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  • Leslie 122 keeps blowing fuse

    I recently acquired a mid 1960s RT-3 from a church. This is my first Hammond and I have only had it for a few weeks. It came with two benches, a PR-40 and a Leslie 122 (with original Hammond warranty paperwork/brochure/songbook). The PR-40 works well except for some vibration with low bass tones/chords. Necklace reverb in working order. At the church, I noticed that the rotors on the Leslie would spin, but no sound or response from half-moons. When I got it back home (another story), I noticed that it would spin on high, but no tubes or low. Switching between tremolo and chorale makes no difference. Motor is stuck on high. There was a 1.5A 125V slo-blo fuse in there and it was blown. Replaced with 1.5A 250V mdl twice with same results. The fuses I ordered (mdq) are of a different style than the ones I bought at the local hardware store (mdl), although I was assured they were slow blow. Would that make a difference? Clearly, something is wrong here and I don't want to keep throwing fuses at it. I have ordered new slo-blo 1.5A 250V from tonewheel general (and some oil).

    I removed the power amp to look for any noticeable damage, but didn't notice anything smelly or out of the ordinary. In fact, it looked immaculate on the inside. Almost like a piece of art. However, this is the first piece of electronics from the 60s that I have ever looked at and am not sure what something faulty would look like. I have only used a voltmeter a few times, but am willing to experiement and listen.

    Where do I go from here?

    Thanks!

  • #2
    To find the short circuit, it is necessary to eliminate the circuits one by one.
    First disconnect the secondary wires starting with one of the red wires (High Voltage before bridge rectifier).

    If you are tired of changing fuses, you can install an incandescent lamp serial in the power line.
    As long as the problem is not found the lamp will bright when you turn on the power.

    JP

    Comment


    • #3
      Originally posted by 32Pedals View Post
      I recently acquired a mid 1960s RT-3 from a church. This is my first Hammond and I have only had it for a few weeks. It came with two benches, a PR-40 and a Leslie 122 (with original Hammond warranty paperwork/brochure/songbook). The PR-40 works well except for some vibration with low bass tones/chords. Necklace reverb in working order. At the church, I noticed that the rotors on the Leslie would spin, but no sound or response from half-moons. When I got it back home (another story), I noticed that it would spin on high, but no tubes or low. Switching between tremolo and chorale makes no difference. Motor is stuck on high. There was a 1.5A 125V slo-blo fuse in there and it was blown. Replaced with 1.5A 250V mdl twice with same results. The fuses I ordered (mdq) are of a different style than the ones I bought at the local hardware store (mdl), although I was assured they were slow blow. Would that make a difference? Clearly, something is wrong here and I don't want to keep throwing fuses at it. I have ordered new slo-blo 1.5A 250V from tonewheel general (and some oil).

      I removed the power amp to look for any noticeable damage, but didn't notice anything smelly or out of the ordinary. In fact, it looked immaculate on the inside. Almost like a piece of art. However, this is the first piece of electronics from the 60s that I have ever looked at and am not sure what something faulty would look like. I have only used a voltmeter a few times, but am willing to experiement and listen.

      Where do I go from here?

      Thanks!
      I would do it like this. Pull all the tubes out, and then remove the filter cap. (2 5/16" nuts and plate) Power it up. If the fuse blows, it's a shorted diode in the rectifier or less likely, the power transformer.
      If ok, plug the filter back in and power it up. If it blows, bad filter, if not it's ok.
      Then, add the tubes. The two large tubes are really the only tubes that can draw enough current to blow the fuse. Still, put them all in and test. If it then blows, replace the 6550's.

      Geo

      Comment


      • #4
        Originally posted by geoelectro View Post
        I would do it like this. Pull all the tubes out, and then remove the filter cap. (2 5/16" nuts and plate) Power it up. If the fuse blows, it's a shorted diode in the rectifier or less likely, the power transformer.
        If ok, plug the filter back in and power it up. If it blows, bad filter, if not it's ok.
        Then, add the tubes. The two large tubes are really the only tubes that can draw enough current to blow the fuse. Still, put them all in and test. If it then blows, replace the 6550's.

        Geo
        This is great! This is exactly the type of support we noobs sometimes need. Step by step, methodical troubleshooting advise and techniques. I don't have this problem but just wanted to thank you Geo for illustrating the thoughtful approach one must take to arrive at a logical conclusion. Thanks, magoo

        Comment


        • #5
          Originally posted by einstein magoo View Post
          This is great! This is exactly the type of support we noobs sometimes need. Step by step, methodical troubleshooting advise and techniques. I don't have this problem but just wanted to thank you Geo for illustrating the thoughtful approach one must take to arrive at a logical conclusion. Thanks, magoo
          Appreciate it. Unfortunately many problems can’t be easily diagnosed this way. ��

          Geo

          Comment


          • #6
            Thanks for the response Geo! Got some new 1.5A 250V MDL fuses today.

            Pulled tubes and filter cap. Powered it up and fuse fine without filter cap. The fuse blew after I plugged the filter in and powered it up. I noticed some residue on the bottom of the filter cap when I pulled it out. I will replace the filter cap and let you know how it goes.

            Comment


            • #7
              OK, filter cap was in the mailbox this morning. I put it in, powered it up, and the fuse did not blow. Powered it down, replaced tubes, powered it up, tubes lit up. Put power amp back in cabinet, plugged everything in. Powered up organ, rotors spun, speakers came on and sounded good. One problem, I noticed is that Tremolo/Chorale switch makes no difference in speed of rotors. I powered everything down and let it sit for about 1/2 hour. I pulled the pulled the back of the Leslie at this time. When I powered it back up I noticed that it spins on high, then spins on low for a few seconds before going back to high speed. Tremolo/Chorale makes no difference. Plugged low speed plugs into high speed outlets and rotors spin on low.

              Another thing I noticed is that when I switch to chorale, reverb goes off on PR-40. Being new to Hammonds not sure if this is how they work, but the switch does seem to be doing something.

              What would be the next step in trying to getting the rotors to switch between high and low?

              I hope I gave enough symptoms to make a remote diagnosis. Thanks for the help so far. Progress is being made.

              Comment


              • #8
                The motors going fast, then slow, then fast again is normal for a Leslie set to go fast from the organ. Set to go fast meaning no switching voltage from the Leslie kit in the organ. The reverb changing in the tone cabinet means there is switching voltage to it. So, is there switching voltage going to the Leslie? Also, a bad switching tube 12AU7 in the Leslie would have the same symptom.

                Geo

                Comment


                • #9
                  How would I check if there is switching voltage to the Leslie?

                  Comment


                  • #10
                    The front end of the PR-40 is set up such that it supplies a positive voltage superimposed on the audio G-G conductors that has to be grounded through the organ preamp's output transformer for spring reverb to work. Hammond supplied a kit (I've seen two of them installed.) that you could install in the organ to give remote reverb control from the organ console.

                    The upshot is that you can't have the PR-40 and Leslie 122 connected to the exact same output from the organ. They are incompatible. The usual way to deal with a PR-40/122 combination is to connect the PR-40 directly to the organ's preamp while the 122 and it's control voltage circuits are isolated by the transformer that's part of the Leslie 122 kit.

                    Alternatively, you could use an additional isolation transformer for the PR-40 if you wanted to implement remote reverb control. The necessary circuit is drawn on the PR-40's power amp schematic.

                    If you have the PR-40 and 122 connected to the same output terminals from the organ, weird things could certainly happen. Both use DC control voltages superimposed on the audio signal, but for different purposes. What's happening is that when you switch the 122 to Fast, you are also grounding the reverb control voltage from the PR-40, causing reverb to work.

                    I do service a B-3 that has a 122/PR-40 hookup with Main-Ensemble-Echo, Leslie 122 speed control, and PR-40 reverb control. Very crowded in there.
                    I'm David. 'Dave' is someone else's name.

                    Comment


                    • #11
                      As it turns out, I had the plugs switched in the back of the RT-3. Again, not sure if I mentioned this, but this is the beginning of my Hammond adventure and I am only about a month into it. Once I swapped plugs, tremolo/chorale works on Leslie and reverb does not shut off on PR-40 on Chorale. Everything now seems to be in good working as far as I can tell.

                      Thanks for all of your help. Once Geo said switching voltage was going to the PR-40, something clicked. Then I saw the different wires going to the power strip(?) on the back of the RT-3. At this point I realized that the receptacles were not the same.

                      Amateurs. It must be fun to watch.

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