TL;DR: Bad design and quality problems. EDIT: but can be fixed.

Bought AT40 on GB. The plan was to put Optisolis in it.

First disappointment – dead on arrival. Reason – battery tube is too short and does not make contact with the driver. I thought: not a problem, I need different driver for Optisolis anyway.

The original driver in AT40 is glued. After heating with standard soldering iron, it was possible to remove it. Driver diameter is 21,6 mm. I replaced it with Nanjg 105E, 20 mm, with some copper pieces around the driver it is perfect fit now.

But…

After testing it a bit, a problem with connection was discovered. I thought that the problem was my copper hack around the nanjg. Tried to redo it with no success.

Tailcap is also glued. The only way to insert battery is by unscrewing the head.

EDIT: tailcap is not glued.

Then it struck me. Maybe the problem is with the switch. I took multimeter, turned the switch on, measured voltage between battery positive and an edge of the tube. 3.85V. Seems to be ok, but if you apply some pressure on the battery, voltage reading goes nuts.

EDIT: for the fix see #8

Album with all pictures