Does the light work if you take off the tail cap and use as paperclip or something to connect the battery's negative and un-anodized part of the light (like the tail threads)?
I just fixed a friend new 501b. It had a faulty tail switch. Try removing the tail cap and shorting the negative terminal of the battery to the body. If it lights up, it’s the switch, if the issue persists, it’s the drop in.
Hmm, sounds like its down to the drop in now.
Do you have a soldering iron? You could pull off the led centering disk & just take a look at the led connections.
I believe fasttech has a return address somewhere in the US (Florida maybe?) if they ask you to send it back. I got a 502b from t-mart and within the first hour of use I managed to break it. Luckily for me it was just a faulty connection in the tail cap and I managed to fix it fairly easily.
Good luck getting it sorted out!
It's a P60-based light, its entire reason for being is that the part that makes the light is easily replaceable. Keep it, get your refund or credit (or not, either way you haven't lost much since the drop-in was likely crap anyway), and just pick up a new drop-in from somebody here who builds them.
Before tossing the original drop-in, test it out of the light, sometimes they have problems with getting a good ground connection due to that stupid big outer spring they use in stock form. Hold the driver against the +, and with a piece of wire connect the battery - to the side of the brass pill.
Another thing is that if you're using a protected cell, it could be drawing more current than your cell can supply without tripping. Saying 'I know the battery is good because it works in another light' is completely irrelevant since two different lights are likely to have different current draws.
I think the 4x XML2 SRK has a higher current draw than this 500 lumen dropin. I tried doing the wire trick and same thing, very faint blip on the emitter and that’s it.
The thing is, this light was working when I got it but started having issues same day. From random turning off to not turning on at all now.
Not necessarily so. Especially if you haven't checked these things with the meter. Real data has a nasty habit of making 'common sense'-based guesses look foolish.