[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.