I would first check the volume pot. If you are comfortable tinkering with electronics, get a multimeter and check the resistance on the two channels at the volume pot terminals. If the two values are different, then that is your problem and you would need to change the volume pot.
Though volume control mis-tracking is a common problem among low-end volume pots, there could be several other reasons why your TP20 is unbalanced. But that would need a proper diagnostic check by an experienced technician.
Good luck.
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EDIT: BTW, my TP20 died completely about a month back. We had a power surge that destroyed the SMPS adapter (and also killed my VOIP modem). But instead of cutting off supply, the burnt SMPS adapter passed on the full 220V to the TP20. I got one heck of a shock when I touched the TP20 to see why it stopped playing - lucky it didn't harm my kids who usually use it. Been planning to open it up and check if I can figure out what got blown inside, but couldn't find the time.
If and when I get around to opening it, I will post some pics.
Cheers.