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Leslie 720 speed control issue

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  • Leslie 720 speed control issue

    Hi. I have a Leslie 720 that is acting up. Basically, it wont switch to fast speed. Slow speed and stop is fine but when you switch to fast both rotors just stop. I have verified that all connections and switches are good. If I leave the leslie disconnected for a week then the fast will work for a while and then stops working. Seems like an issue in the circuit to me, like maybe a capacitor but theres only a single gumdrop cap on the speed circuit as far as I can see. I was hoping somebody has dealt with this before. Thanks

  • #2
    We have dealt with this before, but diagnosing solid-state Leslie switching circuit problems over the internet is frustrating unless dealing with someone with some skill in testing components. There are switching transistors, resistors, relays, Triacs, and diodes involved. An intermittent loss of function could indicate a semiconductor problem, but could also be a relay or even a bad solder joint. Semiconductors can work when cool, but fail when hot. The thing is, it's sort of beyond the scope of this forum to teach component diagnostics. You can imagine that after doing that a few times with new members, it's not much fun anymore.

    First, check very, very carefully for cracked solder joints, especially at the motor plug connectors.

    You might try spray cooling spray on a transistor or Triac to see if you get temporary restoration of function.

    Another strategy with some chance of success is simply shotgunning components that switch the fast motors until you replace the right one. Leslie used rather cheap components in building these so it would be hard not to make an improvement.
    I'm David. 'Dave' is someone else's name.

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    • #3
      Hello,

      In these machines there is a transistor circuit that engages brievly slow and fast at the same time when speed go from Fast to Slow. This is the brake.
      Look carefully if both engines are not engaged at the same time on Fast.
      Disconnect Slow motors and watch if Fast resurect.

      JP

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      • #4
        Originally posted by Jyvoipabo View Post
        In these machines there is a transistor circuit that engages brievly slow and fast at the same time when speed go from Fast to Slow. This is the brake.
        No, that's not how it works. When you switch from Fast to Off, there is a timing circuit that engages the Slow motors for a few seconds as a brake, with the length of the brake determined by that single capacitor. But again, this is only going Fast to Off. The Slow and Fast motors should never be powered at the same time. Going Fast to Slow is just like any other Leslie motor stack; the Slow motor turns on at the same instant the Fast motor turns off. However, it's not a bad idea to check to make sure that both motors are not trying to run at the same time.
        I'm David. 'Dave' is someone else's name.

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        • #5
          I swapped the triacs but no change. When I switch from fast to stop the slow motor does kick in for a few seconds for braking. The motors run when connected directly to line voltage. IDK maybe its a zener diode. I should just replace the diodes and transistors and hope for the best. Or just make my own control circuit.
          When the circuit is allowed to sit for a couple of weeks and completely discharge then it works. But something causes it to stop working at some point. Even after a few hours or days it still won't work. Has to be 1-2 weeks. Weird.

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          • #6
            Btw, im im dealing with rough schematics and a vague service manual so tell me what to test and I should be able to do it.

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            • #7
              The Fast motor control circuit isn't that complicated. When the control contacts are shorted, DC control current is drawn through the relay coil (and diode D10) which closes the relay contacts and applies a low-current 120VAC signal to the Triac, which turns the Triac on.

              There are clean, legible copies of the 720 schematic available online. There is a table of control circuit voltages on the schematic.

              The brake timing circuit only controls the Slow motor, so if it appears to be working, you can probably ignore it.

              Diode D11 is a flyback diode, normally off, but if it has shorted, it would disable the relay.

              I'm David. 'Dave' is someone else's name.

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              • #8
                Hello,

                Basic fast control voltage are :
                Pin 7 (Violet) measured from Gnd : = Zero volt on Fast if not ~17V
                Across D11 : ~16V on fast if not = Zéro.
                Are they correct ?
                if Yes, check the relay for an open coil. Resistor should be = 500 OhmS (power off).

                JP

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                • #9
                  Click image for larger version

Name:	20190812_115039.jpg
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ID:	662772I think I found the issue. The large black resistor looking components labeled PA111035. I swapped them with each other and now I have fast and no slow. I cant find that component when cross referencing the numbers on the net. Anybody have these for sale or know where I can get em?
                  Last edited by JD1973; 08-12-2019, 10:17 AM.

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                  • #10
                    Good !

                    This is a reed relay 12Vdc.

                    JP


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                    • #11
                      Sweet! Thanks JP, and everyone else for your responses.

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                      • #12
                        I cant find a replacement anywhere. Even the tonewheel general hospital doesnt know where to get them. Any suggestions? Might have to retrofit a modern relay. This old one is a 375volt reed relay part#137454.

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