M3 pedal drawbar 60hz hum

[Guest]

Hi all, I'm a new member of The Organ Forum and the proud new owner of a 1958 M-3 (later model). Hope one of you experts can help me.</p>

After a good cleaning with DeoxIT, oiling and resoldering 2 broken TG contacts, the organ is sounding good, except for one problem: a loud 60hz hum, heard only when the pedal drawbar is pulled out.</p>

Volume of the hum is raised and lowered by the expression pedal. Touching the wire going from the pedal drawbar to the amp alters the hum slightly but does not remove it.</p>

Likely suspects? Any help will be greatly appreciated.</p>


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[bluetantra]

Re: M3 pedal drawbar 60hz hum



You may need to adjust the pedal cutoff control, located on the amplifier chassis. Push the expression pedal to its loudest position and pull the pedal drawbar all the way out to 8. Press the highest pedal and release it. Wait about 15 seconds for the note to fade, then slowly turn the pedal cutoff adjustment till the note just disappears.</P>


John</P>

[Guest]

Re: M3 pedal drawbar 60hz hum

Thanks John, but I'm not sure that's it. What I'm hearing is not a low, musical tone, it's a nasty, loud buzzing hum.

[Guest]

Re: M3 pedal drawbar 60hz hum

sounds like a capacitor problem, but that's coming from someone who doesn't really know hat he's talking about

[geoelectro]

Re: M3 pedal drawbar 60hz hum



The M-3 has an air capacitor on the pedal drawbar. It is supposed to be fully covered with a metal sheild. It may be missing. You shouldn't actually see the capacitor when you move the drawbar. Also, that shield may be misaligned and touching the capacitor. Do you get any pedal tones?</p>

George
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[Guest]

Re: M3 pedal drawbar 60hz hum

[quote user="geoelectro"]

The M-3 has an air capacitor on the pedal drawbar. It is supposed to be fully covered with a metal sheild. It may be missing. You shouldn't actually see the capacitor when you move the drawbar. Also, that shield may be misaligned and touching the capacitor. Do you get any pedal tones?</p>

George
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[/quote]</p>

Ah, so that's what that cylinder is! Yes, George, thanks — the metal shield is present, and no, I do not actually see the capacitor when I move the drawbar. I will check to see if the shield is misaligned. And yes, I do get all the pedal tones, although the hum is nearly as loud as the tones. Sorry, should have mentioned that in my original post.
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[Guest]

Re: M3 pedal drawbar 60hz hum



[quote user="thekennel"]Thanks John, but I'm not sure that's it. What I'm hearing is not a low, musical tone, it's a nasty, loud buzzing hum.
[/quote]</p>

I am an M3 owner and I think that IS what you are hearing. When a bass pedal is held down (or the pedal cut off is not adjusted) the sound is not very musical, low C sounds very like a 60 hz hum. Adjust the pedal cut off first. It is one of the 2 pots (steel knob 1/4" dia) on the amplifier, it is marked 'pedal'. Push the pedal drawbar in, and turn it to see if the sound goes away or gets louder.
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If the pedal cut off is not the problem, and you do have a for real 60 hz hum the capacitor that would likely be causing that is one of the large can shaped ones that sit on top of the amp beside the tubes.
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[bluetantra]

Re: M3 pedal drawbar 60hz hum

In any type of service work, it's wise to check the easy things first. If the electrolytic cap is the original, it certainly could be suspect, but checking the pedal cutoff adjustment is easier than soldering in a replacement capacitor. Antique Electronic Supply (www.tubesandmore.com) carries a wide variety of values in the original style aluminum can multi-cap.

[Guest]

Re: M3 pedal drawbar 60hz hum

[quote user="everiman"]

[I am an M3 owner and I think that IS what you are hearing. When a bass pedal is held down (or the pedal cut off is not adjusted) the sound is not very musical, low C sounds very like a 60 hz hum. Adjust the pedal cut off first. It is one of the 2 pots (steel knob 1/4" dia) on the amplifier, it is marked 'pedal'. Push the pedal drawbar in, and turn it to see if the sound goes away or gets louder.
</p>

If the pedal cut off is not the problem, and you do have a for real 60 hz hum the capacitor that would likely be causing that is one of the large can shaped ones that sit on top of the amp beside the tubes.
</p>

[/quote]</p>

[quote user="geoelectro"]</p>

The M-3 has an air capacitor on the pedal
drawbar. It is supposed to be fully covered with a metal sheild. It may
be missing. You shouldn't actually see the capacitor when you move the
drawbar. Also, that shield may be misaligned and touching the
capacitor.</p>[/quote]

Man, this forum is great! You guys are the best. OK, here's what I've discovered:</p>

George — when I removed the metal shield, a new, extremely loud hum resulted. Replacing the shield (which was not misaligned, not touching the air capacitor) removed that terrible sound, but I was still left with the original hum -- which, by the way, is between A# and B, two octaves below middle C, so for sure a 60hz hum (I think, ha ha).</p>

Everiman — When I checked the pedal cutoff, it had already been turned clockwise as far as it could go. Evidently someone even more ignorant than I had been monkeying with it. I turned the cutoff counter-clockwise until I heard the pedal tone come in under the unpleasant hum we're dealing with here. I moved it back clockwise just to the point that tone stopped and the pedals were working. (After I solve this hum problem, I'll go back and adjust the cutoff properly.)</p>

So where does this leave me?
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[geoelectro]

Re: M3 pedal drawbar 60hz hum



Two things... Make sure all the pedal wires are properly connected to the tone generator. Often in shipping, a wire can break off causing pedal problems. Secondly, you may have a power supply filter problem. This is somewhat more complicated. I usually grab a 30 MFD @ 450Vdc and with the chassis turned over, I parallel this cap across each filter turning it on each time to see if the hum is gone.</p>

George
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[Guest]

Re: M3 pedal drawbar 60hz hum

[quote user="geoelectro"]

Two things... Make sure all the pedal wires are properly connected to the tone generator. Often in shipping, a wire can break off causing pedal problems. Secondly, you may have a power supply filter problem. This is somewhat more complicated. I usually grab a 30 MFD @ 450Vdc and with the chassis turned over, I parallel this cap across each filter turning it on each time to see if the hum is gone.</p>

George
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[/quote]</p>

OK thanks again for your guidance. Wish me luck -- and yes, I'll be VERY careful!
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