I have an MOX8 that I bought new in 2012 that has an issue with its headphone output.  It has had the problem for some time but I don’t usually use the headphone output so I’d forgotten about it until recently.  Basically, some of the sounds don’t play correctly through headphones.  While many of the sounds play perfectly well, with many others I seem to only be hearing the reverb and not the sound itself.  There is also some crackling noise on some of the sounds.  I have tried different headphones and the problem persists.  Everything sounds fine when I play out to my studio monitors.  At first I thought this was the headphone jack, but given that some sounds are fine, it seems like it could be some internal issue.  Any suggestions/ thoughts are welcome.  Thanks!

There are a few possible causes of what you’re experiencing…

A lot of headphones come equipped with a 1/8"/3.5mm plug, and have to be adapted to the 1/4"/6.35mm jack on the MOX. Some of the adapters are unreliable. If you’re using one, you might try getting another.

Your description of “with many others I seem to only be hearing the reverb and not the sound itself.” would usually indicate that there is phase cancellation occurring between the right and left channels. That can happen when the common ground connection to the PHONES jack is lacking. It’s especially noticeable when the source is mono - if so, and the two channels are very closely matched in level, there will be almost complete cancellation of anything that’s in phase, leaving only out-of-phase sound such as effects which were added to the original sound. Try putting slight pressure on the headphone’s plug in various directions, and see if the symptom changes - that could point to a bad solder joint, for example.

By the way, with stereo sources, loss of the common ground won’t result in the degree of cancellation that would occur with a mono signal, but it’s usually still quite evident. On the sounds that “play perfectly well”, are they actually in stereo? Can you pan sounds fully left or right and have them sound that way in the headphones?

Problems in the electronics (headphone amp circuit) are possible, but less likely. There’s very little additional circuitry from the line outputs (which apparently are working correctly) to the PHONES output.

Indeed, 3.5mm jack often cause that problem, especially if the headphones are in fact speakers and microphone for a smartphone. A prolongator cable can solve the problem.
If there is distorsion in the output signal, and the synth is quiet old, the problem can be in the MUTE circuitry, that progressively will stop working correctly.
Simple but radical solution: get the schematics, find the 2 transistor that bring the audio signal to the ground, and remove them.
I did it on all my Yamaha synths, with great results (Motif es rack, motis es6, psr s950, s30, s80, Modx...)