I recently did a trade deal with a member here for a PVS14. Upon arriving I noticed the gain control knob got extremely hard to turn at the point towards the upper limit of its rotation. In other words the last say.....1/4 turn was extremely tight to turn.
Upon very close inspection I noticed a small crack running from the knob outward for roughly 1/4-3/8".  
Here's my questions........ with the crack being where it is, has my unit lost its "purge" or is the battery compartment separate from the tube compartment and also is the battery compartment something I can replace myself?

Yes it's all the same compartment. Yes you can replace it yourself.

Just go easy when you remove the pigtail and the auto-off lines ( need some needlenose pliers )

They are not all that expensive.

Yes, you have lost your purge. You can DIY with a can of air duster ( just to get rid of moist air in the unit ). Or don't bother about it.

For someone that has never been inside a PVS-14 it can be a little daunting so let me add a little detail to clear the air. It is fairly easy.

You need a clean work area.

There should be 4 screws at the corners of your battery compartment. I have seen these in Philips and in hex key styles - regardless, those 4 screws are what holds it together.

The new housing probably does not include a new O-ring seal so you need to buy a new one or recover the old one.

I like to use the plastic sticks we use in electronics manufacturing for prying the pigtail contacts off of their connections rather than pull them with the pliers. If you don't have the plastic stick then you can substitute a small flat screwdriver.

Your old battery compartment is now separated.

For reassembly, I get the pigtail contacts in place with the needle nose pliers and then press them in place with the stick. One of them is in a difficult spot so I an take a few tries because the pigtail has a mind of its own - just take your time and don't get frustrated.

Get the battery compartment aligned with the main housing and GENTLY work the screws in a little at a time until the O-ring compresses and the gap closes. This is where most guys that botch their installation start to cry. They either crank one corner of the compartment so they crack it or they over tighten the whole thing. Just snug it until the seal is compressed.

Losing a purge is not a spiritual failing on your part. It happened. Get past it. Get the compartment replaced and we'll walk you through the purge.

New guys are great! I really appreciate the response and explanation on the process. The unit in question is a Night Combat Solutions and I was thinking of contacting them about replacing the battery compartment for me but after your responses I think I will tackle it myself.
Now the $100 question.........where is the best place to buy the replacement and what should I expect to pay as far as a far price. By the way......it's a single AA unit. Thanks again guys!

They used to make a CR123 battery housing shell. You could solder a couple wires and transfer your electronics over. Was a easy DIY project and the shell was cheap. Google for it to see if it is still sold. I did one years ago. But yes you lost any purge you had. As David said canned air is the way to go. I have the pro kit and bottles of nitrogen. I still use the canned air deal on my own gear. Quick, easy and efficient.

A new single battery lower housing is 2 to 3 humdred bucks. Just shop around as many dealers sell them.

Here are some videos that might help. It will also help if this is your first time take you phone and take pics of each step when you take it apart. It really is not complicated. Also if you just by chance have a portable dehumidifier put it in a room and just get the humidity down real low when you reassemble. I use a shop vac to vac everything out as a reassemble. Also if you touch lens on the inside clean them with a lens cloth and lens cleaner. Not windex

But you should not have to complete disassemble to replace knob.

Video1

Video2

Looking at the videos, I'm pretty confident I can handle this job!
Now to find a quality replacement that don't break the bank.

If you use your unit in nasty environments then you'll need to tackle this right away. If you have some time then wait - one of the guys puts a couple on the EE from time to time for $75 each.

You don't know whom he is do ya? I'll pm him and ask. Thanks

I don't remember his ID. Does anyone else remember? He was listing two per week for a while then he dropped out. For the total amount he was listing it looked semi-permanent but it's possible he's out. I would definitely shop, though. I got one for $115 when I needed one. You could always be proactive and put a WTB ad in the EE. That might flush him out.

Was it this one?

Battery Housing

It was exactly that one. He kept relisting like he had a bunch. I recognize the ID now that I see it.

To be on the safe side you could take a cell pic when you take off the battery housing. From what I've seen people usually try to install the pig tails upside down.

I have not seen anyone do that yet. Does anything smoke or does the unit just fail to operate?

Sent an email. Will advise upon his response. Thanks guys

Follow up. The guy had one left. New ITT in the plastic bag. Got for $75 shpd! Thank a lot guys. It should be here Friday.
Now, feel free to share your tips on purging! LOL

I like to do a rebuild on a dry day, if possible, but I see your state indicator and I know you might have some trouble waiting for a dry day. If you can't wait for a dry day then you can run the A/C to dry the air.

I use an almost empty can of "canned air" type product. The chemical mix has little to no moisture in it. I don't use a fresh can because it blasts so hard out of the can that I get propellant inside the PVS-14 and I don't want it to leave residue on the tube or lenses. I use the cans that everyone else wants to throw in the garbage because they are "empty".

1) Gently flood the PVS-14 through the purge port using the straw that comes with the can.
2) Replace the purge screw but don't torque it.
3) Let the internal atmosphere settle for several minutes to dilute the remaining moisture.
4) Repeat steps 1, 2 and 3.
5) Repeat steps 1 and 2.
6) Torque the purge screw.

Now that's the kind of instructions even I can understand! I really appreciate you taking to time to share this!  Thank you!!