Broken anti reverse latch

[mattmanic]

Hey,

 

I just built my self an AB mosfet, I installed it, reassembled the gearbox and went to test it. It fired 2 shots then jammed with the piston about half way back. Naturally I stripped it all down again and reopened the gearbox and then my anti reverse latch fell out in two pieces.

 

What could've gone wrong? Is it possible that I've miswired it somewhere and the motor's polarity was reversed causing it to spin into the anti reverse latch? Would it've gotten two shots off before it met with it?

 

 

And which brand should I replace it with? Is Guarder OK? I've found them for £6.50. Systema is £11, Prometheus is £15.

 

It is a CA SCAR with (as far as I know) a stock gearbox and a JG motor. I've got a Li-Po arriving tomorrow so I want it fixed soon

 

Thanks

Matt

[Utty]

CA SCAR with (as far as I know) a stock gearbox

There's your problem. CA internals tend to be hit or miss.

 

Just bung in a new ARL. Personally I use stock ACM ones in some of my guns, and the nifty Modify ones in the rest. The Modify ARL has the spring attached with a C-clip, so is less fiddly during mechbox maintenance.

[mattmanic]

So this has nothing at all to do with the mosfet? Seems a bit of a coincidence that as soon as I add that something decides to break.

 

I'll get a modify ARL then, it's only £12. Although if I could be bothered I could make my own out of a bit of 8mm steel. I'll just get the Modify one I think...

[Utty]

I have no idea, I've never heard of something like that happening. I can't see how the mechbox would be able to cycle correctly for a couple rounds before the ARL broke, if the motor polarity had been reversed. Maybe the MOSFET isn't working properly, the AB function may be reversing the motor polarity for too long?

 

All I know is CA internals aren't known for their excellent quality, so one can expect some malfunctions sooner or later.

[Stealthbomber]

If you're worried about the mosfet why not simply check the polarity of the juice going to the motor?

Disconnect the wires to the motor and attach a digital multimeter to them.

Attach the red probe to the wire that usually goes to the front motor terminal and the black probe to the one that goes to the back.

Pull the trigger.

If the meter reads "9.6v" then you're fine but if it reads "-9.6v" it means you've got things the wrong way around.

 

I'd bet it's just a coincidence though.

[mattmanic]

Found the problem, I whacked out the multimeter as advised and discovered that the gear box body is "live". So I'll have to break it open again and rewire the whole thing by the looks of things.

[Stealthbomber]

FWIW, that's not always the end of the world.

 

The body of a lot of AEGs is often live.

On some AEGs a screw through the trigger contacts will make the gearbox shell live and then, if the gun has a metal body, it'll make the whole gun live.

Alternatively, some motors are built so that the -ve terminal shorts against the motor housing which, in turn, makes the motor cage, gearbox and anything touching it live as well.

 

I had a big problem with this on my M14 EBR.

As soon as I hooked up a battery it started firing. WTF?

Turned out that the entire gun is "live" and the back of the motor cage was touching one of the motor terminals and making it fire!

 

Another common hassle with this is if you've got tightly-fitting batteries in a metal space such as a stock-tube.

If the insulation on the battery is worn through it can short against the metal surface and cause problems.

 

Your biggest problem, and it IS a pretty big one, is that if your mosfet is living in a part of the gun where there's metal you absolutely CANNOT risk letting the mosfet touch any of the guns bodywork cos it could ruin it.

 

It is often unavoidable to have a gun that is live.

As I say, often the way the trigger contacts are designed means they'll always make the gearbox live and that, in turn, means a metal gun is live as well.

It's nothing to worry about really.

I mean, by way of comparison, you entire car is "live" too.

The body is the negative terminal for all electrical connections.

 

So, yeah...

Have a look at it but don't spend too much time trying to figure out why the body is live cos it might just be unavoidable.

The main thing to worry about, once you know it's happening, is to make sure not of your wiring is damaged, none of your connectors (especially Deans) can short against the body, your motor terminals aren't touching the motor cage, there's no damaged insulation on your battery and, most importantly, that your mosfet isn't touching anything it shouldn't be.

[mattmanic]

Well, I rewired the whole thing with proper high temp 16 gauge wire, it only just fits in the channels, and I don't seem to be getting anything through the body anymore but I've also checked my mosfet out as the P one got very hot and melted the electrical tape I'd used to insulate it. It was also blowing fuses as soon as I plugged the battery in before which would lead me to believe there was a problem with one of the 'fets. Luckily, you could only buy them in packs of two from RS so I can just make another one up with my spares. I'll do that then see what's happening with it.

[mattmanic]

Right! New fet installed, new wiring done, stock wiring holes hacksawed to fit new stuff. I made a recording of before, with an 8.4v mini and after with 16awg wiring throughout, an AB mosfet, deans connectors and a 7.4v 1600mah 25c lipo.

 

The quality is a bit rubbish but you can definitely hear on semi it's much quicker with the trigger response. They were recorded on my phone, which for some reason thinks it's a good idea to record in .amr which the forum doesn't recognise when you try to attach it. So, you'll need to download them and change the file extension to .amr ! Then you should be able to play them with media player.

Before.mp3

After.mp3