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Leslie 122 - Distortion and Low Volume

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  • Leslie 122 - Distortion and Low Volume

    Greetings!

    I'm trying to help a buddy get his 122 back in shape. The symptoms are distortion and generally lower than normal volume. When played very softly things seem ok, but as soon as I push some input from my Hammond it distorts badly.

    A video of the symptoms:
    https://www.youtube.com/watch?v=RMfs...94JFEw&index=1

    I've transplanted the amp into an otherwise 100% healthy 147 cabinet and it still exhibits the problems (thus ruling out speakers or any non-amplifier components). So far the only thing I've tried is swapping all tubes with known healthy ones. No difference, unfortunately. The amp has clearly been worked on in the past; there's a new relay and replaced capacitor can, as well as several resistors and other new-ish caps under the hood, but I have no idea of its history.

    Pictures are linked below (sorry, I couldn't get them uploaded to the forum in anything but a tiny thumbnail). Anyone have any guesses where to start measuring to narrow things down...or even better...some likely culprits? Appreciative of any leads.

    https://drive.google.com/open?id=1tR..._08MoNbqOUI1k5
    https://drive.google.com/open?id=1WD...BxBHNbQ5PIU0f2
    https://drive.google.com/open?id=16D...AITiu8BX1btfUj
    https://drive.google.com/open?id=1wN...iXI0J4qbo6yXAB
    https://drive.google.com/open?id=14L...ZPtRNWNG8QL8cB


    Best,
    Wes
    Attached Files
    Last edited by weslan; 07-10-2018, 05:52 PM. Reason: Fixing picture attachments.

  • #2
    Hopefully it's something else, but the last time I heard a sound like that, it was a bad output transformer. Again, that's just a general impression, and everything else should be checked first since it's difficult even for expert techs to diagnose a bad OT. It tends to be a diagnosis of exclusion. Time for a complete amp diagnostic. But it sounds like something is arcing as you increase volume. Could be a capacitor or bad connection, but could also be a shorted primary winding in the OT.

    If it is the OT (which, again, I'm not saying it is), ClassicTone makes an excellent replacement.

    It's also a little odd that someone paralleled that Sprague Atom with the can cap. Did the can cap have a bad section, or was someone trying to increase the capacitance of the first stage? Paralleling a good cap is the cheap & dirty fix when one section goes bad. Best practice is to replace the can when one section fails.
    I'm David. 'Dave' is someone else's name.

    Comment


    • #3
      Hello,

      I have exactely the same waiting for a new relay.

      It is very difficult to check a transformer. The easy way is to replace it and there is often no other issue.

      But, if you have not done any measurement, you must have to start there.

      1- Measure all the resistors for value in the +/- 10%.
      Conditions : Power Off. Dischage can capacitors. Desoldering is not necessary.

      2-Check Zero volts on grid of the output tubes : Voltage on each side of the 390k should be Zero. Check bias = 25 Volts (approx) on the 150Ohm/10W resistor.
      Conditions : Power On. No signal. Volume = 0. Tubes in place. Turn power off immediatly in case of incorrect values.

      3-Refer to schematics and check all other voltage.

      On the pics i see some remains of ancient repairs. It is clear this amp have an history.
      Don't worry. The amp that work badly have no history.
      But, you must have to check all the connections. Looking for a bad solder or a broken wire. Start with the amphenol plug.

      JP

      Comment


      • #4
        You're not trying to drive a 122 with a Hammond set up for a 147, are you?
        1963 C-3 147 Leslie
        1972 X-77GT 2 - 77P Leslie
        Kurzweil K 2000

        Comment


        • #5
          Hi Tim,

          Nope - I've installed both 147 and 122 jacks on my B3 and definitely know they don't play well together

          David and others: Thanks so much; I'll try to get some measurements of a few spots in the circuit and report back with my findings. Thanks for your tips.

          Best,
          Wes

          Comment


          • #6
            Greetings,

            I had a few minutes to take some measurements this afternoon. Obviously things are not well...

            Cathode resistor reads around 13v.
            Voltage off rectifier reads around 314v.
            Pin 5 on OC3 reads around 308v.
            Pin 3 of the 6550 closest to the transformers reads around 304v.
            Pin 3 of the other 6550 (closest to the 6 pin connector) reads 8v (not a typo).
            I then swapped the 6550s with known good ones and remeasured -- same thing.

            Thoughts/feelings? David, would these readings be consistent with a shorted OT? What about the filter capacitor section? That paralleled Sprague makes me suspicious. To answer your previous question, I'd never seen this amp before yesterday, so I have no idea why someone put that Sprague in there.


            Thanks as always.

            Best,
            Wes

            Comment


            • #7
              Maybe one of the 0.0047uF caps to ground from the 6550 plate (pin 3) is shorted. I would tend to think that that would make the fuse blow, but maybe not. You did check the fuse, right, to make sure it's not 20A or something?

              It's always possible that this 122 amp has multiple issues. Many do at this point.

              Have you measured the DCR of the OPT primary? You should have ~74Ω from center tap to one side and ~82Ω from center tap to the other.

              But if one side of the OT primary was open, you'd tend to see higher B+ voltages, not lower, unless there's also a power supply problem.
              I'm David. 'Dave' is someone else's name.

              Comment


              • #8
                Originally posted by weslan View Post
                Greetings,

                I had a few minutes to take some measurements this afternoon. Obviously things are not well...

                Cathode resistor reads around 13v.
                Voltage off rectifier reads around 314v.
                Pin 5 on OC3 reads around 308v.
                Pin 3 of the 6550 closest to the transformers reads around 304v.
                Pin 3 of the other 6550 (closest to the 6 pin connector) reads 8v (not a typo).
                I then swapped the 6550s with known good ones and remeasured -- same thing.

                Thoughts/feelings? David, would these readings be consistent with a shorted OT? What about the filter capacitor section? That paralleled Sprague makes me suspicious. To answer your previous question, I'd never seen this amp before yesterday, so I have no idea why someone put that Sprague in there.


                Thanks as always.

                Best,
                Wes
                The first stage of the filter is bad. This reduces the overall voltages. The extra cap is on the second stage so it doesn’t affect the first stage problem.

                Geo

                Comment


                • #9
                  Originally posted by geoelectro View Post
                  The first stage of the filter is bad. This reduces the overall voltages. The extra cap is on the second stage so it doesn’t affect the first stage problem.
                  Yes, I misread it in the photo. The extra cap looks like it's on the 10uF stage. A bad first stage explains the low voltages, but not the 8V on the 6550 plate.
                  I'm David. 'Dave' is someone else's name.

                  Comment


                  • #10
                    If the output transformer is suspected to be bad, then why can it not be tested good or bad with an insulation resistance meter, just like an electrician or electrical tech would do with a suspect electric motor!

                    Comment


                    • #11
                      Originally posted by geoffbrown View Post
                      If the output transformer is suspected to be bad, then why can it not be tested good or bad with an insulation resistance meter, just like an electrician or electrical tech would do with a suspect electric motor!
                      I have the same appreciation.
                      But it is not a short.
                      It is a cutted wire. This is why only one 6550 is working.
                      I think it is possible to repair (autopsy required).

                      But because the other possible issue is a bad tube socket, this diag needs confirmation : Disconnect transformer primary wires and check continuity.

                      JP

                      Comment


                      • #12
                        Originally posted by geoffbrown View Post
                        If the output transformer is suspected to be bad, then why can it not be tested good or bad with an insulation resistance meter, just like an electrician or electrical tech would do with a suspect electric motor!
                        I'm not aware of exactly how insulation resistance meters work, but inside output transformers, you have large AC voltage swings, and it can sometimes take a large swing to break through damaged insulation and create the arc between two windings.

                        I looked up insulation resistance meters and looked at the specs, and they are simply handheld high-value Ohmmeters that only apply the internal DC battery voltage. You could detect a short to ground with that, but not necessarily a short between windings that only appears under high voltage peaks.

                        In any event, weslan has apparently not yet checked the OT in this amp for continuity -- or hasn't reported what he's found.
                        I'm David. 'Dave' is someone else's name.

                        Comment


                        • #13
                          Originally posted by Jyvoipabo View Post
                          But because the other possible issue is a bad tube socket, this diag needs confirmation : Disconnect transformer primary wires and check continuity.
                          You don't need to desolder the primary wires to check them for continuity. I do this without removing any wires all the time.
                          I'm David. 'Dave' is someone else's name.

                          Comment


                          • #14
                            Hello! Thanks so much for all the replies.

                            A few things:

                            Just FYI, there's a 4 amp, 250v fuse in this thing that is not blowing in the current state.

                            I have Ω 81.4 DCR from center (OC3 pin 5) to the happy-ish 6550 (pin 3). It's open (OL) between center and the unhappy 6550 (pin 3). Thoughts on replacing the entire filter can and taking out that Sprague? Probably looking at a new OT as well given we are not getting resistance between the OT and that 8v 6550?

                            Possible sequence of events: Could that bad first stage of the filter eventually lead to problems with the OT?

                            Comment


                            • #15
                              I run across many amps with a bad first stage cap. It means the amp has been running on much lower voltage but for how long, who knows. In most cases once the filter is replaced and the voltages come back to normal the output tubes usually need replacing. I’ve long suspected that running them at the lower voltage damages them somehow. Now if a tube over conducts badly it “could” damage the tranny but usually the fuse would prevent that. Given the age of these amps anything is possible.

                              Geo

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