Reverse the cables from the turntable to the preamp. If the volume reduction moves to the right channel, it's not the preamp.
I've seen selector switches go bad in spots, so while there are just wires, things can go wrong with individual inputs on preamps. Take the extra minute and check everything.
You know the how it goes. The one thing you don't check is what ends up being wrong..
But yeah, I suspect the cartridge as well...
I changed the turntable connection to a different input on my linestage preamp and there was no change; the right channel was still much louder than the left. I then swapped the left/right channels from the tonearm cable to my phono preamp, and the problem switched sides. At this point, I figured the problem was either the cartridge, tonearm cable (which is relatively new), or the phono preamp. I then swapped back the left/right channels on the phono preamp, and the problem disappeared. Everything is back to normal now.
I suppose I'll just wait til the problem occurs again (if it happens again) to see if I can determine what's wrong. If it was either a cartridge, tonearm, or phono preamp issue, isn't it odd that the problem just fixed itself? Strange.
Another issue could be the cantilever on your cartridge is out of position. This could be caused either by mishandling or defect in the cartridge.
Get a pair of magnifying lenses and look at the cartridge straight on from the front. The cantilever should be evenly positioned from left to right in the cartridge body with no tilt on the stylus. If it appears off, this could be generating the volume difference since the far end of the cantilever will be out of position inside the body of the cartridge.
If this is the case, sometimes removing the stylus assembly and reinserting (if removable) will correct the problem. Otherwise you might need a new stylus assembly.
Also check the weight and anti-skate settings on your tonearm, and double check the overall alignment.
Are you using the arm lift to drop the stylus into the zerodust?
If you are hand holding the zerodust and pushing it up on the stylus it will cause the cantiliever to be stressed side to side and can damage the stylus. One of my local dealers stopped carrying these because of his customers damaging their stylus.
I had this problem with a Sumiko Bluepoint Special and cured it by gyrating the cantelever around in a very small circle. I don't know why this worked but as a last resort I was just screwing around with it trying to find sht was wrong and got lucky. It has croped up once again with the same cart and was cured the same way.
Thanks for everyone's responses. I figured out what was wrong. It appears the connection from the cartridge to the tonearm cable was the culprit. When I originally attached the very thin tonearm leads to the pins on the cartridge, I just slipped them on; they are a bit on the loose side I'll be the first to admit. Obviously the connection wasn't very good, causing the sound from the left channel to fade out. All I did to fix the problem was just wiggle the connections and everything was ok.
So how do I prevent this in the future? I was thinking of using a pair of needle nose pliers and just crimping the connection to make it fit tighter. I'll only do this as a last resort because the cartridge pins are SO small and I don't really want to damage anything. Are there any other options for me? Any suggestions would be appreciated!
The old toothpick trick works perfectly. It prevents you from squeezing the clip too much and flattening it out. A round wooden toothpick works best, just the shape you want and it "gives" a little.
Don't use needlenose pliers unless you also use a light touch. Finger pressure is often enough IME. Like any electrical connection, clean surfaces and a snug fit do work best.