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Discussion Starter · #1 ·
So I have a friend who has a Cyma AK, and it's got some problems. Specifically, it does not fire if you pull the trigger too quickly, it works if you pull it slow in semi, but it's pretty iffy in automatic.

By "pull the trigger too quickly" I do not mean rapid-firing while in semi-auto.

He said that it sounds like it's trying to fire, but it just doesn't have the power, I don't think it's a problem with the gearbox, because it works sometimes, and he says it doesn't sound so much like it's jammed as much as like a weak motor or something.

Anyway, I'm gonna go take the thing apart with him today, but I'm curious to see if any of you airsoft gurus has something to tell me before I go messing with this thing's innards.

I'm guessing it's a problem with the selector plate (whatever that is) or a bad switch in the trigger, or loose contacts somewhere. I don't think it's the battery, as he's replaced it already; he said the new battery did help a little, but it's acting up again, so I think it's something else.

Any advice would be helpful, thanks!
 

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Check:

Trigger contacts maybe too much carbon deposit ( w/ multimeter using continuity test with probes on the motor terminals its quicker if there isnt a mosfet)
GB grease (more current draw)
GB shims (more current draw)
 

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Discussion Starter · #3 · (Edited)
Ok thanks. I do have a multi-meter, I should try that. Didn't think of it, so thanks. Although to be honest, I'm not sure exactly what you said, 'cause I'm not that smart. ;)

There's no mosfet, it's entirely stock. No upgrades. I do plan to grease the GB; I'm not equipped to shim it, but I'll check it.
 

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I have seen this a lot at my local field lately. Though most of the "weak sounding shots" were due to the hopup system. (Combination of bucking and chamber.)

Now when you say it does not shoot, is does not shoot a BB or it does nothing?
 

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If the gun has no mosfet, and it's had a bunch of BB's through it, I'll go with the notion that the trigger contacts are cooking, or cooked. Even with a multimeter, if you hit a sweet spot they'll still test ok or give you some continuity, but will arc like crazy when full battery voltage is applied.

You only have three components in the electrical system on that gun. It should be a version 3 gearbox on an AK, so the trigger contacts should be accessible without opening the gearbox. They're going to be the two red wires going into the front of the gearbox by the nozzle end. Now, with the gearbox out of the gun, and safely secured without being able to hurt anybody (disclaimer), put a jumper across those trigger contacts. If the gearbox cycles correctly, it's the trigger contacts that are burnt up. If not, and the motor still sounds weak, it's either the motor or the gears.

If you want to test the motor, hook it directly to the battery using proper polarity and the motor is secured in a vise, or clamp (disclaimer). If the motor goes like hell, it's good and you have a mechanical issue in the gearbox...

Oh yeah, to expand on what Plazmaburn was saying, I've seen it as well where the hop up was way over cranked.
 

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Sounds too me like the battery is too weak to properly cycle the gb. There could also be something wrong with the gb making it draw more amps.
 

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The rifles I have dealt with were all brand new. Must have been sitting in a warehouse or something as the buckings were hard and a couple looked like they were installed wrong.

End of the day, i got them fixed with new buckings.
 

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Discussion Starter · #8 ·
Ok, thanks ya'll. It was the trigger contacts.
The funny thing is, we did take the entire thing apart, because we didn't know you could get at the contacts without opening the gb until we checked here, AFTER completely dissembling the gb. So, lesson learned, and now we know firsthand how the gb works.
But yeah, the contacts were all black and grimy. Not good, eh?
 

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Nice! That's a relatively easy fix. Are you going the extra mile and popping in a cheap mosfet?
 

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Discussion Starter · #10 ·
Well, it's not my gun, and honestly, I'm not sure what a mosfet is. And what do you mean by "cheap?"
 

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A mosfet is a switching circuit in the form of a an electronic gate that lets you use a small current to control a larger current...which is how it saves your contacts. Electrical relays do the same thing, but use a mechanical coil, whereas a mosfet is electronic.

In a simple mosfet, there is three wires and is usually installed inline with the negative or 'black' wire from the motor to the battery. The small signal wire comes back from the trigger contacts (and battery +) and is used to switch the mosfet. When you pull the trigger, instead of now carrying the entire amperage load of the battery, the trigger is just switching a small signal voltage and the mosfet is doing the heavy switching.

Think of it like a gas pedal. Small effort by your foot (trigger) moves a great big machine...but by switching a throttle plate, not by pushing the whole car. If that makes any sense? (better analogies are welcome)

Anyhew, the added benefit of an electronic switch is the speed at which it engages and releases as opposed to that split second of mechanical arc that happens with mechanical contacts. In the vacuum tube business we call it tube lag. Where it takes just a smidge of a second for the tube to react as opposed to solid state or digital switching which is virtually instant.

Now a simple mosfet will simply switch current. It won't have active braking (which is super hard on a motor) or pre-cock the gun (which is super hard on the gears), it will simply switch the current. No burst firing either...that's a different circuit. But, it will help those trigger contacts last almost indefinitely and the trigger will feel more crisp and defined.

My latest version of mosfet featuring the TV18 diode (I think) and the 3034 chip is costing me around 2 dollars CDN each to make at this time. Ok, maybe 2.50 with the heat shrink and wire. Nonetheless, even 20 bucks is super cheap insurance against a burnt up gun. :)
 
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