Premium SK68 (14500 Zoomie)

No crazy MT-G2 or XHP50 leds in this build, this build is focused on making a cheap and nice ZTF (Zoom to flood) light that can used a practical daily carry

I’ve always liked all kinds of ZTF lights, since they offer the versatility of bright and large flood beam for lighting up a big area or a focused pencil beam for long distance use, one of my favourite lights, while I have played with its larger SK98 (18650) cousin, this version is almost the same but takes in a 14500 cell (AA battery size) instead

The best part? You can probably snag a Sk68 for less then 10 bucks (Heres one that costs $6)

ZTF, unfortunately lose a lot of light when compared to their more tradition reflector/TIR variants as a lot of light is lost at the sides of the LED, whereas a reflector “collects” the light emitted from the side, losses of more then 50% and up are common, depending on the zoom level and design

Here is a typical SK68

So i just happened to have a SK68 lying around for sometime, this particular SK68 was one of the very first lights i purchased when i got into the hobby, so it carries a sentimental value as well

Almost all flashlights come with a form of protective layer in order to protect the light from scratches or wear, in aluminium lights, this is called hard anodizing (or HA), where an electric current is applied to a chemical solution to coat the light and forms a protective oxide layer

This light comes with probably type 2 anodizing (unlike the more durable type 3a found on more expensive lights)

I decided to strip it off to expose the bare aluminium, but how do you remove anodizing from a light?

The easiest and fastest method is to use sodium hydroxide (common in drain cleaners from your home store), it chemically reacts with the aluminium to remove the anodizing quickly and within minutes, please remember to wear gloves and wear googles, you do not want that chemical to touch you whatsoever

Once all anodizing is removed, it is a simple matter (abeit a lot of patience) to sand off the metal using 500, 800, 1000 and 1500 grit sandpapers, followed by polishing it with metal polish (autosol in my case) which will result in an almost mirror finish

When sanding, it is important to change directions and sand first horizontally, then vertically if not swirls may be present, essentially when you are sanding, you are “scratching” the surface, which higher grit of sandpaper will “scratch: away the scratches made by the previous grit, you keep moving to a higher grit and you will essentially reach a grit where the scratches are almost invisible

With enough patience, you can get something like this

SK68 when compared to its larger SK98 cousin has a smaller flood area, due to it physically having a smaller aspheric lens and the LED being further away from the lens (the nearer the LED to the lens, the larger the beam will be)

So in this case, LEDs with higher surface brightness relative to size should be used, in this case, smaller LEDs like XPGs or XPEs or larger dedomed LED (dome removed from LED), this will result in less light being wasted as the lens only has to focus a smaller LED

So I set out to improve the available flood and increase its throw slightly, doing some rough sketches and visuals, I figured that I could file off about 2-3mm of the top of the pill so that the sliding head can slide closer to the led, thus improving flood, concidentally, the driver itself took up almost 3mm of space at the bottom, causing the pill to fill up the gap when screwed in tight, thus equalizing the space used for sliding, the result, the LED is now closer to the lens.

I inserted a rubber o-ring on the outside and a glow o-ring in the inside of the head so that when the head is screwed in, makes the head protrude out slightly, increasing its throw slightly (it eats into the flood improvement above, but the net gain is still good), the graphic shows the before and after as well as simulated beam shots

The original driver that came with the light was removed and replaced with a 7135 based driver, with 8x 7135 380ma chips for a total of approximately 3amps, this driver comes loaded with the guppydrv firmware, a very nice and versatile driver that allows you to select your own modes (22 modes to choose from)

This driver does not fit as the SK68 pill is smaller then the diameter of the driver so I insulated and taped the driver to the pill

Unlike the other SK98, this Sk68 came with a hollow pill, which is sort of a lottery then buying these kinds of flashlights, to fix this, I stuffed thermal cubes into the empty space between the driver and the pill and stacked a few copper discs until it was flush with the “hole”, the thermal cubes are elastic and springy, so provides back pressure for the LED star to rest against

Copper also has much better thermal conductivity then alumnium, so in my case i actually “upgraded” the heat sinking capacity of the pill

A 20mm notigon copper DTP star was cut and hand filed to fit into the 15mm of space on the from of the pill, here it is with a XPE2

After testing with the XPE2, while it throws really well, its flood mood is very dim, and the flood beam is VERY weird, with many black zones in the beam, in essence, a very ugly beam. I reflowed the new osram square 4000k 92 cri to the star, at 3a, it should be able to put out about 500-700 lumens? to prevent artifacts in the beam, I put a sandpaper “gasket” on top of the LED, as well as blacktaping ALL shiny surfaces in the head

To improve throw, the osram was dedomed by slicing it with a sharp knife. I sliced it as close as I could and left it there (no more spare to risk)

As with all my lights, I very much prefer forward clicky switches, it was a PITA to get the original reverse clicky switch out, and I wrecked the plastic gasket trying to get the switch out.

Fortunately, the gasket was not needed, the forward clicky will fit with a little filing and rounding of the switch’s square profile, one of the leads at the switch must be insulated as it makes contact with the tail, insulate the spring side of the switch

Finally, the light is ready to go, next to the MT-G2 T10T
I replaced the original tail clip with the tail clip from the T10T, it is much better and does not “rip” your pants like the original clip does”

The flood beam is very uniform, MUCH better then the original beam that came with the light (forgot to take before modding)
The zoom pattern is like an “H”, projecting the die pattern of the osram

Also, because the dome was sliced, there is little if no tint shift, resulting in high cri light, both flood and zoom!! Imagine a high cri spot beam


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