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  • Univox U1011 Hum

    Hello to the Music Electronic Forum.

    Let the show begin...

    Ok, I bought a used amp with hum and I can't find the source.
    I figured I’d see if maybe you guys might be able to help locate what I'm missing.
    So, here’s some info and what I’ve done so far.

    The amp is an old Univox U1011.
    From using my scope it looks like a 120hz hum.

    So, the hum.

    1. With both volumes down turning the bass up brings in the hum.
    2. With the bass turned down turning up the channel one and/or two volumes produces no hum.
    3. With the bass turn up, and the hum humming, turning the channel one volume knob from 0-10 changes the hum from a lower pitched hum to a higher pitched hum. Note that the channel two volume is at zero.
    4. With the bass turn up, and the hum humming, turning the channel two volume knob from 0-10 does nothing to the hum until you hit 10 and then the hum goes away.
    5. With the bass up, channel two volume at 10, turning the channel one volume brings hum back slowly and at 10 gets pretty loud again. Funny thing. To me, this hum sounds likes it’s in between the lower and higher hum spoken of earlier (#3).
    6. With the bass up, channel one volume at 10, turning the channel two volume has the same effect as in #4, only with the volume control switched.

    So, what have I tried so far?

    If you remove V3 (the 2nd gain stage) the hum goes away.
    If you remove the phase inverter the hum goes away.

    All tubes swapped out – no change. The tubes I swapped out for the power tubes were a used set and probably not a great matched quad, however, with that said the tube swaps made no change.
    (holding off on buying a new quad of 6l6gc’s for the moment)

    I’ve checked the board, pots, etc for bad ground connections, etc – no change.

    I’ve moved wires around, etc – no change.

    I’ve even effectively removed the reverb and tremolo out of circuit by removing the tubes and disconnecting wires from the board, etc – no change.

    I disconnected the rectifier diodes and substituted another rectifier set up – no change.

    The stock amp is essentially using a bus ground (via the circuit board) starting at the filter caps and terminating at the input jacks.
    I figured I’d try isolating the totem pole stack and power tube grounds from the board, leaving only the three 22uf caps going thru the board ground – no change.

    Which reminds me, this amp used two 2x47uf caps for the totem pole stack and one 3x20uf cap.
    They’ve all been replaced.
    I’ve double checked these for wiring issues, etc, and all seems fine.

    Actually, all the electrolytic caps have been replaced.

    Now, in order to isolate the bias supply circuit from the buss ground on the circuit board I’d have to cut the ground trace on the board.
    Essentially I'd just cut the ground trace in one spot and provide a ground for that section.
    To reverse it I'd just use a short piece of wire or flat copper and solder it over the broke trace, so no biggy really.
    (holding off on doing that for now)

    The switching/shunt input jacks and grounding have been double checked.

    I’ve put the chassis top/cover on – no change.

    My other amps have no hum issues plugged into the same receptacle and sitting in the same location.

    I cleaned the pots, checked their values, etc – all seems fine.

    And, I may have missed something I did and forgot to mention.

    Oh, the power transformer has a small amount of mechanical buzz.
    Both the PT and OT run without getting overly hot. I’ve also checked all the transformer windings to make sure there weren’t any shorts, etc.

    I suppose I could have got a bad 'new' filter cap, I haven't tried subbing in a different caps to double check the new caps yet.

    Hope this wasn’t too long winded, but I figured I might as well try to answer some questions I might get ahead of time.

    I've attache a schematic: Univox_U-1011_LEAD_AMP.pdf





































  • #2
    With all the caveats about safety, have you measured the plate voltages of the preamp tubes for AC noise? Actually I would start measuring at B2. When you say "totem pole stack" is that at B1 and B2 on the schematic? Are the 47uF caps in place of the 100uF caps on the schematic?

    Comment


    • #3
      Hello glebert, thanks for chiming in.

      Yes, B1 and B2 totem pole stack. I guess I should have stated it's a voltage doubler setup though.
      When I got the amp it had two 2x47uf caps, each wired in parallel.
      They have been replaced with two 2x50uf caps - each 2x50uf cap wired in parallel.

      Measure the plates of the preamps for AC noise?
      Like 60hz AC or 120hz pulsating DC or ?
      No I haven't done that, but I'm not quite sure I know what mean for sure though either.
      How would I go about doing what you asking and I'll give it try.

      Comment


      • #4
        I'm not clear what you have there. You say 2x50uF in parallel. Is that a sub for one of the 100uF caps? And if so, what of the other 100uF? It might help to draw on the schematic what you actually have going on and upload it. Also, if you replaced the 47uF because they were bad or old- fine. If you replaced them just for a couple extra uF, that's not necessary.
        "Yeah, well, you know, that's just, like, your opinion, man."

        Comment


        • #5
          Originally posted by tubemonster View Post
          Hello glebert, thanks for chiming in.

          Yes, B1 and B2 totem pole stack. I guess I should have stated it's a voltage doubler setup though.
          When I got the amp it had two 2x47uf caps, each wired in parallel.
          They have been replaced with two 2x50uf caps - each 2x50uf cap wired in parallel.

          Measure the plates of the preamps for AC noise?
          Like 60hz AC or 120hz pulsating DC or ?
          No I haven't done that, but I'm not quite sure I know what mean for sure though either.
          How would I go about doing what you asking and I'll give it try.
          Assuming you have a DMM, it should be able to measure smallish AC noise like 60 or 120 Hz and ignore the DC component. Set the voltage to AC volts, probe the B2, B3, etc. voltages relative to ground. With no input there should be no AC voltage (ideally). It gets a little weird because sometimes by probing your meter leads can act as antennas and couple noise onto the node you are probing. If there is AC on the preamp plates it is going to end up going out to the speaker because it is in the audio frequency range.

          Comment


          • #6
            No schematic needed - the caps are like these: https://www.tubesandmore.com/product...f-electrolytic

            Each multi-section/can-type cap is wired in parallel to double their value.
            (50uf+50uf making 100uf)

            The third multi-section/can-type cap contained three 20uf caps. I couldn't find one with three so I picked up one with four and just used three.
            Although I may end up using the extra 20uf section just after the choke.
            I actually tried that too, but it didn't help the hum either.

            Comment


            • #7
              Originally posted by glebert View Post

              Assuming you have a DMM, it should be able to measure smallish AC noise like 60 or 120 Hz and ignore the DC component. Set the voltage to AC volts, probe the B2, B3, etc. voltages relative to ground. With no input there should be no AC voltage (ideally). It gets a little weird because sometimes by probing your meter leads can act as antennas and couple noise onto the node you are probing. If there is AC on the preamp plates it is going to end up going out to the speaker because it is in the audio frequency range.
              Got a couple few DMM's. I'll give that a try tomorrow - never done it before (on purpose anyways).
              Now I'm curious. Kind of like the reverse of checking for leaky dc.

              Comment


              • #8
                Originally posted by tubemonster View Post
                ......Each multi-section/can-type cap is wired in parallel to double their value.
                (50uf+50uf making 100uf)............
                Yes. I got that, but there are two 100uF caps in that circuit. So, you have two of the dual 50uF can caps?

                "Yeah, well, you know, that's just, like, your opinion, man."

                Comment


                • #9
                  So 120Hz means power supply ripple. But that doesn't necessarily mean the caps are bad. Certainly grounding issues can arise. But if you think caps, instead of just swapping the caps and hoping, get out the scope and look at the power rails. How much ripple is present? At the OT CT I expect a few volts of ripple, but by the screens hardly any. Is that your case?

                  Unless ALL of this model has this hum, I don't tend to think re-engineering the connections is needed.

                  Is V3 the one with the two sides wired in parallel? If pulling that stops the hum, then everything after it is innocent.

                  The bias supply is half wave, so any hum it introduces will be 60Hz.

                  SO pulling the input 12AX7 has no effect?
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment


                  • #10
                    Originally posted by The Dude View Post

                    Yes. I got that, but there are two 100uF caps in that circuit. So, you have two of the dual 50uF can caps?
                    Ah, sorry about that. Yepper - got two.

                    Comment


                    • #11
                      Originally posted by Enzo View Post
                      "So 120Hz means power supply ripple. But that doesn't necessarily mean the caps are bad. Certainly grounding issues can arise. But if you think caps, instead of just swapping the caps and hoping, get out the scope and look at the power rails. How much ripple is present? At the OT CT I expect a few volts of ripple, but by the screens hardly any. Is that your case?"
                      I have not checked/measured the ripple on the power rail yet. I will set up the scope and check that out.

                      Originally posted by Enzo View Post
                      "Unless ALL of this model has this hum, I don't tend to think re-engineering the connections is needed."
                      I am unsure if the hum is inherent to this amp or not, it's the first I've owned or have worked on.

                      Originally posted by Enzo View Post
                      "SO pulling the input 12AX7 has no effect?"
                      Pulling the the first 12ax7 input tube has no effect on the hum at all.



                      Comment


                      • #12
                        Originally posted by Enzo View Post
                        The bias supply is half wave, so any hum it introduces will be 60Hz.
                        Yes, this it true and I do appreciate the heads up reminder.
                        Mainly I was thinking if I ended up rewiring the ground setup that I'd pull the bias supply off the board too.
                        But, definitely looking for input on this too, know what I mean.
                        I'd rather not change anymore than I have too, but...
                        I'll change as much as needed to get rid of the hum if the hum can be got rid of.

                        Comment


                        • #13
                          If you are going to use a scope to probe rails make sure your probes and scope are rated for the DC voltages you are probing. That 625+V feed to the power tubes could blow some stuff for sure.

                          Comment


                          • #14
                            Originally posted by glebert View Post
                            If you are going to use a scope to probe rails make sure your probes and scope are rated for the DC voltages you are probing. That 625+V feed to the power tubes could blow some stuff for sure.
                            Ya, buddy. They've got some juice on these 6l6gc's for sure.

                            But, from this thread I've already got a couple more things to look into.
                            It will probably take me a couple days to do it, here and there type thing.
                            But it's all most appreciated.

                            Comment


                            • #15
                              I'm still planning on using my oscilloscope, but for now...

                              Audio Cyclopedia by Howard M. Tremaine

                              Ripple May Be Calculated:

                              Percent ripple voltage = ac/dc * 100
                              where,
                              ac volts is the measured ripple voltage,
                              dc volts is the measured output voltage.

                              The amount of ripple voltage in decibels below the maximum voltage is:
                              20Log10(dc/ac)
                              where,
                              dc voltage is the maximum voltage under normal load conditions,
                              ac voltage is the ripple voltage under the same conditions.

                              It's my understanding the tests for ripple are done at Max or the expected normal operating conditions.
                              I did this under idle conditions, because I'm getting hum at idle and didn't th1ink it was necessary to bring it up to max current load (I could be wrong though).

                              I used my multi-meter in DC for DC.
                              For AC I used the same multi-meter with a .1uf cap between my probe and hot connections.
                              I got the following:

                              Just Before the Choke:

                              DC = 330.8 , AC = 3.269

                              % = ac/dc * 100 = .99
                              db = 20log10(dc/ac) = 40.10

                              Just After the Choke (1st 22uf):

                              DC = 329.7 , AC = 0.12

                              % = ac/dc * 100 = .03
                              db = 20log10(dc/ac) = 68.78

                              2nd 22uF:

                              DC = 315 , AC = .013 to .04 (approx.)

                              % = ac/dc * 100 = .004
                              db = 20log10(dc/ac) = 87.69

                              % = ac/dc * 100 = .013
                              db = 20log10(dc/ac) = 77.93

                              3rd 22uf:

                              DC = 279.7 , AC = .004 to .02 (approx.)

                              % = ac/dc * 100 = .0014
                              db = 20log10(dc/ac) = 96.89

                              % = ac/dc * 100 = .0072
                              db = 20log10(dc/ac) = 82.91

                              If this is the incorrect way - please let me know and correct me.
                              I'm doing my best to get better at this electronics stuff, but it's a hobby, not my day job, know what I mean.

                              Hopefully I at least get an E for effort

                              I'll be messing around doing this with the scope next.
                              To be honest, up to this point any amps I've built or repaired that have had hum issues has been pretty much the status quo.

                              Comment

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