Why is this happening?!

[The_Lord_Poncho]

So - I've re-built a V2 gearbox - nothing fancy, standard wiring and trigger contacts, no mosfet or anything like that.

Connecting up the motor wires to a motor outside of the receiver to test trigger function - spins the motor absolutely fine.

Assemble into a lower receiver, attach the grip, motor etc, and it fires a single cycle, then nothing more. No motor function at all - it's not a lock-up of the gearbox, because after removing the gearbox from the receiver, and connecting the motor to its wires outside of the gun, the motor will not spin when the trigger is pulled - completely dead.

Disassemble the gearbox, check all wires are undamaged, solder joints good etc, and then reassemble the gearbox - test the motor outside of the rif - spins fine. Pop into the receiver, and again - single cycle, then everything once again dead. Repeated this process another two times - same results!

Any ideas? This is made all the more painful as its not a quick change spring gearbox, and the ARL is particularly murderous in this example.

 

[Jez_Armstrong]

Case is applying pressure to the trigger unit casing it to not function, the trigger might still feel like it's doing something because it's using the tension from the trigger spring itself but not actually pushing the trolley in 

 

[The_Lord_Poncho]

Case is applying pressure to the trigger unit casing it to not function, the trigger might still feel like it's doing something because it's using the tension from the trigger spring itself but not actually pushing the trolley in 

Thank you - would this be the case when it happily spins the motor with the motor out of the grip, and fires one shot/cycle (only) when the motor is in the base of the gearbox, before then not spinning the motor in or out of the gearbox? Perhaps the shock from that one cycle of the gearbox is enough to shift the position of the trigger assembly, such that the trigger can no longer actuate the trolley?

 

[Fatboy40]

... it fires a single cycle, then nothing more.

Does this occur for both semi and full auto?

 

[The_Lord_Poncho]

Does this occur for both semi and full auto?

Mm - that is an excellent question - I have no idea, I've only ever attempted to test in semi so far - i'll try full auto tonight and see. Are you thinking that maybe the cut off lever is involved somehow?

 

[Sewdhull]

Look at the cut off lever and fire selector plate. These together with the trigger trolley allow single fire. Otherwise you'd get auto all the time.

The receiver can pinch both the fire selector plate and the cut off lever.

Is the fire selector controller installed with the receiver?

 

[Fatboy40]

Are you thinking that maybe the cut off lever is involved somehow?

Ding ding ding, we have a winner  

That's the joy of mechanical trigger contacts and no "controller" in the gearbox that has optical sensors for the sector gear. Controllers, optical sensors, and "cycle control" make life so much easier.

 

[The_Lord_Poncho]

Look at the cut off lever and fire selector plate. These together with the trigger trolley allow single fire. Otherwise you'd get auto all the time.

The receiver can pinch both the fire selector plate and the cut off lever.

Is the fire selector controller installed with the receiver?

It is indeed- it's a standard AR lower receiver, with a standard selector switch. I will take a look for pinching, however would this explain why after it has fired a single cycle once fully installed, the trigger continues to fail to activate the motor once the gear box has then been removed fully from the lower receiver?

Ding ding ding, we have a winner  

That's the joy of mechanical trigger contacts and no "controller" in the gearbox that has optical sensors for the sector gear. Controllers, optical sensors, and "cycle control" make life so much easier.

I'd really rather not spend money on this - it was only ever supposed to be a project from leftover parts - but you do make a compelling case to get a cheap T238 and chuck it in there if I can't sort it out any other way!  

 

[Fatboy40]

I'd really rather not spend money on this - it was only ever supposed to be a project from leftover parts - but you do make a compelling case to get a cheap T238 and chuck it in there if I can't sort it out any other way!  

Agree that it sounds like the perfect test bed for playing around with a very cheap controller from T238, maybe even bought from AliExpress to make costs lower  

 

[The_Lord_Poncho]

Agree that it sounds like the perfect test bed for playing around with a very cheap controller from T238, maybe even bought from AliExpress to make costs lower  

I've bought a basic T238 1.9 from aliexpress before, with good results actually. It's been in an AR for a while now, and five or six mornings of game play later its still going strong. Certainly makes things quite snappy. I previously bought it as it only worked out an extra £10 or so in comparison to the new trigger switch+ wiring that I needed and I fancied trying it out.

I had a play last night again with the gearbox - COL seems to be moving correctly, and seated in the right place etc, but I haven't fully convinced myself that it is not the culprit and may not just be playing nicely with the trigger switch - one its all screwed into place on the gearbox shell, its tricky to see the two interacting. I have also wondered if its the trigger contacts themselves - potentially the shock from the first shot making the trolley jump to the side so that all successive pulls of the trigger move the trolley contact behind the other two contacts, not between to complete the circuit  (if that makes sense). I've bent the two contacts slightly to try to make them a more inviting womb, and will see if it makes any difference tonight when i get a chance to instal into the lower receiver again.

 

[The_Lord_Poncho]

Ok, update - gearbox placed back into receiver, auto selected, and blats off all day long. Switched to semi, a single shot/cycle, then dead. COL must be messing with the trigger switch somehow, but I'll be damned if i can sort it. A T238 now ordered from Aliexpress for the sum of £21.50!

 

[Jez_Armstrong]

Like with my theory, auto would work because it's free to cycle, in semi the semi auto cut off lever would jump the trolley back over the trigger and get stuck 

 

[The_Lord_Poncho]

Like with my theory, auto would work because it's free to cycle, in semi the semi auto cut off lever would jump the trolley back over the trigger and get stuck 

yep - looks like it!  In your post and that from Sewdhull you mention variously the casing is too tight, or the receiver is too tight. The trigger switch and CoL are actually the ones that came with the gearbox and it was working fine before i stripped it down to swap a few bits out - everything swapped except for the CoL, trigger, trigger switch (how many times has that been said!) - so in my mind at least, this indicates that the problem is not with the gearbox case itself, but rather Sewdhull's suggestion that the receiver is somehow squishing the selector plate and/or CoL. An SRL gearbox in an old metal G&G LR300 receiver - possibly just a compatibility problem with the only rectification being the fitting of the cheap FCU - or is there another way do you think?

 

[Sewdhull]

You haven't cycled the gearbox with the receiver off have you? Just run the motor? I was thinking you had cycled the gearbox...

I would do this and observe the cut off lever and the trigger trolley, check the trigger itself catches the trolley again. 

The fire selector plate if it moves ok, wont be the issue. The cut off lever can be trapped in the receiver but its more likely the trolley getting lifted and not getting dropped again. You should be able to see it all happen through holes in the gearbox case near the cut off lever

or cycle just the trigger, trigger trolley and contact block with the sector gear and cut off lever in place in one half of the gearbox.

A new trigger block and cutoff lever costs very little tho

 

[Pseudotectonic]

cut off lever spring?

 

[The_Lord_Poncho]

You haven't cycled the gearbox with the receiver off have you? Just run the motor? I was thinking you had cycled the gearbox...

I would do this and observe the cut off lever and the trigger trolley, check the trigger itself catches the trolley again. 

The fire selector plate if it moves ok, wont be the issue. The cut off lever can be trapped in the receiver but its more likely the trolley getting lifted and not getting dropped again. You should be able to see it all happen through holes in the gearbox case near the cut off lever

or cycle just the trigger, trigger trolley and contact block with the sector gear and cut off lever in place in one half of the gearbox.

Thanks - yes, you are correct, i hadn't 'manually' cycled the gearbox first out of the gun - probably ought to have. But without the box having a QD spring, and an ARL that is a particular nightmare, I was being lazy and trying to minimise the number of times i needed to close the gearbox up. Looks like I failed on that particular quest. Thank you for your detailed explanation - this is super helpful - i'll investigate further.

cut off lever spring?

 It seems to be in place ok - if you mean the tiny spring that goes between the selector plate and the top of the COL where it pokes through the gearbox casing slightly? possibly a stronger spring could help?

 

[simonh]

just be careful with the t238 I had lots of "fun" making mine work as it should. The error I was getting was with the selector gear, but it was actually the selector plate needed trimming, took me ages to figure it out.