C3-distortion at low volume

[dweisskeys]

Hey All,
Hoping you are all staying healthy.
I've got a late 50's C3. As per the subject I am hearing low level distortion at all volume levels. When playing a 1 low note and 1 midrange-high note it's as if there are extra voices in the chord, like an overdriven amp would sound like. It's more or less across the full range of the keyboards and any drawbar. I'm using just the 8' by itself for testing.

What I believe I've ruled out:

1). The Leslie - I've got a 21h, a 145 and a Motion Sound Pro-3 (run from the GG terminals). They all have the same issue in varying degree

2). I'm using a swapped preamp from a B3 that my organ tech put in a month ago for test purposes which was supposedly known in good working condition. Same issue my other pre, but it's gotten worse lately. I can't reach the tech since the Corona has struck and nobody will come near my place in NY State now anyway. Who knows when that'll change.

What's left? My tech skills are limited. I'm a player, but I can solder and take things apart.

Thanks for your kind advice.

Daniel

[Sweet Pete]

The one tube common to all audio in the preamp is the 12AX7.Try cleaning that socket and pins,or replacing that tube.
I cleared an issue much like you describe this way; more than once.
House calls not a good idea for either parties at this time,or the near future.Maybe July?

[Drawbar Dave]

Also check cable run to 6 pin socket and check Leslie cable for continuity on signal and ground.

[dweisskeys]

The one tube common to all audio in the preamp is the 12AX7.Try cleaning that socket and pins,or replacing that tube.
I cleared an issue much like you describe this way; more than once.
House calls not a good idea for either parties at this time,or the near future.Maybe July?

Thanks, Pete. Cleaned it all, swapped it. No difference.

[dweisskeys]

Also check cable run to 6 pin socket and check Leslie cable for continuity on signal and ground.

Thanks Dave. 2 different Leslie cables ( 145 & 122) and a 1/4 inch output from the GG terms all sound alike.

[Sweet Pete]

Thanks Dave. 2 different Leslie cables ( 145 & 122) and a 1/4 inch output from the GG terms all sound alike.

Maybe try one G and ground for the 145/1/4" and both G/G for the 21H? Have you determined the preamp distortion with headphones on the G/G's?
Debris in the expression control housing,or wires within wrapped too tightly near the axle will cause distortion and shorting.
Was the dynamic range of the pedal similar to the original preamp,with the fitting start and end points?

Cold solder joints will act up,I reflow all the preamp solder on the 'tabs' from control cabling to make sure.

[dweisskeys]

Maybe try one G and ground for the 145/1/4" and both G/G for the 21H? Have you determined the preamp distortion with headphones on the G/G's?
Debris in the expression control housing,or wires within wrapped too tightly near the axle will cause distortion and shorting.
Was the dynamic range of the pedal similar to the original preamp,with the fitting start and end points?

Cold solder joints will act up,I reflow all the preamp solder on the 'tabs' from control cabling to make sure.

Ok, thanks.

I didn't check headphones, no. The GG outs went to the Motion Sound speaker.

I believe the dynamic range was similar to the original pre, yes.

Sorry, where exactly are the tabs I should reflow? Those connected to the drawbars?

[Sweet Pete]

Reflow the solder on the 'spare' preamp connections,since that work is the most recent.Revisit the work most recently done is always a good place to start.
Check for preamp distortion with headphones to eliminate other possible issues,downstream in the signal flow.Maybe the 'tech' failed to 'button it up' right?

[David Anderson]

I second looking at all solder joints AND all ground connections. I once worked on a Rhodes with a distortion problem that had supposedly been looked at by several people, and no one could figure out what the problem was. I found a suspicious-looking solder joint in the grounding, and that was the problem. Many of the ground connections on the tone generator are made via screws, and you should check the screws on the preset panel as well.

[dweisskeys]

OK, thanks! I notice that the connection called BN GRD has nothing connected to it. No wire hanging loose in the area. Is that a problem?

[Sweet Pete]

Not all AO28 had a BN GRD tab. Early units had no pilot light so the 6.3V and ground taps are different.
That might be a later AO28,so an unused tab would make sense. Not sure that would have any bearing on your distortion though?
Get the solder pencil out and reflow the connections,and follow drawbar dave's advice.
A cold solder connection on pins 1 or 6 of the Leslie cable will cause distortion.
David concurs,three suggestions to check solder and cable connections from people who have been there,done that and found other people's mistakes.

[David Anderson]

OK, thanks! I notice that the connection called BN GRD has nothing connected to it. No wire hanging loose in the area. Is that a problem?

Probably not. That ground connection is primarily for your tone cabinets. It would be needed, for example, for 21H speed switching to establish a common ground between organ and tone cabinet. However, the main audio ground connection between the audio sources in the organ and the preamp is in the braided shields around the two coax cables from the Vibrato Select tab box.

[Jyvoipabo]

Hello,

I think you may got a cross talking distortion.
I had the same problem on a guitar amp whose filter capacitors were tired.
But it mostly happened at high level.

However if the filters capacitors are very tired it can happen at low level.

What about is the condition of the capacitor of the Power supply ?

Confined JP

[tpappano]

Any chance you could make a video or audio clip showing what it sounds like? You mentioned an “extra voice” effect, could this be a situation where a stray strand of wire is shorting between two preset panel busbars?

[dweisskeys]

I appreciate all this great info from you all. Thank you. Jyvoipabo, I think you're on to something. On closer inspection of the sound it seems like there's low volume sonic hash playing along with every note.It's especially obvious at low volume and builds up quickly. At louder volumes not so much. It's somewhat inconsistent in terms of pitch as you move up and down the keyboard. tpappano, I will upload a couple of examples. It's just iphone but you can hear it with headphones. I'll set up mics if that's not sufficient.

Here's what I've done so far.

1. Reflowed all preamp solder
2. Checked grounds (that I could spot)
3. Checked GG terms with headphones - it sounds less distorted but it's still present.
4. Examined preset panel. Everything looks OK but I'll revisit for stray wires.
5. Examined expression control housing for tight wires and dirt.

Here's a dropbox link with the examples. There seems to be a tiny upload size limit for direct uploads. keep in mind that the volume is barely cracked. https://www.dropbox.com/sh/pilnjndb6...JxazK0WZa?dl=0

[tpappano]

Interesting sounds... I'm not sure how much of a 'tinkerer' you are, but one thing one could do to prove whether the noise is in the preamp or not would be to patch the signal directly from the matching transformers into a hifi or guitar amp, or similar, and verify that the sound is clean and free of artifact before it goes into the preamp. In the first clip I thought I might have heard something my A100 does, when you hold a single low key then add an upper key, at a specific number of steps between them, I hear an 'extra' pitch besides the two being held. I had accepted this as a Hammond quirk, but I just tried the same thing on the RT3 and D152, and they both are clean as a whistle. I'll double check the A100 when I get home in a little while, I could be imagining the whole thing 8)