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I'm more of a software guy than hardware, so I haven't used mixers much. I was surprised at what I was finding w/ a small used Behringer Eurorack UB-1202 I grabbed on eBay. I was running a single channel from my player into each input to test with a 1/4" TS plug. Everything was working fine until I got to the paired (L&R) line inputs #9-#16. When I plugged the cable into each of the left-side inputs, all was good. But when I plugged into each of the right-side inputs I could barely hear a signal. I double-checked the pan settings and they were all centered. I tried jiggling the plug, although the odds of all four having a bad jack was infinitesimal. The markings on the mixer show "mono" for the Ls and "BAL OR UNBAL" for the Rs. I might have a misunderstanding of the inner workings, and I was not able to find documentation to give me a definitive answer. But I was expecting a TS plug would work the same in both the L and R inputs. My two theories of why that isn't true a

1) A TRS plug is needed for either a balanced or unbalanced signal because the R side inputs connect to the RING not the TIP. (If this is true, then is it true for all inputs that can be both BAL or UNBAL - they always need a TRS plug?)

OR

2) There's some auto-switching between the channel inputs of the same line whereby if you plug something into the R channel, you have to have something plugged into the corresponding L channel.

Appreciate any augmentation to my knowledge on this topic.

Thanks,
- DK