troubleshooting for a lost channel

I recently lost all sound from my right speaker, but only for the phono rig. How can I go about figuring out which component is to balme? I have a rega p1 (about 4 years old) cambridge 540 preamp and a cambridge 640 amp.

Start by reversing (left output to the right input) cables from the TT to the pre. If the problem moves to the other channel it is the TT or cartridge. If the problem doesn't change put those cables back and reverse the cables from the pre to the amp - then the speaker cables.
Now that I re-read your question and it only happens with the TT That pretty well says it all. It's either a bad cable or cartridge. If you swap the right and left outputs of the cartridge you will know it is the cartridge or tonearm cable or something else related to the TT
all it took was removing the wires connected to the cart and putting them back on. the blue one may have slipped off. its not the best connector, just a "C" shaped head that snugs onto the cart prong. good thing i didnt have to switch the left-right outputs on the, white, blue and green cables from the tonearm, and i have no idea whos who there.
thanks for the suggestion anyway, it worked!