Trigger contact melted
 

Okay, Im looking for the cause of this problem,

here is the context:

One client bought a new AEG. He took it home and fired off a few rounds, and then it "just stopped shooting". Now this AEG I've seen personally, probably seen it gamed at least twice and been fired 5000rd ish with lipo, no problems.

So he comes to me, I try to fire off a few rounds, no sound. Contacts aren't even touching each other because it made absolutely no sound. So I take apart the gearbox, and here is what I see

http://img535.imageshack.us/img535/985/dscf1984z.jpg

The trigger half of the contacts have utterly melted the plastic around it. NO OTHER PART OF THE CIRCUIT WAS BURNED. Now I know this can happen if the contact hits volcano temperature, but how this happened is not known to me.

A few things:
-He used 7.2v Nimh batts, not a lipo
-Gun fired perfectly before

What could cause so much current to build up in only this one part of the circuit? Any ideas? I'm asking so that replacing this part won't just repeat the problem.

Thanks

Poor shimming or a motor adjusted too tightly can create a huge current load. The result is massive current arcing as the trigger comes close.

Electrical arcing can easily reach temperatures high enough to literally shoot a hole through the metal. He's lucky only the plastic melted, so it can be saved. I think I have a couple of old smoked trigger circuits around here with holes burned through the metal contacts.

He probably jammed the gun and kept pressing the trigger. When you opened the gun, you might now have remarked that you unjammed it. Especially if he was using a cheap mini 9.6 that was barely charged. Not enough to spin the gun, but enough to melt the trigger.

Is there a fuse in the circuit?

Further to the above, the OP noted that the gun has shot 5000 rounds with lipo in the past. So...it could be the way the new owner is interacting with the trigger. I am making no judgement, but ensure that he is fluidly and fully pulling the trigger. Messing around with partial pulls is the best way to cook your trigger contacts (because of the arcs that Illusion describes).

yeah... a 9.6v NiMH even on a well tuned AEG WILL melt the contacts
The reason is melted the trigger and nothing else is because the trigger is your point of highest resistance in the electrical system. And resistance = heat. Holding the trigger down for really long bursts at that voltage would cause that kind of damage for sure.

Here's a pic I took of my buddy's FULLY upgraded M4. prometheus gears, shimmed properly, magnum motor, 350fps, 9.6v 4200mah NiMH batt. Shot around 1200rpm. This was after about 8000 rounds, always short bursts.
http://photos-c.ak.fbcdn.net/hphotos..._3456408_n.jpg

And Amos MELTED his triggerblock on his RS Type 97B by using a 9.6v 5000mah NiMH batt lol

Solution : MOSFET switch or triggermaster!

just to clarify, it was a 7.2v 3000mah nimh battery. too strong maybe?

It was a 9.6 battery not a 7.2, besides i doubt the 7.2 could barely flip the moter so no that would not be to strong, it would be to weak.

Thunder did he mod the RS so the battery would fit(somehow) or did he just use an external peq box?

I ran my M249 off a 7.2v 4200mah battery before I had a triggermaster, at 370fps it gave me a nice ROF of 900rpm :)
However a 7.2v mini battery would likely do nothing but make your motor click lol

And he had a HUGE AN/PEQ box on the type 97B

What he do drill holes into the side of the gun lol

My mistake, it was a 7.2 volt, Andrew over here is the client.

The 7.2volt was a nimh, not even a lipo, and it still shot about 1500 rounds before doing this Andrew says.

So I'm assuming this is a current overload from lack of power now.

Well it depends on what material the trigger block is made of, I know some airsoft plastics are more prone to heat than others. Clearly there was little arcing going on, but there was massive heat buildup. All that means is that the plastic had a low melting temperature, and the metal piece was just a very poor conductor, thus is built up heat very quickly. If he was doing long bursts it would definitely melt that thing.

Once again, solution: MOSFET or SW-COMP
Alternatively you could replace it with a G&P switch, they're pretty tough

http://img.photobucket.com/albums/v2...e/DSCF5193.jpg
http://img.photobucket.com/albums/v2...e/DSCF5196.jpg

And the wires were tucked around the body gaps :)