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Troubleshooting: Stampede Firing Uncontrollably

For those of you who upped the voltage

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#1 Zack the Mack

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Posted 04 November 2010 - 01:35 PM

I just finished a Stampede with 14.5v battery pack, and when I pull the trigger, the blaster fires uncontrollably. Unlike the common catch-spring problem that SGNerf identified and troubleshot, this is caused by a "race" condition between the plunger tube and the actuator gear.

I'm posting this thread to get some help solving this problem and chronicle any solutions for future modders. I'll put up photos as soon as I can.

Here's the problem:
The Stampede's plunger tube is pushed forward by a large gear which is about 2/3 toothed and about 1/3 smooth (untoothed). When the motor rotates the smooth part in, the plunger-tube assembly can slide freely and is pushed backwards by that weak black spring you can see through the window, which resets the blaster.

Normally, the gear turns fairly slowly, exposing the smooth part long enough for the spring to fully reset the plunger assembly. This releases the white motor switch, cutting power to the motor.

However, when the voltage is increased, the toothed part of the gear might rotate back in before the plunger-tube assembly has fully reset. This means that the white motor switch is never released, and thus the blaster fires uncontrollably. This can also cause the gearbox to become misaligned, which creates further problems and can damage the mechanism.

This is the "race" - the plunger tube needs to fully reset before the untoothed part of the gear rotates out.

Even if you haven't upped the voltage on your Stampede, it's likely that this problem could happen anyways once the lubricant wears off and the plunger-tube assembly returns more slowly.

I tried replacing the black spring with a more powerful one, and it still didn't reset fast enough. Any ideas?
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#2 pjotrkuh

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Posted 04 November 2010 - 02:55 PM

Maybe you should use a lower voltage, I use 12 volts and a stronger plunger spring without any problems....
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#3 MavericK96

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Posted 04 November 2010 - 03:56 PM

SGNerf, Spartan, and myself have been doing testing on this issue over at the NerfSG forums. We concluded that it can be due to a number of issues, but here are some of the likely causes:

1. Mangled/misplaced catch control spring (seen below)

Posted Image

Posted Image

(Photos taken from Nerf SG forum)

Either reset the spring, or replace with a stronger one (a pen spring works well if it's the right size)

2. Black plunger return spring not strong enough (I know you said you tried this, but stretching this spring helped me solve my problems)

3. Needs additional lubrication on the plunger o-ring (usually if you've replaced the o-ring with a stronger one, such as the one in the Stampede Upgrade Kit that SGNerf sells)

Messing with these things fixed the problem for me. I've been using 2x 7.2v battery packs (~16-17V fully charged) in my Stampede and had the auto-firing issue, but after doing these fixes it's working great now.

Edited by MavericK96, 04 November 2010 - 03:57 PM.

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#4 Zack the Mack

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Posted 15 November 2010 - 12:35 PM

I've fixed the problem!

I replaced the stock spring with one from a Big Bad Bow. I had to cut off about two coils to make it fit, but it works fine.
The increased spring strength requires more force to compress and thus the gearbox turns more slowly. Less momentum on the main gear means that it has more time to reset!

The blaster fires hard and fast, and I haven't noticed any heat or damage in my postmortem.

Long story short: If your Stampede is firing uncontrollably and your catch spring is fine, upgrade the spring.
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