Okay. Nothing stands out so far as the clear cut issue. There are some tiny details in your pictures that causes me a few suspicions, but we’ll start methodically and with the standard assembly procedure.
afaik, there should be a retaining ring for the driver, but who knows with Lumintop. The assymetric grooves/scratches on the driver suggest to me that its loose and the ‘keying’ isn’t engaged causing it to not sit square/flat inside the head.
I don’t see in your pictures the inner signal tube either. Two things… Check the outside of the inner tube all the way around, are there any scratches or spots missing the black anodizing?
Here’s the full procedure to troubleshoot an FW light.
Clean all the threads and tube ends where they make contact with the circuit boards. Make sure there are not any nicks or burrs or splinters near these surfaces. The retaining ring would be the most common place to find this type of defect. If there are any, you should take care of them before proceeding.
Next assemble the tailcap and tighten the retaining ring thoroughly, but not overly tight.
Remove the pocket clip for now, it can only cause problems.
I’m not 100% sure the FW21 has it, but there should be a tiny white oring on the inner signal tube. Note if that’s there and assemble the tube to the tailcap. Again tighten very snuggly.
Insert a battery into the tube while tailstanding and get your DMM. Turn it on to the DC voltage function. With the red probe carefully on the + contact of the battery, and the black probe on the shiny aluminum threads, verify you have 3-4.2V (it should be the same as your battery by itself). If you have 0V, stop here. If in range, continue. (Note: based on the video, I expect to see voltage here, but this should be checked anyhow)
Remove the battery from the tube and set out of the way. Now turn your DMM to either the beeping continuity function (if equipped) or just the standard resistance/ohms position.
This can be a slightly tricky step depending on how coordinated your fingers are… Place one probe sideways across the shiny exposed edge of the inner signal tube, and the other to the shiny threads of the outer body tube. There should not be continuity here yet. The meter should not beep or display anything other than ‘OL’ or ‘OOR’ or whatever it is that it displays when you hold the two probes apart in the air. This is a very important step to get right considering your symptoms.
Next, with the DMM touching the same points, depress the tail button. The meter should now beep or display a value near 0 ohms as long as it is held down. Typical is ~0.5 FYI.
If everything checked out to this point, it would appear the issue lies within the head and contacts there.
Next, with your DMM still in the same mode as the last step, perform the same test when touching the probes to the outer gold ring on the driver and the shiny part of the threads in the head. There should NOT be continuity here. If there is this is the problem. Finding the root cause may be complicated though.
Since there is not a retaining ring holding in the driver, check with whatever small tool you have if the driver wiggles or can be rotated around in the head. If this is the case (meaning they forgot the ring and/or the glue, then this is your problem. Getting the missing part can fix your issue.
At this point, if everything checked out, I’d expect the light to work. If it’s not, and still being erratic, I would suspect a bad driver.
