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Thread: How would you upgrade a Hammond M3 plug to 3 prong grounded?

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    How would you upgrade a Hammond M3 plug to 3 prong grounded?

    I have an old Hammond M3 which has developed a lot of ground hum, perhaps due to added wire lengths as part of a chop I performed. I noticed that when I touch the chassis the hum abates significantly (probably dumb to do, I realize). This leads me to believe it's a grounding issue. I was thinking of replacing the original 2 prong plug with a three prong one, and attaching the ground lead on the new plug to the M3 chassis. Would this be a mistake? Is there a better way to wire this? Is this project ill advised for any other reasons?

  2. #2
    Senior Member Wes's Avatar
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    1 members found this post helpful.
    There's lots of stuff I the forum about this, including important hazards to be ware of, but I will immediately caution you that the third prong on a power plug is for sAfety, it is not a magic hum killer. This equipment had neither a grounded plug nor a hum problem when new.

    If you have a signal ground loop due to a poor chop, a safety ground will not fix it. If you screwed up the wire shielding or generator grounding, it won't fix that either. If you placed signal wires in the magnetic flux of a transformer, that too will not get fixed with a safety ground. I actually have that last problem in my l111 chop.

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    Junior Member UncleClark's Avatar
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    1 members found this post helpful.
    I noticed that when I touch the chassis the hum abates significantly
    My comments are pretty much the same as what "Wes" said:

    Since you said you did a chop on your organ, it's possible you have a SIGNAL GROUND problem.

    Missing shield connection somewhere?

    Missing wire?

    Bad solder / bad connector?

    Aside from the hum I would add a modern 3 wire power cable with POWER GROUNDS to all the chassis, manuals, etc. POWER GROUNDS are a safety issue aside from any hum you might have.

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    Member MihevicB3's Avatar
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    1 members found this post helpful.
    Quote Originally Posted by UncleClark View Post
    Aside from the hum I would add a modern 3 wire power cable with POWER GROUNDS to all the chassis, manuals, etc. POWER GROUNDS are a safety issue aside from any hum you might have.
    Adding the third prong safety ground is O.K., however, if you have high leakage current in the power transformer, it is possible to burn up the transformer with the addition of the third prong ground since this current now has a path to ground. If you add the third prong ground, I would add fuse protection on the power transformer. As Wes mentioned, there is much info on the forum on this topic, including how to measure the amount of leakage current you have from your power transformer.

    John M.
    1956 Hammond B3
    1963 Leslie 122
    Two Pr40ís
    One JR-20 (for fluid reverb signal)
    Hamptone LEQ3B
    Trek II Reverb
    Trek II String Bass

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    Member ShadyJoe's Avatar
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    Quote Originally Posted by MihevicB3 View Post
    Adding the third prong safety ground is O.K., however, if you have high leakage current in the power transformer, it is possible to burn up the transformer with the addition of the third prong ground since this current now has a path to ground. If you add the third prong ground, I would add fuse protection on the power transformer. As Wes mentioned, there is much info on the forum on this topic, including how to measure the amount of leakage current you have from your power transformer.

    John M.
    Fix the hum, then add the ground.
    When I become dictator, those who preach intolerance will not be tolerated.

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    Administrator Admin's Avatar
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    Quote Originally Posted by MihevicB3 View Post
    As Wes mentioned, there is much info on the forum on this topic, including how to measure the amount of leakage current you have from your power transformer.
    Just scroll down this page and you'll find several threads on this subject in the Similar Threads box.

  7. #7


    Thanks everyone for your input.

    Quote Originally Posted by UncleClark View Post
    My comments are pretty much the same as what "Wes" said:

    Since you said you did a chop on your organ, it's possible you have a SIGNAL GROUND problem.

    Missing shield connection somewhere?

    Missing wire?

    Bad solder / bad connector?

    Aside from the hum I would add a modern 3 wire power cable with POWER GROUNDS to all the chassis, manuals, etc. POWER GROUNDS are a safety issue aside from any hum you might have.
    This sounds like it may well be the problem. Everything was cool once I completed the chop initially in Portland, and found a position where the wires didn't pick up interference from the transformer as Wes suggested. I've since shipped the organ to my new home in Brooklyn. Somewhere along the line between shipping it and carrying it up to my 3rd floor walkup, the problem (and others to be addressed in another thread) developed. I checked the continuity on my connections since then and eveything seemed okay, but I'll double check them as it seems likely something may have been pulled out of place in the move. There's one connection in particular I suspect is the culprit.

    After all your input I'll follow Shady Joe's advice and fix the hum BEFORE I add the 3 prong chord.

    Also, I read some similar threads regarding this and have a question about fusing. What type of fuse should I use, and where should I put it?

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