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Thread: Leslie 122 - Distortion and Low Volume

  1. #11
    p Piano Jyvoipabo's Avatar
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    Quote 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

  2. #12
    fff Fortississimo David Anderson's Avatar
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    Quote 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.

  3. #13
    fff Fortississimo David Anderson's Avatar
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    Quote 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.

  4. #14
    pp Pianissimo weslan's Avatar
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    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?

  5. #15
    fff Fortississimo geoelectro's Avatar
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    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

  6. #16
    fff Fortississimo David Anderson's Avatar
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    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.

  7. #17
    p Piano geoffbrown's Avatar
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    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 !

  8. #18
    fff Fortississimo David Anderson's Avatar
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    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.

  9. #19
    p Piano Jyvoipabo's Avatar
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    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 at 06:48 PM.

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