The volume knob part seems like the most likely candidate.

Hi Jason,

Thanks for responding. You're saying it's the pot, right? Do you know the best way to deal with it?

It's a really clean, new-looking unit so it would surprise me if it had already become dirty inside. I'm worried it got bumped or something and that there are other malfunctions awaiting me as I dig deeper down.

-Russell

Well, if it was a cold solder joint - I wouldn't think it would always be at the same rotational area where the sound starts working. It would go in/out as it was moved. Could be some other component - but we start getting into less likely scenarios. Second hand - I have no idea what that keyboard has been through. It's relatively easy to make outward appearances mask the history of that keyboard.

If it is a dirty pot - the fix is not going to cost much and you can have someone take a look at it. Typically there's a minimum bench fee which will be what you'll be spending. The tech can tell you what they see in terms of general health of the insides if you ask for that.

Thanks a lot!

I'll take it to someone to have a look.

FWIW... Buy some Isopropyl alcohol (cheaper than deoxit) and spray it into the crevice around the base of the control, then gently rotate the knob back and forth through its full travel. . It won't do any harm, but will flush out any gunk (dried coca cola for example) that might be fouling the resistive wiper blades.

Its worth a try for $5 before paying a $100 bench fee.

I used this a lot when buying "vintage" effects pedals. Clean and pristine on the outside, a museum of beer and coffee spills on the inside. If it is just a dirty pot, most of the time it works.

Hi Antony,

Thanks for the reply. I have both isopropyl alcohol and Deoxit D5. Is one better than the other? I hesitated to put anything inside because I am assuming the electronics are much more delicate than the guitars I use the Deoxit for. If there's no risk, I'd try it, but if there is any risk I'd probably rather take it to somebody.

-Russell

Obviously switch the power off before you do this.

Isopropyl is used all the time for safe cleaning of electronics. Its a mild solvent that evapourates quickly and leaves no residue. This makes it useful for loosening and flushing out gunk in pots, which is generally born of other liquids like coca cola, beer, coffee, nicotine tar etc or it could simply be house dust.

Its your decision ultimately.

Ack, no! First thing to try is to rotate the pot back and forth 50 times.

Solvents, whether alcohol or Deoxit-D always risk breaking down the potentiometer's lubricant. Deoxit-D is also meant for metal surfaces and not carbon traces - that's why you don't see them mentioning pots here:

https://caig.com/deoxit-d-series/

However, you could attempt to clean a pot with Deoxit-D then use Faderlube afterwards:

https://caig.com/fader-f-series/

If you're desperate and have nothing to lose, the use Deoxit and see what happens. Worst case the pot turns to a grindy mess, as if there was sand inside.

(I used to repair big polysynths professionally. My profile pic is the guts of a CS70m I worked on 😉 )

If this were my keyboard, I'd ohm out the pot to verify that's the issue and pull it replacing it with a like part. I'd probably pony up for the service manual from Yamaha anticipating it may come in handy for the future. This gives me an orderable part number I can get an original pot - but I could also get one that matches spec from elsewhere. For things like this, I usually get original parts unless I suspect there's a systemic issue with quality of the selected parts. I haven't heard of this being a common failure - so I would probably first suspect "misuse" by the prior owner over other theories and order the original part. There's not a huge premium for the simple parts like this. Shipping is usually the bigger number. This is where a "package" buy with the service manual CD may help justify the whole thing for myself.

I've got the proper equipment, training, and experience to do this work.

My advice has been and continues to be to have it serviced. This helps save your time and avoids the chance to learn through mistakes. Yes, there's a cost. I'm not sure what your financial means are - so this is a decision. I assume - if you're asking - then it would be best to have someone with more expertise fix the keyboard.

If you had the expertise to do this - then it would likely be done. And if you know enough to be "dangerous" - but still not up to the task. Well, this could be a good learning experience or a bad one.

No judgement or elitism here. I want you to have a good outcome beyond all else.

Thanks Michel and Jason.

Jason, yes I'm planning to have it serviced at a Yamaha authorized place nearby. Luckily the seller in California from whom I bought it online has said he'll cover the repair cost if it's not too crazy. I am good with wood and do a lot of things myself but this is clearly a different world and I might be dangerous if I went in there today. I'm curious and in another time would love to learn, but right now this synth is just a musical tool for me, and I'd like to get it done right and be done with it.

Michel, that makes sense about the lubricant. I will definitely hold off on spraying anything in there without knowing what the heck I'm doing, especially since the seller says he is willing to pay.

Thanks to everyone again for your thoughtful replies! They have been hugely helpful---it's possible I might have tried something and regretted it if not for all these perspectives.

-Russell

@Russel... I always err on the side of caution, and if you don't feel confident about something, don't do it. For future reference you could read up on Isopropyl, dedicated contact cleaner and di-electric lubricants. There's no bad juju with these.

@J - I was surprised to learn the Master Volume control is actually on the Land R analogue signal paths (audio pot). I would have guessed it was controlling some voltage sensor on a processor chip, (thats also how I imagine the Assign Knobs work). So when I first read the OP, I thought it might be a loose PCB jumper, as others have previously reported.

I don't have the MODX schematics to reference but previous boards placed the volume (slider) in the signal path before buffers going out to the Main L&R/Phones. For input - they used a stereo knob with the same general topology for setting the level - so a similar type of part previously used for audio in could be used for audio out in order to "knobize" what was previously sliders.

It also fits with what is being reported as the behavior - even though an educated guess.

Having a tech look at it means I don't have to be spot-on with my description as the tech will have proper references to address this problem - or just will see what's in front of them when examining the PCB and components.

Assign knobs as encoders are certainly in the digital domain. So you imagine correctly there.