Hammond A-100 low freq hum goes away while touching the "C" terminal post

[chopper0001]

I have changed tubes and many of the suspect caps in trying to reduce the very low frequency hum. The hum is audible through both the internal speakers and the Leslie 251. The hum increases and decreases with expression pedal operation. Drawbar positions make no difference on the hum level. The tremolo, revere, and percussion controls do not seem to have much if any effect. When I touch the terminal labeled "BN C" with brown wire on the AO-28, the hum goes away completely! Given this information, is there another possible solution besides having my wife keep her finger on the terminal while I play?? :) It's driving me (and her) crazy!

[Jyvoipabo]

Hello,

As far as the preamplifier is concerned, the vibrato signal is sent from V3 (12BH7) by the terminal C to the delay line. The delayed signal is scanned by the scanner and returned to the D terminal preamp.

Since Terminal C is an output, what you are explaining is unexpected.

Pull the wire going to C terminal for solder check. Idem for all the wires of the ground circuit.

Move gently V3 on is socket. If the hum disappear the problem is not so far.

If the circuit after terminal C is involved, the buzz can be louder on chorus C1, C2, C3 than V1, V2, V3

You can also try to disconnect the BN wire, which will disable the vibrato and may be the hum will disappear.

JP

[David Anderson]

We are not told the history of this organ. Was there a time when it didn't hum?

One confusing thing about A-100 wiring is that brown wires are used both for Ground wiring and Vibrato Line Drive. I have encountered several situations where A-100s were miswired due to confusion over wire color.

[chopper0001]

I am the new owner, and the previous owner does not have any historical info that would help. I'll try to follow that brown wire up and see where it goes. I can see that it makes it's way up into the tone wheel generator area.

[chopper0001]

The brown wire on C terminal does in fact go to the bank of vibrato caps. Solder joints seem good. I'm going to re do any questionable solder joints and see what happens. One thing I noticed is that on my AO-44 reverb amp, there are three wires connected to the far right speaker terminal. A black wire goes directly to the speaker, another wire goes to ground the swell pedal housing, and a brown wire goes to the GND terminal on the AO-28. From that terminal, a second brown wire goes to the brown wire coming out of the 6 pin Leslie adapter box. Does this all sound normal? A wiring diagram I found must be for a different reverb amp, as it shows 8 terminals on the amp instead of the 6 that I see on my AO-44. None of the terminals on the AO-44 is actually labeled as a ground.

[gtojimmy]

Chopper, I have the same identical problem with an A-102. Recapped preamp, power amp, and I get a 60hz hum with a higher 180 (3x) peak, which I find with the tone generator not running, but organ powered up,and keys depressed. Comes out of the main speakers, and/or leslie, and i've unplugged the leslie to take out of the picture. I believe that brown wire is coming from either the reverb amp or the vibrato line...(can't remember). Guess this gives me a new place to poke around. My wife's always looking for something to do, so maybe I can put her to work and telling her to hold her finger "right here" !! lol

[KC9UDX]

I've got a guitar that does this whenever I'm not touching the strings. It's been doing this since I got it over twenty years ago. Hopefully within a year I'll actually get a chance to take it apart and fix it. But it's kind of low priority. My Lowrey desperately needs some work. By the time I get that done, my Hammond is going to need work again.

[chopper0001]

I had one of those. My quick fix was running a wire from my guitar ground into my shoes :p . Luckily I was never shocked.

[KC9UDX]

I did something similar here. But my wire came loose naturally as soon as I hit the record button. Stupidly fumbling round trying to reconnect it distracted my playing.

https://youtu.be/mI92t-4yOXI

[chopper0001]

This may not be related, but the hum adjustment pot on the power amp has no effect whatsoever. Even while the hum is silenced by touching the C terminal, turning the hum adjust pot full travel both ways has no effect......

[David Anderson]

No, those circuits are completely unrelated.

[tpappano]

This probably doesn't apply to your issue but it happened to me: Reworking a M3, all functions worked but an obnoxious AC hum was present. putting my finger near the scanner/pedal terminal on the amp nulled out the hum! Long story short, the ‘key' was missing from the base of a 6V6 and it was plugged in wrong. Plugging it in correctly eliminated the hum 8)

[David Anderson]

Thinking about this a little more, I would make sure that all the grounding associated with the vibrato/chorus system properly connected, including a ground wire to the vibrato depth selection switch assembly.

[gtojimmy]

Chasing my tail some more on my hum...organ powered up, without tone generator started, drawbars all out on preset b+, 3 keys depressed.
noticed that pulling the 6x4, hum goes away.
Also, after removing cover, noticed that the terminal connection at the preamp coming from the vibrato scanner raises the volume of the hum by about 3x when touching the solder joint on the amp with a screwdriver tip. Just passing my hand over the general area of that terminal will cause the volume to rise somewhat!
Had just recapped this AO-28, and it seemed to work good for a couple days then the hum crept in. It does have most of the "Hammond" tubes as I've not changed them.
Other than the hum, everything works fine...percussion, vibrato, etc. Organ has a lot of grunt, but this hum is driving me nuts!
Could it be nothing more than a bad tube in the preamp? I have no one around that can check them, so would it be prudent to simply change them with new?

[David Anderson]

Yes, if you pull the 6X4. the hum will go away -- along with all the audio because you've just disabled the power supply. That's kind of like unplugging something and discovering that it doesn't work. Messing around with high impedance circuit points with hands and screwdrivers will definitely cause hum. That's why there's cover over those terminals. You'd get the same results poking any AO-28 with the same stick.

I can't entirely rule out tubes, but they would not be anywhere the top of my list of causes of hum, other than pins that are not making good contact.

One of my main diagnostic rules is: If there's a problem, look for mistakes in any work done after the organ left the factory. You have just recapped this AO-28. I would start there.

[gtojimmy]

David, pulled the amp back out and gone through my work, and while "cleaning up" a little, saw nothing to indicate an issue. What is the "mortality rate" on new electrolytics? Is it possible one has failed, causing the hum?

[David Anderson]

I have heard of new electrolytic capacitors failing, but I haven't personally seen it in almost 30 years working on tube gear. But I only use capacitors from companies with known records of quality like Panasonic, Nichcon, United Chemicon, BC Components, F&T, Sprague Atom, etc...

Installing one backwards is always a danger if you're using discrete capacitors, but is harder to do if you're using the CE Distribution cans.

I do test all capacitors with my test gear prior to installation.

[gtojimmy]

Put the amp back in, and after warming up for a minute or two, hum was very subdued! Started normally and sounds great! Now percussion is not working!! I still have a microphonic tube and most are Hammond tubes, so I'm going to change the tubes in the preamp, then if perc. is still not working, i'll deal with it ! Miss the days of the corner drug store's tube tester !
David, I appreciate the help on this.

[gtojimmy]

Checked amp and found V7 a little skewed in it's socket...musta bumped it doing the amp work. Reseated and fired up, and everything now works like it's supposed to ! Still don't know what i did to cure the hum...assuming maybe a cold solder joint, or ??
Thanks for the support folks!