Question why my stereo amps 'balance' = volume left & 'volume' is volume right

Jan 30, 2022
1
0
10
Hi guys! I picked up a used Marantz PM6007 amp for next to nothing last summer and finally got around to setting it up. Sounds great, but to my amazement the balance/volume is completely messed up. The small 'balance' knob is controlling the volume level of the right audio channel,and the massive 'volume' knob is controlling the left audio channel. It is of course usable as is, but also annoying as hell since it's impossible to know when the R/L channels are in the correct balance.
I have zero knowledge of the inner workings of these things, so I'd like to know the probable cause of this, and whether that is worth fixing by taking it to shop. I would like to at least take this to repairs without revealing in person there that I am a clueless idiot.

Thanks for any help in advance!