First thing first:
1. try any other stereo headphones (earbuds, whatever,) to see whether right channel is on. If not, you have problem with your source right channel - could be right channel of amp is gone or headphone jack out (line out, whatever set-up you've got) is gone. If that's the case, post your finding, I'll try to narrow it down.
2. Cable on MS1 - tried to twist it around jack or close to the right cup? Does it go intermittently on-off? If yes, cable gone open circuit on right channel.
3. Right driver on MS1 is gone (wire is lose from solder tab - not bad, easily fixable) or, worst case scenario the driver voice coil is burned open (bad, cannot be fixed DIY).
The "pop" you describe causes a bit of concern though, it looks like a source problem or overdriving headphones into short circuit (but then left channel should go as well).
See how you go.
