this button blends the two stereo channels together to produce monophonic sound this blend minimizes rumble and surface noise in old monophonic records ,the button must be out for normal stereo listening
if you aee making a tape recording,engaging the mono button may affect the signal being recorded-depending on whethre the tape monitor button is alsoengaged,then pressing mono will blend the input signal into mono,and the monophonic signal will be recorded.lf the tape monitor is engaged,then pressing mono will affect only the sound that you hear as it returns from recoder the signal going to the tape will be recorded in stereo