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Thread: Leslie 760 distortion at low volume, both amplifiers

  1. #1
    Newbie ColinS's Avatar
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    Leslie 760 distortion at low volume, both amplifiers

    I've picked up a lot of good information from the forum, but this is my first question.

    I have a Leslie 760 connected to an L100 using an adapter which I built to the specification and schematic from CaptainFolback's website - with the exception that the AC power is connected to the Leslie by a separate cable and only audio/control circuits are connected through the 'Leslie' cable.

    All worked well last year, but now there is distortion from both the high and low frequency drivers, as if the input signal is too high, although the volume from the Leslie is modest - much lower than the volume from the L100 internal speaker.

    I've checked all wires and connections, replaced the speaker-organ cable and generally checked connectors and plugs throughout, but to no avail.

    I'm wondering if there may be a fault in the 760 and though I have a service manual, I do not have a list of test points and test voltages etc.

    Any suggestions would be welcome.

    Many thanks,

    Colin

  2. #2

    At one stage I burned out my 760, the ensuing thread was really long: http://www.organforum.com/forums/sho...ht=transformer
    but Forum Member Expiano provided loads of voltage references in the process.
    I think from memory I may have a copy of a digital model of the active crossover and maybe the power supply... not sure about the amps... which runs in free troubleshooting software called "LTSpice."
    Have a look through the thread for voltage readings first, it takes a little while to get used to the software.
    -1959 C3 and PR40- -1964 Busillachio Harmonium- -1964 M101- -1967ish Leslie 122-
    -1975 T500 (modded..chopped, and reassembled!)- -DIY 760 FrankenLeslie/rat hideout-

    -1980 Electrokey Electric Piano- -Yamaha electric Harmonium (early 80's?)- -1990 Jansen GMF150 amp-
    -1992 Korg 01W/fd- -1992 G&L S-500 geetar.

  3. #3
    Senior Member jimmywilliams's Avatar
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    If nothing has changed on the input side of things ... Low volume and distortion together can be indicative of sagging voltages which would indicate filter capacitors getting weak in the power supply piece - unless the rare chance that you fried both the upper and lower speakers. Did you try running the organ into a different Leslie, or try ringing a different instrument into the 760? That would help determine whether you have a problem external to the Leslie.
    Jimmy Williams
    Hobbyist (organist/technician)
    Gulbransen Model D with Leslie 204
    Farfisa Compact / Leslie 860 and Combo Preamp III / Hammond Porta-B

  4. #4

    Had this on my 760. Clean out the master volume on the crossover with deoxit. Secondly I had intermittent crackle if I tapped the lower back panel. I ran a ground wire from one of the screws on the power chassis to each grounded screw on the amp boards. Works fine now and plays cleanly.

  5. #5
    Newbie ColinS's Avatar
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    Thank you all for advice.

    Yesterday morning I took the signal input from off the crossover and attached it to each power amp in turn - amps and speakers working fine.
    Then refitted all connectors in their normal positions and tried it to make sure all was reconnected - whole thing working as well as ever!!!!
    'Played', if I can call my efforts at generating melodious sounds, for an hour or two without no problems then the fault returned, just as a 'real organist' was due to arrive. I quickly pulled and replaced the same connectors (which are all good and tight) and it was well again.

    I will eventually try to tabulate the test voltages from the epic thread when Brandon burned out the 760, or print of a big schematic and note the voltages at the appropriate points. I can't really move other instruments or speakers into the fray as we're at my sister in law's home and she already finds a Hammond (even a small one) and a 760 enough to cope with.
    I guess the problem is possibly dust/oxidation, maybe a failing component or a faulty joint on the crossover board. We'll be in town today so I'll pick up some contact cleaner and remain optimistic.
    If that fails close examination, or replacement of the offending board next!

  6. #6

    As you're near the UK go for Servisol Switchlube. I think Maplin.com sell it. The Deoxit is the US brand and will take forever to arrive. I crimped all of the interboard connecters on mine so they were a tight fit. One other thing you can do is reflow the solder joints under the boards expecially around those pins and the wirewound white resistors. It may be as something heats up (and those resistors do) that a solder joint opens up.

  7. #7
    Junior Member jkleban's Avatar
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    Quote Originally Posted by tonewheel1966 View Post
    As you're near the UK go for Servisol Switchlube. I think Maplin.com sell it. The Deoxit is the US brand and will take forever to arrive. I crimped all of the interboard connecters on mine so they were a tight fit. One other thing you can do is reflow the solder joints under the boards expecially around those pins and the wirewound white resistors. It may be as something heats up (and those resistors do) that a solder joint opens up.
    I purchased a 760 about a year ago and had the same issue... lots of distortion at low level.... I tapped into the crossover and sent the signals to a guitar amp and NO DISTORTION... so, then I connected the output of a guitar amp to the the horns and no distortion... so, by matter of deduction, the AMP was FRIED....

    Now, the 760 is a tranny amp so I purchased a stereo power amp (100 watts) and just tapped into the cross over into the amp and then back to the speakers from the amp....

    It SOUNDS GREAT now.

    Jim
    The Lamb Laid Down on MIDI
    www.lldom.blogspot.com

  8. #8

    Quote Originally Posted by jkleban View Post
    Now, the 760 is a tranny amp so I purchased a stereo power amp (100 watts) and just tapped into the cross over into the amp and then back to the speakers from the amp....

    It SOUNDS GREAT now.

    Jim
    Lovely! I bet it's great for LP's too....
    -1959 C3 and PR40- -1964 Busillachio Harmonium- -1964 M101- -1967ish Leslie 122-
    -1975 T500 (modded..chopped, and reassembled!)- -DIY 760 FrankenLeslie/rat hideout-

    -1980 Electrokey Electric Piano- -Yamaha electric Harmonium (early 80's?)- -1990 Jansen GMF150 amp-
    -1992 Korg 01W/fd- -1992 G&L S-500 geetar.

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