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Leslie 720/540 + 20 Amp fuse + lightning = my new project!

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  • Leslie 720/540 + 20 Amp fuse + lightning = my new project!

    Last week I was fortunate to acquire a pair of "dead" model 720 and 540 speakers. On checking out the 1st 720, it would not turn on. The small 'control' transformer in the power supply had an 'open' primary winding and also one of the pcb tracks that fed the primary with line voltage was simply gone, evaporated without a trace! I swapped the power supply with one from a 540 and the amps came to life. 'Fast' speed worked OK but 'low' speed would kill the audio and the motors would not run. The little diode across the low speed reed relay coil was shorted and when low speed was commanded, the control power would be overloaded, causing the amp power relay to drop out. Replaced the diode and #1 was fully back in business 8)

    720 #2 though, a little more interesting! Opening this one up, one of the two amplifier boards had been almost completely *incinerated*! I swapped the board and output transistors with one from a 540. Most of the speaker wiring in the lower part of the cabinet was burned up but the motors and their wiring were OK. I reworked the speaker wiring. The power supply worked OK and had just 'browning' of the pcb under the large power resistor. Presumably because the supply was grossly overloaded while the amp board destroyed itself. Lo and behold, we have both sound and two speed motor operation, but lacking low end from the bass rotor. The 10" woofer voice coil was sent into another dimension when the exploding amplifier applied DC voltage to it. My favorite local speaker guy (Speakerworks, here in Tulsa) should have my new driver this week.

    #2's forensic analysis: Lightning induced voltage came in (or left) through the 11-pin cable getting into the input of the amp board, shorting the driver transistors. This applied 30 vdc to the driver transformer primary causing it to overheat and catch fire. This in turn caused the output transistors to fail and put DC on the woofer, blowing it. Primary contributing factor toward all this damage? The 2.5 amp amplifier power fuse had been replaced with a *20 amp* fuse (which never did open up, by the way)!

    Epilogue: These speakers being "2-channel", I followed a recommendation on the forum to use an active crossover to split my M3 and/or E100 audio between the rotary and non-rotary channels. I cobbled up a version of the one by "Electronic Instrument Services", modified to make use of the +29 vdc supply available on the Leslie's 11-pin connector. *Very nice*, the bass from the Leslie's 15 just blows the E100's original bass away! The organ's woofer simply cant speak well because the organ's open back doesn't function as a proper speaker enclosure. This is my first experience with a Leslie speaker, and now I really get what everyone means! I realize now a Hammond without one is like a '69 Dodge Charger with a 2-bbl carb and single exhaust. With one, adding dual 4-bbl and dual exhaust!

    Thanks to all here for all the great info about the features and application of these speakers!
    Tom in Tulsa

    Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720
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