Mono output disappears when second Stereo output is engaged

I am thinking of a Roland digital synth. The Left output works. The Right output works. But when both are engaged at the same time for stereo, the mono output disappears. I am wondering if there is a particular part that manages this, or perhaps it is caused by more general problems like old capacitors. Thanks.

I think this needs more information to be able to say anything. Which synth is it? Does the manual mention anything specific about mono output? All synths that I know share a single socket for left and mono output, so if there is a synth where these are actually separate sockets, I could imagine that the mono socket is still muted when L/R sockets are used.

Thanks. JV-1080. What I mean to say is - there are 2 main outputs. The Left and Right. Left is Mono and Right, when added to it, is Stereo. Both work individually, but when the Right is “added on” the Left disappears, instead of remaining and becoming a stereo pair. So it’s like the machine can’t handle both at once, as if the Right shorts out the Left.