Bosch driver board repair

[dark_hawk]

Hi,
I'm trying to repair this Bosch GSR 10,8 V-LI driver board, and I'm at my wits/knowledge's end.

https://www.boschtoolservice.com/de/de/bosch-pt/spareparts/akku-mittelgriffschrauber-gsr-10-8-v-li-3601J92U00

The fault is intermittent which is a bad fault to try to find. Sometimes the board works fine, other times it does not work at all, and other times it works at like half speed for the motor.
The machine does not have variable speed when holding the trigger half way, it's either on or off, disassembled the switch to make sure of that.

What I have been able to gather so far:

1- The IC TC1411NE is mosfet gate driver, pin 1 and 5 are Vin, Pin 4, 8 ground, pin 2 is the input, pins 6,7 are outputs, and pin 3 is NC.
When Pin 2 input has 4.2v, pins 6,7 (outputs) has 6 volts, so the gate of IRL7833 is at 6 volts and the motor is running at full speed.
When the gate is at zero volts the motor is off, but i've been able to measure volts ranging from 2.5-5v at the gate, the motor then is working slower, and mosfet and the double package diodes are getting hot.

2- Pin 2 (Input of the mosfet driver) is connected to pin 1 of an SMD SOT23-6 (marked V6 on the board) with a marking MA, and its the same voltage as pin 2 of the mosfet driver. I have no idea what that smd package is and what it does, my guess is that it gets some enable signal from somewhere and pin 1 gets pulled high to 4.2v which in turn enable the mosfet driver, just a guess though.

3- When shorting the input of the driver to any 6v rail, and motor runs at full speed no problem, remove the short, it's back to the original state, either not working or working at half speed.

4- Shorting R30 resistor (right side of the board) also makes the motor run at full speed no problem. The resistor value is correct out of board.

5- I've checked all diodes and capacitors on the board and all seem fine. Also checked most of the traces with no breakage in the traces I can see.

How do I proceed?

Thanks.

[fzabkar]

MAx and A2x could be dual PNP and NPN transistors.

NST/NSV3904DXV6, Onsemi, dual NPN transistors, 40V, 200mA, marking MAx, SOT−23/SOT−323:
https://www.onsemi.com/pdf/datasheet/nst3904dxv6t1-d.pdf

NST3906DXV6T1, Onsemi, dual PNP transistors, -40V, -200mA, marking A2x, SOT−23/SOT−323:
https://www.onsemi.com/pdf/datasheet/nst3906dxv6t1-d.pdf

TC1411/TC1411N, Microchip, Inverting/Non-inverting, 1A High-Speed MOSFET Driver, 4.5V - 16Vin, 8-Pin MSOP/PDIP/SOIC:
https://ww1.microchip.com/downloads/en/DeviceDoc/20001390F.pdf

"8E527" in the SOT89 package appears to be a 3-terminal regulator. The marking is probably "8Eyww" (yww = Year/Week).

http://markingcodes.com/search/c/8e

IRL7833, International Rectifier, MOSFET, n-channel, 30V, 150A, 3.8mOhm RDSon, TO-220AB:
https://www.infineon.com/dgdl/irl7833pbf.pdf?fileId=5546d462533600a4015356600d71257b

I expect that the variable speed is achieved via PWM. That is, the motor current is being switched on and off rapidly. This means that you would be measuring the average voltage at the gate, not a constant voltage.

[Circlotron]

Whoever did that board layout should be put in jail.

[dark_hawk]

Whoever did that board layout should be put in jail.

I don't have any experience laying out boards, but It has to fit inside a relatively small enclosure and be a specific shape to be secure between the two halves.
I won't be too harsh on the one who did the layout.

[dark_hawk]

What would be the purpose of the 224a Low-power quad operational amplifiers in a circuit like this?

[fmashockie]

What would be the purpose of the 224a Low-power quad operational amplifiers in a circuit like this?

Something needs to be controling the gate driver thats controlling those MOSFETs.  Quad-OP amps can sometimes be used as PWMs.  Maybe that is what it is doing?  I would follow that input (pin 2) to see what is controlling this.  A scope would be helpful to troubleshoot this issue.  You need to see if the gate driver is working properly and being controlled properly.  Can't really do that without a scope.

[dark_hawk]

I think I phrased my question in the wrong way.

What would be the purpose of the Quad-Op amp/Mosfet/ Mosfet driver in a screwdriver without speed control switch or a potentiometer in the trigger to control the speed.
Granted there is a kind of a potentiometer in the trigger but it's not working, it's connected to V28 on the board which is unpopulated, which means there is no speed control here.

Reason for asking is, if the "Quad-Op amp/Mosfet/ Mosfet driver" has no purpose here, then I can force the input of driver to be ON by shorting either resistor R30 or R12. Which will turn on the mosfet and let the motor run.

Edit: One purpose would be of course to use the mofet and the diodes to keep the motor at max voltage regardless of the input voltage. The should be some other temperature protection going on, but can't find anything resembling a thermistor on the board.

Also, MA and A2 are not transistors, and not Schmitt-Trigger Inverters, and I have no idea what those parts are.

[fmashockie]

I think I phrased my question in the wrong way.

What would be the purpose of the Quad-Op amp/Mosfet/ Mosfet driver in a screwdriver without speed control switch or a potentiometer in the trigger to control the speed.
Granted there is a kind of a potentiometer in the trigger but it's not working, it's connected to V28 on the board which is unpopulated, which means there is no speed control here.

Reason for asking is, if the "Quad-Op amp/Mosfet/ Mosfet driver" has no purpose here, then I can force the input of driver to be ON by shorting either resistor R30 or R12. Which will turn on the mosfet and let the motor run.

Edit: One purpose would be of course to use the mofet and the diodes to keep the motor at max voltage regardless of the input voltage. The should be some other temperature protection going on, but can't find anything resembling a thermistor on the board.

Also, MA and A2 are not transistors, and not Schmitt-Trigger Inverters, and I have no idea what those parts are.

The MOSFET is being driven by a PWM of some kind to control the speed of the motor.  You might not be able to adjust that speed, but it still needs to be controlled nonetheless.  It is an electronic screwdriver right?  That is likely going to have a motor that is designed to rotate at a relatively slow speed.  So you need a PWM circuit to control that speed.  The other TO-220 next to the MOSFET (1st photo) is likely the flyback diode.  If I was trying to fix this, I'd be interested in what is driving that MOSFET gate driver and is it working properly.  Again, you'd need a scope to troubleshoot this effectively.