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My first Hammond...electrical short!

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  • My first Hammond...electrical short!

    I did it, my first Hammond is home! A new to me 1961 A101! Now the reality starts to set in. At some point I will probably replace the original caps but first, I am reading (and feeling) 27 volts on many of the internal component chassis! Can't imagine this is proper. Any advice to begin to diagnose that?

  • #2
    27 volts from what to what? Is it AC? DC?

    We need a bit more information to work with.
    I'm David. 'Dave' is someone else's name.


    • #3
      Thank you David.

      I am reading 27 volts AC from many surfaces of component chassis to ground including the tone generator, amp and preamp. This is the situation when the run switch is on and the Leslie 122 is not connected. When I connect the Leslie, the voltage increases to 56 bolts on those surfaces and includes the Leslie amp.


      • #4
        There is a procedure in the front of Hammond Service manuals describing how this should be measured. If you are reading directly with a meter, you may be reading phantom voltages.
        BTW - kudos for even checking the safety of the equipment. Most people just plug and play.


        • #5
          More diagnostic info...
          I was curious so I flipped the AC plug 180 degrees. With the run switch OFF I get 124 volts across the aforementioned surfaces! When I turn the run switch ON it reduces to 68 volts.


          • #6
            With two-wire equipment, there is often a "floating" AC voltage on the chassis. It depends, to a certain extent, on the winding symmetry of the various power transformers. As you've discovered, if two power transformers share the same audio chassis ground, the measured voltage will change, since the two combine. And if you flip the plug, you'll get a different reading because you've now moved the point of winding asymmetry.

            The important question is how much current can be delivered from un-grounded chassis to ground, and as Jaim says, there are instructions in the Hammond Service Manual involving connecting a 10k Ohm resistor between chassis and earth ground (with a 0.01uF capacitor in parallel with the resistor) and measuring the AC voltage across the resistor.

            This "floating" or "phantom" AC voltage can be found on the chassis of all vintage two-wire equipment, guitar amps, HiFi amps, etc... You'd find it on Dynaco amps, Fender amps, H.H. Scott amps, Gibson amps, etc...

            The key is to distinguish between a floating voltage and actual current leakage from the power transformer primary winding to the chassis. The latter is what's dangerous since it can provide enough current to hurt you.
            I'm David. 'Dave' is someone else's name.


            • #7
              Thanks again. I found your reference in the Hammond Manual and will make up that tester.


              • #8
                If the Leslie amp still has its ‘death caps’ in place, they will also try to float the chassis at half the line voltage. If one of them has failed shorted a dangerous shock hazard can exist.
                Tom in Tulsa

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