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Leslie 122 - Distortion and Low Volume

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  • #16
    I think the filter can and the problem with the OT are unrelated problems. Bad filter cans are common, and they usually don't short out; they simply vanish from the circuit like there's no capacitor there at all.

    The open primary winding could be due to a manufacturing defect, running the amp without a speaker load, or a catastrophic tube failure combined with the wrong fuse. (I found a bad OT in a 122 amp with a 20A fuse in it; the normal fuse would have prevented this damage.)

    If you're very lucky, you might be able to open up the original output transformer and find that it's a bad connection where the lead-out wire is attached to the winding, but I haven't gotten lucky that way. The good part is that, IMO, the Classic Tone replacement for Leslie amps actually sounds better than the originals. Take careful note of where the wire colors go before removing the original OT. This will help you on reinstallation.

    What weslan found is why I always do basic checks on the transformers on a Leslie amp I'm rebuilding before I get to work. You don't want to rebuild it and then find it's still not working or blowing fuses.

    Yes, you'll probably need a new filter can, and put a 1.5A or 2A Slo-Blo fuse in it.

    For Geo: For the 6550s, I don't know. I've seen amps running on low voltage where the tubes were fine. But if you have an amp that's been running with low voltage for a long time, chances are that it hasn't been serviced at all for a long time, too, so the tubes may just be worn out. They might have been worn out before the capacitor failed.
    I'm David. 'Dave' is someone else's name.

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    • #17
      The Insulation resistance tester commonly known as a "Megger" around the world is a handheld battery powered ohmmeter, but generates much higher voltages for testing purposes.

      I use a Kyoritsu 3132a at work which has working test voltages between 250v and 1000v some use much higher voltages.

      Every trade and profession has its own way of doing things,but it appears that people in the audio repair industry dont use Insulation Resistance Testers to prove transformers and chokes good or bad, but I can't see any reason not to.

      Of course the person operating such equipment needs to know what they are doing, as selecting too higher voltage will damage the component being tested !

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      • #18
        I'm aware of Meggers, and they have their uses, but applying a static DC voltage to an output transformer winding won't necessarily reveal a fault, the reason being that, as a result of the large AC voltage swings inside an output transformer, different points along the windings will actually be at different instantaneous voltage potentials. Charging the whole winding to a static DC potential would not reveal an arc that happens due to those AC voltage swings, so I think that there's a good reason that tube amp techs don't use them for this purpose. I don't think it's because they're ignorant of them.

        For example, you can have a bad output transformer that shows no DC faults with the amp at idle, even with >500VDC on the tube plates; the fault would only show up when you pass an AC signal through it, and the signal would have to be large enough to break through the insulation resistance and arc.

        I do know of tube amp techs who've used Meggers to test power transformers for leakage from the primary or secondary to the laminations and mounting flanges, but that's usually not what we're testing for in output transformers.
        I'm David. 'Dave' is someone else's name.

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        • #19
          Hello,

          Measured values of Output Transformer windings (primary) with an ohmeter are :
          Brown to CT(Red) = 72.2Ohm
          Blue to CT(Red) = 79.3 Ohm

          JP
          Last edited by Jyvoipabo; 07-13-2018, 11:48 AM.

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