I recently made a repair for this issue on an AR12 USB. Since there seems to be no publicly available info anywhere about this problem, I figured I would share a writeup. The VU meter is a modular subassembly and is likely the same across this family of mixers, so there's a good chance it is also applicable to the AR8 and AR16. The mixer I worked on had boards dated 2016 inside, no idea if the newer C versions use the same hardware or are susceptible to the same failure. In my estimation the issue didn't affect audio performance, but it's **** annoying to look at so let's fix it.
TLDR: the VU meter module is attached by soldered pins to the top face of the fader board. Remove and inspect it to find the damaged SMD components (for me it was Q501, R581, and R582--they were very obviously burnt up). Replacing the damaged components restored proper function of the meter.
You will need to be--or find someone who is--comfortable with electronics repair, and good with a soldering iron. Overall disassembly is relatively painless and these devices seem to be designed with repairability in mind, but surface mount components are tiny and this is not a beginner-level operation. The normal disclaimers apply: I am not affiliated with the manufacturer and this is not in any way official service information. Proceed at your own risk, use your own diagnostic skills, etc.
Start by pulling off the faders and knobs, remove XLR port screws, and jack nuts (use an 11mm socket). It will be easier to unscrew the XLR ports before removing the jack nuts since they help hold in place the board that bears them. On the AR12, you can skip this step for channels 1-4, as they are on a separate circuit board than 5-12 and the superchannel. Remove the plastic side rails like you were going to rackmount the unit (4 small phillips screws) to expose all the bottom panel screws (10 larger phillips screws on the AR12, 6 on the bottom and 2 on each side) and remove them. The power supply is attached to the bottom panel, so when removing the panel you will need to unplug the power input wires and the power output cables that go to one of the circuit boards.
With the bottom panel removed and the mixer face down, you will see several overlapping circuit boards. The digital interface board is dead center. To remove it, unhook the big ZIF ribbon cable and the four-wire harness that goes to the rear USB port; take out the single screw, and squeeze the wings of the two plastic retaining clips to release the board. Next remove the larger input/output board in the top left quadrant. There are two screws holding it down, and they are different lengths so don't mix up where they go! Remove the ribbon cables (one is ZIF, the other is push-in) and desolder the single wire that runs down under the board (good idea to mark the spot where it goes since it's not specifically labelled). Carefully lift the bottom edge of the board and maneuver it free. This will expose the top edge of the larger, left side fader board below.
Next, remove the 8 screws retaining the fader board and the two push-in ribbon cables on its right edge. There is a notch on the bottom edge of the fader board where you can pull it upwards. The SD card holder sticks up through a cutout in the board, so be careful not to strike it as you remove the fader board. With the board free, you will find the VU meter module is attached to its top face (knob side) by a small row of soldered pins.
You are now entering the delicate and diagnostic part of the repair. Desolder the pins that attach the VU board--you can attack these from either side, but I found the rear side of the fader board had better access than the front where there are electrolytic capacitors right next door. If you go from the front, remove the friction-fit segmented lightguides first (they pop right off). The VU board is entirely surface mount: LEDs on the top side and driver components on the bottom. There is an adhesive backed foam sheet over the driver side. As you remove this foam, you can "read" it to see where points of excessive heat may have left scorching. This will help locate the failed components.
On the mixer I repaired, it was the right channel meter that had the "stuck" lights. On inspection it was very clear where the problem was as the failed components were severely burnt. Viewing the driver side of the VU board, they were located near the middle of the left edge: Q501 (a 2N3906 transistor marked "2A") and the resistors R581 and R528 directly above it and in the same circuit. Tracing the board showed that there is a matching transistor (Q500) on the right side of the board, whose accompanying resistors are split up with one right next to it (value code 511, forgot to note the label number) and the other in the same row as the two that were burnt on the unit I repaired (value code 221). I assume that if you have a stuck light on the left channel these components are the ones you would find damaged. Since the failed resistors were too burnt to read, I used this mirrored circuit to determine the correct values of the resistors: R581 is 510 ohms and R582 is 220 ohms. Carefully desolder and replace the failed components, stick the foam sheet back on, and solder the VU meter board back in place on the fader board.
Reassembly is, as the old saying goes, the reverse of removal. Fader board back in place, 8 screws, ribbon cables. Sneak that I/O board back in, making sure the loose wire doesn't interfere with the jacks, 2 screws in (remember they are different lengths), ribbon cables in, and resolder that one wire to the spot you marked earlier (I recall it has a "+" label on the board). Interface board pops back on its mounting posts, one screw, big ribbon cable, USB header cable. Plug in the power supply cables to the I/O board and rear panel, screw the bottom panel back on, and reattach the side rails. Put your tongue at a scientific angle and fire up the mixer. With any luck the stuck meter lights should be gone.
