systema electronics issues

[mcguyver]

i had the opportunity to work on systema m16a3 PTW ver. 2 last week. the owner had several systemas that had failed due to a problem with the low-resistance switch assembly. the problem came to my attention as i have many years experience in the electronics and electrical field and started out as a discussion but progressed into me tearing open one of these beauties. here's what i found on the inside:

systema uses a low-resistance switch assembly couple with interlock switches for a realistic "bolt release/motor interrupt" circuit to lock out the motor when the mag is empty. the trigger is basically a microswitch that either sends voltage (or not) to certain electronic components to allow the motor to turn and fire the gun (or not, like when the mag is empty). one of these components is a circuit board in the stock of the gun that contains some transistors, resistors and a 20 amp fuse. this board connects directly to the battery and passes all the voltage and current necessary to operate the gun's motor and electronics. the basic operation is you select a fire mode, which sends voltage thru the "bolt release sensor" to the base terminal of the transistor and triggers the collector/emitter junction to close, thereby sending voltage and current to the motor, thus firing the gun.

the problem that i encountered when the gun landed on my bench was that as soon as you connected the battery, the gun would fire. you did not need to select a fire mode or press the trigger. the problem really could have been anywhere in the system and only a methodical and thorough search would find the problem. decided to start with the dry-contact switch board and see where that leads me. to access it fully for testing, you need to remove the stock of the systema and pull both wiring harensses outside of the gun to disconnect them. one harness has three wires that contain the +,- and - base control lead for the transistors and is very fine #26 awg wiring. the other harness is for the motor and is a + and - #16 awg wiring. once the wiring harnesses are diconnected you can pull the entire assembly back thru the stock and out the battery door to proceed with testing.

once i had the assembly out i could start with my testing. i noticed immediatley that the whole assembly was encased in an aluminum heatsink with a single screw holding the circuit board firmly inside. i removed the screw and slid the board out to see what lay within. i notice several surface-mount resistors and 2 transistors along with a 20 amp ato-mini fuse. the transistors were a 50N06 transistor which i cross-referenced via the NTE website to an NTE2395 part number. the data that i pulled up for the NTE2395 was that it was an n-channel 50 amp mosfet transistor. the switching circuit for the systema uses 2 of these in parallel giving the switch a theoretical capacity of 100 amps. this is not a reasonable figure for the actual required current draw for a running motor in an aeg, but on initial start-up of the motor the current may approach these levels for a fraction of a second. i also noticed that there was no thermal bonding compound (heatsink compound) between the transistors and the heatsink. as there was only one screw holding the board in the heatsink (it went thru the mounting hole of one of the transistors), this left one transistor with effectively no thermal bonding to the heatsink and left it vulnerable to heat damage.

i immediately began to test the circuit for continuity and noticed an extremely low resistance (6 ohms) thru the switch. this told me that the switch was closed, an impossible situation for an undamaged and unpowered transistor in this arrangement. i isolated and tested each transistor and found both to have extremely low collector/emitter resistance (28 and under 10ohms respectively). i expected to see resistances in the high kilo-ohm or even mega-ohm ranges. i saw the problem as here and proceeded no further as this was the "smoking gun". i took the board to the electronics tech we use for contract work and gave him the board and told him my tale. he told me he's not surprised as these particular transistors are sensitive to voltage spikes and rough treatment (excessive heat, etc.). replacements were very common and apparently many products in many different areas use these transistors. about 10 minutes and 2 new mosfets later the board was ready for re-installation. i insisted that the new transistors be thermally bonded to the heatsink, so a dab of heatsink compound was applied to each transistor and the whole board was re-inserted into the heatsink. now the switch assembly was ready to be re-installed in the gun and tested.

with a 7.2v battery charged up i loaded the mag and proceeded to test the gun. it fired flawlessly. i tried differing bursts on semi and full auto and ran thru several mags to see if the gun would make a hiccup. it didn't. i was satisfied that the problem was solved and ended my repair to the systema there.

i've thought about what may have caused the problem to see if there was a way to keep it from happening again. the best hypothesis i have is this: 1 transistor failed first (the one not thermally bonded to the heatsink from factory) and the other transistor failed very shortly after the first one as the motor would have run on immediatley after the first transistor failed. the high current and resulting heat failed the second one closed as well, leaving the circuit in the state it was when it arrived on my bench. but why did the first one fail? my best guess is it resulted from alot of semi-auto firing in rapid succession. this would pass alot of current in each start cycle of the motor and this would show up as large, quick pulses of current. the transistors would both have heated up, but with only 1 partially bonded to the heatsink, the failure of the unbonded one first was inevitable. this is supported by my lower collector/emitter resistance, meaning more current passed thru longer on that transistor and literally "fused" the collector/emitter junction closed. this caused the motor to run on and the second transistor would have failed in seconds.

the big question now is will this happen again and and what can be done about it? the answer is it may, but with proper thermal bonding, the transisitors may very well last a long time and never fail again. for factory original ver.2 guns this may yet happen, if you do rapid semi-auto fire and the conditions are right. but then again it may not fail. full auto fire is preferable in this instance as there is only one start-up cycle per burst and the running current of the motor is much less than the start-up current.

for systema owners who have factory original ver.2 guns, should you do anything now to your guns. i say no. if it hasn't failed yet, it very well may not. but if it does, you can take comfort in the fact that the repair is very easy and very inexpensive. the repair cost should only be about $50 to $60 and parts are easy to get. i have not seen a ver.3 systema's circuit to see exactly what has been changed, but from what i hear it is an expensive upgrade and solves this issue. do i believe a poor quality battery caused this issue and would cause it again. the answer is no, but it doesn't pay to have a $2000 gun and a $20 7.2v rc car battery from Bob's Hobby World. a quality battery can be had with little problem from wgc or your local battery supplier.

i hope this gives some info to those who own systemas (a small but fearsome force) and helps you out if you need it.

[thePiRaTE!!]

Says you Sir! http://www.bobshobbyshop.com/

very thorough... nice. Wonder if Systema know this. Perhaps a little consultation on one of their new products is in order :wink:

[MadMax]

Is there any diode protection to prevent the impedance spike from a disconnected motor from applying a high voltage to the driver board?

I like to protect anything driving DC through coils with a reverse biased diode in parallel to the component to clamp any inductance jump. I'm not sure if it's an issue with DC motors, but anything with a coil in it like a solenoid valve or even some mechanical relays can pop out a pretty high voltage spike when current is quickly interrupted.

You also get a lot of noisy commutator related noise on DC perm' motors. There should be a capacitor across the motor terminals. While the noise is mostly an issue with R/C cars (RF interference) you can get more brush wear and it can be a bit hard on transistors.

Is it possible to screw the heat sink tabs on the FETs into the heatsink? Thermal junction compound helps a lot, but it's conductivity is still crap compared to conduction thru aluminum. Minimizing the gap between the FET tab and the HS can make a big difference. However, if the AEG worked for awhile before it burned out, the thermal issues must be marginal. A goopy application of TJC should bring you well into the working range if the transistors died because of thermal issues.

[mcguyver]

the lack of a blocking diode was my first suspicion, but after looking at the switch and reading the tch articles on the z-shot website, i don't believe that was the issue here. i agree that inductive loads should be shunted thru a diode to protect related electronics as the inductive spike can be in the millions of volts. systema uses a dynamic braking system for there motor, which can simply be a resistor engaged across the motor terminals to dissipate the cemf present as the motor continues turning on inertia after the running voltage is removed. this braking system, couple with the spring load, will reduce the cemf drastically, leaving the collector/emitter junction the job of blocking the remaining spike. this may have led to a failure as well, but it's impact was negligable compared to the other problem. as far as diodes go, systema should use a seperate blocking diode system to accompany thier electronics, but it would have to carry the full start and run current of the motor, and this would be impractical due to the size of components required. their system overall is good, with the issue being mainly on the switch board. i am interested to see the reliability of the repair, but if factory assembled switches generally last, this one is far better.

[JohnnyDo]

I am interested to know the NTE cross reference of the mosfet used to fix the board and it'll serve as a good reference here on the forum for other users.

I also agree with MadMax that having a cap accross the terminals of the motor to help with the RFI, as well help with brush life. I guess this info would only be good to electronics guys and gun doctors but when you think of a $2000 gun .. a $0.10 cap is nothing if it helps the life of the motor.

Macguyver:
"but it would have to carry the full start and run current of the motor"

if you have a diode over the motor terminals to such to block any voltage spike once power to the motor was stopped

(+)--|<---(-) where the () are the motor terminals .. why would you need the diode to hold hte start/run current of the motor? The diode just needs to handle the current of the voltage spike once the field colapses. Not steping on toes .. just asking.

[mcguyver]

true, to a point. at the instant the field collapses, the cemf will be nearly as high as the emf generated by the motor supply voltage. as the field continues to collapse, the voltage generated will be very high as time will be very small. a large diode like a 1N4005 will be good for 1000 of those volts prior to breakover, and will absorb a large spike in excess of that. you also have to factor in some motor reversal (small) and the effects of the dynamic braking (resistive) to hte inductive load. your net current (for a short time) will be high. most will be wasted as heat thru the braking resistor and the rest will be blocked at the collector/emitter junction. i believe that is why systema chose not to use a blocking diode as a seperate component. they must have felt that between the braking resistor and the 2 fets, that should be plenty. and besides, a large inductive spike is far more likely to damage a processor than a fet, and without seeing the processor, i can't confirm how much direct diode protection it would have. i can tell you though that the fets have none.

the website is www.nteinc.com they have a cross reference area. they only supply data for their part #'s and not the original that you input. as far as a cap across the motor leads, there may be one, but i haven't opened it up to see. at any rate, either there is one or not. neither concerns me too much as rfi is generally a non-issue for aegs and brush life extension is based on so many other factors of much larger importance, that it isn't worth my effort to install a cap in a difficult (and probably tight) spot and one that is prone to damage from a variety of things.

[MadMax]

How is the braking resistor connected? If it's switched in before current is interrupted (make before break) then there's somewhere to dissapate the field. If the current is stopped before connecting the resistor, you still can get a induction jump.

Really, there should be a high current rated diode in series with the braking resistor always connected to the motor terms. That way there is no time allowed for the field collapse to pop a high voltage to the FETs.

You don't need a high voltage diode. As long as there's an easy current path, there's no reason for a high voltage spike. The inductance of the motor only tends to drive a constant current. If there's a low impedance path, there will be a low resultant voltage.

All the resistor does is offer a place to dissapate a bit of heat (the energy stored in the field).

What is the rating on the braking resistor? I would guess that it should be somewhere in the 5ohm range to keep the voltage below 100v (assuming 20A just before cutoff). Because it's only 5ohms, it can't be left in parallel to the motor. It has to be switched in when the current is cutoff (if you don't use a diode).

[mcguyver]

the braking resistor wasn't located on the switch board and may be located in the grip. as far as opening the grip to see the motor, etc. i didn't do that. they may even have a blocking diode there, but i didn't look for that. to be honest, i would think that systema may use another fet to switch in a braking resistor and it may be located either in the grip or on the fire selector circuit board. to access that board would require a teardown of the lower receiver that was beyond the scope of this repair. perhaps there is an issue there that systema has resolved when they update the PTW with a new electronics version, and i may yet see it.

[Ryan-John]

Hey Mcguyver! I was wanting to buy the Systema Switch Assembly for my CA M15A4 CQB, but which one? the Switch Assembly or the Switch Assembly Ver.2? Also I would ofcourse need to buy the Systema M16 Selector Plate.

If I can't use the systema sa, how would this Guarder do? >> Guarder Switch Assembly

Used with this >> Guarder M16 selector plate

[ILLusion]

That part has nothing to do with the Systema PTWs, which is what this thread is about.

All of the Systema Switch Assemblies which (I'm assuming) you're referring to are all for version 2 gearboxes. The difference is in whether your gun is front (handguard/LAM) or rear powered (butt stock).

You don't *need* to buy the Systema M16 selector plate to use with the switch. Unless yours is broken or burnt out, there isn't a real need to replace it.

The Guarder one will work like the Systema one, but the wiring is just not the same low-resistance silicone wiring of the Systema one. It doesn't really make a difference until you go to extremely high power setups.

[Ryan-John]

Originally Posted by ILLusion

That part has nothing to do with the Systema PTWs, which is what this thread is about.

All of the Systema Switch Assemblies which (I'm assuming) you're referring to are all for version 2 gearboxes. The difference is in whether your gun is front (handguard/LAM) or rear powered (butt stock).

It will be front powered once I get the G&P PEQ2 Battery Case w/ 9.6v 3300mAh Battery. What does this mean?

[ILLusion]

that means you get the one that's for.... front powered!!

Some links for where you're shopping from may help us help you.


http://www.wgcshop.com/pcart/shopper...m=SYS-ZS-07-16

Systema part # ZS-07-16

[fasterbassdrums]

man you guys are smart! i gotta quit smoking pot, or take more schooling or something. props guys

[mych]

good post!

[mcguyver]

Since this thread is 3.5 years old, and the Gen 1 has long since been discontinued, there is not much good in there anymore, unless you own a Gen 1 PTW that requires repair. If that is the case, it should be upgraded with Gen 4 (2008) electronics.