# switch audio input leads - on amplifier, on turntable, at cartridge. Holding input lead with finger will produce a hum - is it the same level on both channels. That should get you close to the problem source.
# 2. If at cartridge, check alignment of needle and record grooves.
+
=== Update (04/15/2022) ===
+
You can remove the cartridge and touch the terminals that the cartridge connects to. This will eliminate everything except the cartridge. Make sure you do it while the turntable is running as most have a mute switch that shorts those connections while it is changing/stopping. If the needle was crooked, the cartridge might not be far behind.
+
+
If you still get one channel with a hum and not the other, then it is either the mute switch or a connection to the output jacks, or weak center connector to the RCA plug = assuming the plugs are fully inserted. Twist and push to be sure.
+
+
I don't see a mute switch, and only 2 of the 4 cartridge wire will produce a hum. The other 2 are ground. Also, don't touch the table with the other hand as you are doing this. It may severely reduce the hum.
Process of elimination.
# switch audio input leads - on amplifier, on turntable, at cartridge. Holding input lead with finger will produce a hum - is it the same level on both channels. That should get you close to the problem source.
# 2. If at cartridge, check alignment of needle and record grooves.