CMEE P90 Airsoft Replica Trigger Upgrade

CMEE P90 Trigger v2.0

The trigger is a replacement for the standard crappy P90 electro-mechanical trigger of Cyma, Cybergun, Classic Army, Tokyo Marui and similar AEG airsoft replicas. 

It improves trigger response and the feel of the trigger, in both semi and full-auto. It also prevents trigger jams that often occur in semi-auto.

It requires a MOSFET, such as Perun Compact, Gate PicoAAB, Gate PicoSSR 3, Gate Warfet, Gate X-ASR and similar.

Printing Recommendations

PETG is recommended for all parts, except for the trigger button (number 4 on the photo) which is the best to be printed in NYLON or PC for durability.

Additionaly Required Parts

1. A micro-switch like the one on the photo below.
2. Thermal shrinking tubes, 1-2mm diameter
3. Signal wires
4. A simple MOSFET, such as Perun Ultra Compact, or Jefftron Micro

Instructions

1. The Kit

1. Trigger box with the switch (green) and two trigger signal wires.
   
   NOTE: Once inserted, do not try to remove the switch from the trigger box, as the plastic clips that hold it are likely to break in the process.
    
2. Lid (box side)
3. Slider
4. Trigger button
5. 2x thermal shrinking tubes (to insulate the trigger signal wires contacts after soldering them to your MOSFET)

2. Removing the Original Switch

Remove the original switch by unscrewing the 3 screws circled in the photo.

Keep the screws 2 and 3 for later.

Screw 1 will not be used any more. Don’t worry, its absence won’t affect the gearbox integrity. If you still insist on keeping it in its place, shorten it by 3 mm or use a shorter screw with lenticular head.
 

3. Scavenging the Springs

Remove screws 1 and 2 and take the old switch apart. 

Take out the two springs: the shorter spring (3) is on the contact slider, and the longer one (4) is on the trigger button.

Keep only the two springs, and you can happily get rid of the rest.

NOTE: The longer spring is usually 8-10 mm long, and its length is not critical. The shorter one varies a lot, and in some models such as Cyma, it can be only 2.5mm long when compressed (8 coils), which can be too short for the new trigger. 

Its optimal length is 5-6 mm (14 coils and is 5.7 mm long), so if your spring is shorter than 5 mm, I'd recommend buying a regular M4 Trigger Contact Spring such as this one https://tinyurl.com/y3r82bcp (or just google “trigger contact spring m4 retro arms"). 
 

4. Installing the Slider

Take the small spring and hook it to the pin inside the slider:

Hook the other end of the spring to the lower hook on the trigger box.

The lower sliding pin of the slide (pins on the outer sides of the slide) must be in the upper (shorter) of the two cutouts on the trigger box:

Rotate the slide towards the switch:

5. Installing the Trigger Button

Hook the other end of the spring to the upper hook on the trigger box.

The lower sliding pin of the trigger button must be in the lower (longer) of the two cutouts on the trigger box:

Rotate the slide towards the switch. Now it should be parallel with the slider above it:

6. Installing the Lid

Take the lid and insert its left side into the groove on the trigger box:

Ensure that the 3 pins (two from the trigger button and one from the slider) are nicely seated into the cutouts on the lid.

Press the right side of the lid until it clicks into place:
 

7. Installing the Trigger

Connect the trigger with the gearbox using the screws 2 and 3 from step 2.

The shorter screw goes near the nozzle:

CAUTION: Do not overtighten the screws since it will break the trigger case!
 

Solder the two signal wires from the trigger to the MOSFET trigger leads.

Use the heat-shrink tubes to insulate the two connections: 

8. Test and Tune the Trigger

Once you install the gearbox back into the receiver, you should test if the safety, single and full-auto fire work as expected. In case it starts shooting full auto on single when the trigger is firmly pressed, you should double check if the gearbox is properly installed, and then file away a tiny bit off the plastic trigger lever, but first you have to figure out where to file.

Put some putty or a chewing gum on the trigger lever, on the front face of the trigger button:

Slide the gearbox into the receiver and push it firmly.

The upper receiver must be removed:

You will see a dent on the putty where the metal trigger lever from the receiver pushes against the trigger button.

Remove the putty and carefully file off a tiny bit from the trigger button where the dent was.

Repeat until you get things working properly:

In case you get only single shot when the fire selector is set to full auto and the trigger is firmly pressed, try gluing a thin piece of plastic on the front face of the trigger button and adjust its thickness as described above until it starts working as expected.