No stereo - one side only

Probably not.

Determining the Cause of the Problem Let's assume our system has a head unit, a crossover, a two channel amplifier and two door speakers with only the right speaker working (the left speaker is not working). If the door speakers are behind the door panels then we want to make sure the speaker is the problem before removing the panel. We'll work backwards starting with the amplifier and ending at the head unit. Testing for a Speaker Problem With the vehicle and the stereo turned off disconnect the speaker wires for both the left and right speakers at the amplifier. Now connect the left speaker to the right amplifier output (which we know works). Turn on the stereo system and make sure the balance is set to the middle position.If the speaker still doesn't work then the problem is somewhere between the amplifier and the speaker Testing for an Amplifier Problem Now we'll need to see if the amplifier channel is bad. Put the left speaker back on the left amplifier channel and the right speaker on the right amplifier channel. This returns the speaker wiring to its original configuration. Again, with the vehicle and stereo off unplug the RCA cables from the amplifier's input and switch them so the left cable is in the right input and vice versa. Turn on the stereo system. Testing for an RCA Cable Problem For any intermediate components between the amplifier and the head unit follow these steps for each one. We only have a crossover so we'll just do this once. Begin by testing the RCA cables between the intermediate component and the amplifier. Unplug both RCA cables from the intermediate component's output. Now plug the working channel's RCA cable into the non-working side of the crossover. In our case the working channel is the right so plug the right RCA cable from the amplifier into the left side of the crossover. Turn on the stereo system.
If the right channel continues to work then you know the problem is in the RCA cables between the amp and the crossover. esting for a Crossover or Equalizer Problem At the input of the intermediate component (in our case the crossover) flip the RCA cables around (left to right input and vice versa). Turn on the stereo system.
If the right channel continues to work and the left channel continues to not work then you know the problem is in the intermediate component. Replace or repair this component.
If the problem is now in the right speaker and the left speaker works then the problem lies before the crossover. Turn off the stereo system and return the RCA cables to their normal places.
Testing for a Head Unit or RCA Cable Problem Pull out the head unit from the dash so you can access the RCA cables. Unplug both RCA cables from the head unit. Do not disconnect any other wiring. Plug the right RCA cable into the left channel of the head unit. Turn on the stereo system.

There is probably a setting that is wrong. The left channel is the default for mono operation. Look in your user manual regarding "PAN" which controls the output going side to side. You MAY be "panned" clear to the right. When you plug into the left, it is the default so in spite of being panned right, they would send signal to the left IF nothing is plugged into right.

your question depends on how good you are at electronic diagnosis (symptom diagnosis) and how adept you are at removing and installing electronc components. first you have to determine why the amplifier has stopped working on the left channel. Make sure that the problem isn't a simple one, take the speakers off the right side and hook them to the left side and see if you get sound. If you do,, then the problem is the left speaker or left speaker wire. If you don't get sound then tyr taking the input that feeds the right channel and pluggin it into the left channel and listen for sound, if you get sound now the problem was the signal from your source not being sent down the left side.
If it still doesn't work you need to move inside the amplfier, but first, get close to the back of the amp and give a sniff, do you smell anything (a pungent burnt wire or rubber smell) if you do, the the chances are good that some components have burned out. Do you recall having a shorted speaker wire on the left side? shorted speaker wires are notorious (especailly older equipment) for burning out the final amplifer stage. If this was an expensive amp it might be worth fixing, if its not worth much then maybe you should consider religating it to use as a monophonic amplifier (one channel) you could use it to power a sub woofer in a hibryd stereo or surround sound system.
I don't know what more I can tell you at this point. If you go through the steps I outlined, and want to tell me the result I shall be glad to respond and maybe give you another direction to head in. good luck and I'm glad you are adventruous enough to want to try.

It's likely there is a fault on the power amp. This is located on the heatsink. A visual check inside the unit might be what you need. Look out for blown parts, bulging capacitors or burnt components. Remember the two channels are generally separate and identical. So identify the two channels. Work back from the speaker sockets if you can't tell. Then compare the left channel parts with the right side components. If it looks odd on the left compared to the right you have found the problem.

First look inside and determine which is the left channel. Then look at the area around the heatsink. Examine the semi-conductors for damage. Remember it's a stereo amp, which means it is just two mono amps put together. So the right side parts will be the same as the left. So if you find something destroyed, it will have a copy part on the other channel. If they are transistors you can test these with an Ohm meter.
Another way of testing is with a screwdriver. If you touch a part (audio) with it (don't short it) it will buzz. So if it buzzers on the right it should (in the same place) on the left.
DO NOT TOUCH POWER PARTS.

It's unclear what type of connectors are on the end of the cables(s) coming from your computer.

This amp only works with mono 1/4" TRS and XLR connectors, not standard RCA connectors.

Did you try the Right one in the Left Channel to see if the Left Channel worked?

Refer to the following manual and read up on the connectors (page 7) and the use of the Stereo/Parallel/Bridge Switch (page 8). When it is NOT in Stereo Mode the unit only uses the Right Channel controls and input.

http://img3.musiciansfriend.com/dbase/pdf/man/m_480824.pdf

STEREO/PARALLEL/BRIDGE switch
Use this switch to select the operating mode.
• STEREO mode
Channels A and B operate independently (as with a conventional
stereo amplifier). The Channel A input goes to the Channel
A output jacks, and the Channel B input goes to the
Channel B output jacks.
• PARALLEL mode
The Channel A input signal is output through both the Channel
A and Channel B output jacks. The Channel B input jacks do
not function. Channel A and B volumes can be independently
adjusted.
• BRIDGE mode
The Channel A input signal is output from the BRIDGE output
jacks. To adjust the volume, you must use the Channel A volume
control knob.

I have had the front, left and right channels go out over time. I have fixed all of them, and each time it has turned out to be a cold solder joint. While I love NAD and will continue to purchase there equipment, this generation is plagued with poor workmanship.

To fix the channel going out, I removed the top cover. Back cover and 2nd card from the left (looking at the receiver from the back side). This card handles all of the audio processing. I have found cold solder joints all over this board which has caused channels to suddenly go quite. I have never found a bad component. Also, I have cleaned the interconnect to be backplane (board that the audio processing board connects to) in the case of the center channel going quite.

If you have poor FM reception, the first board from the left (looking at the receiver from the back side) may have a cold solder joint on the coaxial connection for the FM antenna.

Can you please update this with your connections. I assume that you are connecting the turntables to the mixer? Verify that the mixer is set to send out true stereo. What mixer are you using?
Note that most mixers "pan" all of the channels to the center sending out mono to the left and right. In most live shows, it makes no sense to send out true stereo. The left and right are for the speaker chains. In order to get true stereo out of the mixer, you wil need to pan the input channel for the left side all the way to the left and the input for the right channel to the right.
Dan

Wonder if the return line from both speakers was wired to a common earth, and you've lost that connection? If that happened, you'd end up with the two speakers in series instead of in parallel, and just playing the difference between left and right channels (so very much quieter). Check also the stereo doesn't have a separate fuse (possibly internal) for the power amp stage. A.