Need Help on my ML 26

I was listening to my system when I realized that my left channel had more gain then my right. I traced the problem back to the preamp. Now I am stuck, I don't have a schematic so I can't troubleshoot it. I suspect that a transistor might be the culprit.
Is there anyone out there who had this happen to them or familiar with the topography of the mother board so you can provide me with some ideas where to probe? If you happen to have a schematic, could I get a copy of that schematic?
Any help that would make my 26 healthy again will be greatly appreciated.
This c-o-u-l-d be carbon deposits or trace deterioration on the pot's sweeper. Usually it's at a certain area on the sweeper. But this would happen slowly over time-- not suddenly.

OK, you don't have a schematic, but do you have an Owner's Manual?

I don't think the problem is in the volume potentiometer. It's sealed and of the highest quality. You might try gently rotating the two balance pots back and forth since you don't use them a lot, but they are also sealed. The possible 'switch' devices which MAY be dirty, in order of likelyhood are:
1. The preamp's global gain DIP switches (these are the tiny rocker switches inside the preamp near the front panel.) These should be rocked back and forth (on/off) to make sure they are making good contact (and then of course reset to their original configuration -- unless you want to increase/decrease the main output of your preamp.) There are other DIP's inside to check as well. depending on what kind of input options (phono, balanced) your preamp has.
2. The mono-stereo and the phase switches (you don't use these a lot, so they could be dirty -- gently switch them back and forth a few times)
3. The tape monitor 3 position switch -- gently rotate back and forth.
4. The source selector switch (not likely a problem since you use it all the time ;-)

Don't forget to check your interconnect fittings, and your main output CAMAC adaptors (if you're using single-ended RCA interconnects.)

Peter and Neil,
I will go ahead put into action your suggestions and I should have an answer in the next few days.
I have set my preamp DIP switch to the lower gain since I am running xlr. It was originally set for the higher gain. I wish they had installed the input xlr in my preamp.
Thanks Peter and Neil for all your help and I will be back in a day or two with some feedback on the status of my preamp.
W_barton, I'm glad to hear you are using the XLR main outputs since that allows maximum performance from your preamp. What kind of amp are you driving?
I have a ML 27 and together they are a wonderful match. I have a Macintosh MVP861 as my source player and Legacy Focus for speakers. I just perused your system and I you have a Mac/ML too. I am presently using Kimber kables 8TC as my speaker cables, but I am curious about the straightwire cable. They say that Straightwire and ML are a match made in Heaven. I noticed you have a set too. What do you think of the cables?
And Thank you all who have offered their help with my problem, I really appreciated your time and effort.