stereo receivers

I restore old receivers from the 70's as a hobby to keep me busy in my retirement years. One question. I have a couple of receivers that the Stereo, mono, left, right switch has no effect. Appears to just stay on stereo no matter the switch position. My thinking it is the switch itself. Am I on the right track.
 
Try just one cable plugged into the left input. Does it behave properly when using the mode switch?
Then do the right channel instead and check that.

For example, with signal only to the left input, the mode switch operation should be:

stereo - sound only from left speaker
left - equal sound from both speakers
right - no sound from either speaker
mono - equal sound from both speakers
 
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