No sound from one tweeter - Acoustic Research AR 318PS Speaker

You will need to determine if it's the amplifiers or the tweeter themselves that are not working. If you open up the cabinets and check each tweeter with an ohm meter for continuity it will show if the tweeters are working, in which case the amp is the problem. if the meter shows that the tweeters are blown, you should still hook up another tweeter or speaker to the horn amp to see if it's blown and therefor took out the tweeters.
If any of this sounds complicated, you'd be best off taking the cabinets to the repair shop.

Replace tweeters with units having the same impedance as originals so amplifier loading is the same.

If unable to get exact replacement for unit, then it is important that you also replace the corresponding tweeter(s) on other channel since all speakers sound different and have different efficiency. Result will be unbalanced sound.

In the most general, basic sense, I would be assuming that this is just one speaker and not 2 with this problem, or 3 or more.
Only way to get the ball rolling is pull the tweeter from the cabinet, checking to make sure all wires are connected at the tweeter and inside the speaker.
If that is the case, then you would need to determine if this is the tweeters fault, or its crossover (thing that divides the sound into whats right for the woofer and what the tweeter wants).
If you have a multi-meter, you would disconnect the wires from the tweeter, set the meter to ohms and touch a probe to each speaker terminal. No action means burned out tweeter, 100% action means burned out tweeter, a reading slightly less than the rating on the back of the tweeter (for instance an 8 ohm tweeter would show anywhere from 6 to 8 ohms on the meter) means tweeter is good.
If you have no meter then take the tweeter from the good speaker and swap in place of the bad one, making sure you respect + and - connections.Its a little risky this way,since if there is something wrong in the speaker it may hurt the good tweeter..(the meter is the safer bet). If it works, then you need a replacement tweeter, if not the problem is either wire connections inside the speaker, or something is wrong with the crossover.

The crossover caps may have failed, or the tweeters themselves burned out. Using a small 1.5 volt cell touched across the tweeter leads should produce a sound. If not- they are burned out.

You didn't answer if they ever worked right.

One doesn't usually judge tweeter function visually. They don't actually move much.

Take a cylindrical tube (like from a paper towel) and place it over the tweeter to isolate it from the room sound and the woofer, then place your ear at the other end. Anything? Nothing?

What sort of signal are you feeding them? Let's look (or listen) to the source and how its audio gets to the speakers. Define the electronics, pathing and setup.

Maybe some history. Have you ever tried to blow them up in your enthusiasm for the new rig? If the tweeters are dead, that may have happened. Microphone use and feedback = dead tweeters.

Sound like the tweeter is blown, take to local jbl agent for service.

You have to give the tweeter a good old juicy **** and it will go back to normal.

A Tweeter being out of phase should not have any effect on the sub.

If one tweeter is out of phase with the other there will still be sound in the room as due to the high frequency the unequal time to your ear will mean you won't get perfect destructive interference and the signals won't cancel. Also the two tweeter signals convey much less information in common than a subwoofer or bass speaker.

Regards,
Andrew

they sound like there blown to me

The only way I can think of to check is to disconnect the leads to the tweeter, then check resistance with an Ohm-meter...no reading means dead speaker.