One of my turntables only has sound coming out of left speaker..

It could be like you said cartridge (could be a bad connector and cartridge ok) or bad contacts in amp switch try switching it a few times? also check the phono plugs coming out of the turntable sometimes the metal bit breaks off on the centre pin .If you have a multimeter check across the phono plugs to measure cartridge coil resistance both channels should be the same.You could try swapping the leads around and if the sound swaps L/R that would mean fault on turntable no sound would mean one side of amp down.

1: Faulty cartridge.
2: Faulty connection.
3: Faulty Pre-amp - in turntable or in main amp.

Check the wires to the cartridge, they can come lose and need to be tight. If that doesn't solve the problem check or replace the connecting cables. You can confirm it's not the pre-amp of your amp by just swaping the cables. Then the left speaker should play up. If it is still the right, then there's a fault on the pre-amp. But it could be just bad soldering somewhere. Also take the bottom of the turntable and check the soldering of the cables inside it.

If your left technics sounds good, maybe the problem is the cartridge. Try to interchange the cartridges, right table cartridge by the left one, and listen. If now the bad sound comes from the left table, it´s time to get a new cartridge or stylus. If the problem continues there is something else to look for. Hope this will help. Flavio/Brazil.

sounds like the cartridge lead on the left side of your cartridge is coming off. Or the ground cable of your tone arm or your table is loose.

need to narrow it down, using 1 turn table ONLY with the mixer on channel 1 (the chanel that is working) to test reciever is working properly and sound working on both side of the speakers, this eliminate the first turn table. Next, disconnect the working turn table (1) and connect your second turn table to the same chanel and see if you getting the sound coming chanel 1 just as your first turn table did. If you getting sound coming out from turn table on ch1 than is not the turn table issue. It will be you mixer having problems. You cant solve the problems having everything connected. Testing one turn table at a time using working chanel on the mixer to see if both turn table working properly. If you use the same chanel testing both turn table and the second one does not work than it will be one of your turn table is faulty. Note: if you find and test both of your turn table is working properly than it will be your mixer. good luck and hope that help..

either the cartridge is bad in the phonograph tone arm.  Or the pre-amp in the stereo is bad...remember that the turntable is the only accessory that uses the pre-amp input on the stereo.  I'm betting the pre-amp is bad.

Try some troubleshooting to check whether its the turntable or the the receiver.

What happens if you exchange the output cables of the turntable from left to right. Does the problem shift?
Does the sound changes from left to right speaker?

Similarly try on the inputs of the Receiver.. does the problem shift...?
This way you will be able to isolate the problem, whether with the receiver or the turntable....

Let me know, we can trouble shoot it further....

is it possible to swap the left and right wires on the needle cartridge? does the sound change sides? if yes bad cartridge if you still only get sound from right side try changing around the rca phono plugs