If your chassis is actually coated, make sure that all chassis connections make good contact.
          - Own Opinions Only -

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          • #20
            Is that one of the Fenders that comes with ashielded back panel & is known to make lots of noise if the panel is off but it goes away with the panel on, normally?

            I didn't see you mention if it hums in the cab with the panel on & I know it's a long shot, but can't hurt to try... Given there's a precedent for that. Otherwise I'm iut of ideas.

            Justin
            "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
            "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
            "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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            • #21
              It seems to work fine now! I rewired and rewired it, put shielded wire to both grids of V1. Shaved some coating off. Put the 100R at the tube socket with shielded wire and put underneath the board.

              Was still a bit picking up noise, and the put the cover on like suggested above... And then: fine!!

              Thanks so much for the input!

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              • #22
                A couple of suggestions.

                Does the setting of the volume control affect the hum level?

                I don't see a connection between signal ground and the chassis. On the original layout this was done at the input and speaker jacks. It looks like you have insulated jacks in both locations so try connecting the chassis to signal ground and see if it helps.

                I would be inclined to use a screened cable from the volume control to V1. I notice you did make some improvements to that wire. Connect the shield to signal ground at the volume pot only with the 100R at the V1 end ( as you now have).

                Your spectrum plot does have a fundamental of 50Hz but the 100Hz line is higher. Therefore I think you have both 50Hz (heater or other 50Hz AC source) and power supply 100Hz issues.

                PS: Oh I see kloom posted while I was writing this...oh well!
                Experience is something you get, just after you really needed it.

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                • #23
                  I am still not happy. I moved around the wires again and it still gives too much hum to my taste.

                  Originally posted by nickb View Post
                  A couple of suggestions.

                  Does the setting of the volume control affect the hum level?

                  Yes. Very clearly. Volume at 1 and the hum is very bad. Turning the volume past 9 and it is gone.


                  Originally posted by nickb View Post
                  I don't see a connection between signal ground and the chassis. On the original layout this was done at the input and speaker jacks. It looks like you have insulated jacks in both locations so try connecting the chassis to signal ground and see if it helps.
                  I have one central ground point - close to the input, so it should ground "everything". Also what you mean by the signal ground?? Please, could you explain?


                  Originally posted by nickb View Post
                  I would be inclined to use a screened cable from the volume control to V1. I notice you did make some improvements to that wire. Connect the shield to signal ground at the volume pot only with the 100R at the V1 end ( as you now have).
                  Yes, I have improved this now. Still moving this shielded wire is changing the "sensitivity" to noise a bit...

                  Originally posted by nickb View Post
                  Your spectrum plot does have a fundamental of 50Hz but the 100Hz line is higher. Therefore I think you have both 50Hz (heater or other 50Hz AC source) and power supply 100Hz issues.
                  And that's related to the above?!

                  mmm. what to do?

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                  • #24
                    What happens if you break the connection between the two 47 ohm resistors to ground off your heater wires (puesdo ground), and take that connection directly to chassis at/near the power xfmr mtg screw? I'd also try using a 100 ohm/2W or greater linear pot as hum balance in place of the two resistors, and tweak the balance to minimize hum. We see in your frequency spectrum plot the third harmonic (150Hz) is highest, while also seeing high power supply harmonics, as nickb pointed out.

                    In spite the problems encountered, I do love your rebuild form factor!

                    What are the two rear panel switches? One looks to be a power output select switch (15W/3W), but don't see what the adjacent switch is/does.
                    Last edited by nevetslab; 04-10-2020, 10:46 PM.
                    Logic is an organized way of going wrong with confidence

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                    • #25
                      Originally posted by klooon View Post
                      I am still not happy. I moved around the wires again and it still gives too much hum to my taste.

                      Yes. Very clearly. Volume at 1 and the hum is very bad. Turning the volume past 9 and it is gone.

                      I have one central ground point - close to the input, so it should ground "everything". Also what you mean by the signal ground?? Please, could you explain?

                      Yes, I have improved this now. Still moving this shielded wire is changing the "sensitivity" to noise a bit...

                      And that's related to the above?!

                      mmm. what to do?
                      OK. I see the chassis connection now. Now I wonder how I managed to miss it before!

                      Let's concentrate on the hum at volume on 1. I take it that "1" is zero volume and it's more of a buzz than hum. It might be that the wire from the common ground on the board to the volume pot is picking up noise. As a test disconnect the screened wire that goes to the wiper and connect it directly to the common ground. If the hum / buzz is gone or much reduced then try soldering pin 3 of the volume pot directly to the metal body instead of the wire that currently goes to pin 3 of the pot or use a short wire if need be and put the screened wire back on the volume pot.

                      And do try what nevestlab suggested. Right now the mid point of the two 47 ohm resistors is connected to a rather noisy place and could inject noise everywhere via the heater wiring.
                      Last edited by Boss; 04-12-2020, 08:40 PM. Reason: broken quote repair
                      Experience is something you get, just after you really needed it.

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                      • #26
                        Thanks for your help!

                        Originally posted by nevetslab View Post
                        What happens if you break the connection between the two 47 ohm resistors to ground off your heater wires (puesdo ground), and take that connection directly to chassis at/near the power xfmr mtg screw?
                        I did this and it does not change. The hums is just as much.
                        Originally posted by nevetslab View Post
                        I'd also try using a 100 ohm/2W or greater linear pot as hum balance in place of the two resistors, and tweak the balance to minimize hum. We see in your frequency spectrum plot the third harmonic (150Hz) is highest, while also seeing high power supply harmonics, as nickb pointed out.
                        I tried this. The low frequency hum is not changed (at minimum volume), higher frequency buzz is less or more depending on the setting of the pot.


                        Originally posted by nevetslab View Post
                        In spite the problems encountered, I do love your rebuild form factor!
                        Thanks!

                        Originally posted by nevetslab View Post
                        What are the two rear panel switches? One looks to be a power output select switch (15W/3W), but don't see what the adjacent switch is/does.
                        Yes, one is the attenuator and the other one is a negative feedback switch (on/off)

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                        • #27
                          Originally posted by klooon View Post
                          Let's concentrate on the hum at volume on 1. I take it that "1" is zero volume and it's more of a buzz than hum.
                          Yes, volume "1" is zero volume. No it's more a humming noise. Higher frequency buzz is also there at higher volume settings and changes with moving the grid wire to the volume pot around.

                          Originally posted by nickb View Post
                          It might be that the wire from the common ground on the board to the volume pot is picking up noise. As a test disconnect the screened wire that goes to the wiper and connect it directly to the common ground. If the hum / buzz is gone or much reduced then try soldering pin 3 of the volume pot directly to the metal body instead of the wire that currently goes to pin 3 of the pot or use a short wire if need be and put the screened wire back on the volume pot.
                          I just tried this and no changes to the hum and buzz.

                          Originally posted by nickb View Post
                          And do try what nevestlab suggested. Right now the mid point of the two 47 ohm resistors is connected to a rather noisy place and could inject noise everywhere via the heater wiring.
                          I just tried this and does not change the hum.


                          What I just noticed, is that when I plug into a wall socket that has no ground, you feel the "electricity/voltage" on the chassis. Is this normal??? What could be the cause of this??

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                          • #28
                            Originally posted by klooon View Post
                            Yes, volume "1" is zero volume. No it's more a humming noise. Higher frequency buzz is also there at higher volume settings and changes with moving the grid wire to the volume pot around.


                            I just tried this and no changes to the hum and buzz.


                            I just tried this and does not change the hum.


                            What I just noticed, is that when I plug into a wall socket that has no ground, you feel the "electricity/voltage" on the chassis. Is this normal??? What could be the cause of this??
                            Turn the plug around. scratch that, I'd suggest you have a serious wiring error and need to find this with the amp unplugged. What you describe is common with a transformerless amp or a amp with a failed deathcap. Neither of which I see in your layout. Stop! and find the issue before continuing on
                            nosaj
                            Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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                            • #29
                              Originally posted by klooon View Post
                              Yes, volume "1" is zero volume. No it's more a humming noise. Higher frequency buzz is also there at higher volume settings and changes with moving the grid wire to the volume pot around.


                              I just tried this and no changes to the hum and buzz.


                              I just tried this and does not change the hum.


                              What I just noticed, is that when I plug into a wall socket that has no ground, you feel the "electricity/voltage" on the chassis. Is this normal??? What could be the cause of this??

                              First, never, ever use an ungrounded socket. If it's just a slight sensation likely it's just stray capacitance between the windings. Using an ohm meter to check that the earth from the plug has a low resistance to the chassis. Also check that live goes to the power switch and fuse. Don't just look at wires. Assume that the wire colours can't be trusted so measure from the plug. Check the neutral does not connect to the chassis

                              You hum issue is elusive. Disconnect the grid from the screened wire and 100 ohm grid stopper that goes to the volume control wiper and short the grid to the cathode using as direct a connection as you can manage. What happens to the hum?
                              Experience is something you get, just after you really needed it.

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                              • #30
                                Originally posted by nosaj View Post
                                Turn the plug around. scratch that, I'd suggest you have a serious wiring error and need to find this with the amp unplugged. What you describe is common with a transformerless amp or a amp with a failed deathcap. Neither of which I see in your layout. Stop! and find the issue before continuing on
                                nosaj
                                Thanks. I will go more in detail through the layout and amp!

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