asdf53 wrote on 2025-10-19, 15:13:

boby wrote on 2025-10-17, 10:31:

Is it like this? […]
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Is it like this?

If that is correct numbers for pins then I have ~ 140 kOhm between 1 & 3, and ~ 100 kOhm between 1 & 2
Power is off in this case

Power connected but the motor switch off:
Pin 1: 4.12V & Pin2: 4.12 V (battery is 4.19V )

Again, these are the measurements if the pins are numbered as I marked on the image

Sorry, I didn't see your reply at first.

Is the motor connected or disconnected when you do the measurements? It needs to be connected.

Was disconnected, I will redo it now with motor connected.

asdf53 wrote on 2025-10-19, 15:13:

Did you really measure 4.12V from the source pin of the transistor (pin 2 in your photo) to the negative pin of the battery connector? It should be 0V. If it is really 4V, it means that the white resistor (R010) is broken.

No. That voltage is 0V.
4V is Pin2 on resistor and black wire of the motor, only when I turn on the switch ON. No voltage when it is OFF

asdf53 wrote on 2025-10-19, 15:13:

Also measure the voltage from the drain (pin 3 in your photo) to the negative pin of the battery connector.

4.18V switch ON or OFF (it is the same)

Please tell me what resistance to measure again? With battery connected and do I turn ON the switch or not?

This is what I did:

Case 1: battery ON, switch ON or OFF (it is the same)
pin1 & pin2 ~ 25 Ohm
pin2 & pin3 show 1 (indefinite?)
pin1 & pin3 show 1 (indefinite?)

Case 2: battery OFF switch OFF
pin1 & pin2 ~ 100 kOhm
pin2 & pin3 started at 12 MOhm and dropping bit by bit
pin1 & pin3 started at 12 MOhm and dropping bit by bit

Okay, let's make it a bit more clear. When I say "measure the voltage of X", it means put the red probe on the test point and the black probe on the negative terminal of the battery connector.

Measure the voltage of the following points, according to the pin numbers on your photo:
- Pin 1, 2, 3 of the transistor

Do this two times, first time when the motor is supposed to be off, and a second time when the motor is supposed to be on. Especially the voltage on Pin 1 is imporrant, but if possible, all three pins.

asdf53 wrote on 2025-10-20, 15:58:

Okay, let's make it a bit more clear. When I say "measure the voltage of X", it means put the red probe on the test point and th […]
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Okay, let's make it a bit more clear. When I say "measure the voltage of X", it means put the red probe on the test point and the black probe on the negative terminal of the battery connector.

Measure the voltage of the following points, according to the pin numbers on your photo:
- Pin 1, 2, 3 of the transistor

Do this two times, first time when the motor is supposed to be off, and a second time when the motor is supposed to be on. Especially the voltage on Pin 1 is imporrant, but if possible, all three pins.

Switch OFF (motor supposed NOT to run):
Pin1 0V
Pin2 0V
Pin3 4.18V

Switch ON (motor supposed to run):
same as above

Okay, this means that the transistor does not receive a signal to turn on. When the motor is supposed to run, pin 1 should change from 0V to a high voltage (4V or more).

Use your multimeter in beep mode from transistor pin 1 to the eight pins of the small chip next to the transistor. If there is a connection, then this chip is probably a PWM controller or a gate driver that's used to turn the transistor on and off. Can you identify the model number on the chip?

asdf53 wrote on 2025-10-20, 19:00:

Okay, this means that the transistor does not receive a signal to turn on. When the motor is supposed to run, pin 1 should change from 0V to a high voltage (4V or more).

Use your multimeter in beep mode from transistor pin 1 to the eight pins of the small chip next to the transistor. If there is a connection, then this chip is probably a PWM controller or a gate driver that's used to turn the transistor on and off. Can you identify the model number on the chip?

No beep on any leg. Baterry is disconnected. Number is not very visible
But it could be
TC4056 or TC4050
And then below it
A037944 or A057944

boby wrote on 2025-10-21, 14:01:

No beep on any leg. Baterry is disconnected. Number is not very visible But it could be TC4056 or TC4050 And then below it A0379 […]
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asdf53 wrote on 2025-10-20, 19:00:

Okay, this means that the transistor does not receive a signal to turn on. When the motor is supposed to run, pin 1 should change from 0V to a high voltage (4V or more).

Use your multimeter in beep mode from transistor pin 1 to the eight pins of the small chip next to the transistor. If there is a connection, then this chip is probably a PWM controller or a gate driver that's used to turn the transistor on and off. Can you identify the model number on the chip?

No beep on any leg. Baterry is disconnected. Number is not very visible
But it could be
TC4056 or TC4050
And then below it
A037944 or A057944

4056 seems to be an IC for battery charging. Then this is not the right one. Do the same beep test from pin 1 of the transistor to the large chip with 14 legs on the other side of the board, and if there is also no connection, then test from transistor pin 1 to the pins of the three smaller transistors on the other side of the board (Q2, Q3 and another one).

The purpose of this is to find out which device on the board is supposed to send the control voltage to the transistor. If possible, try to find out the model number of the large chip as well.

asdf53 wrote on 2025-10-21, 14:23:

boby wrote on 2025-10-21, 14:01:

No beep on any leg. Baterry is disconnected. Number is not very visible But it could be TC4056 or TC4050 And then below it A0379 […]
Show full quote

asdf53 wrote on 2025-10-20, 19:00:

Okay, this means that the transistor does not receive a signal to turn on. When the motor is supposed to run, pin 1 should change from 0V to a high voltage (4V or more).

Use your multimeter in beep mode from transistor pin 1 to the eight pins of the small chip next to the transistor. If there is a connection, then this chip is probably a PWM controller or a gate driver that's used to turn the transistor on and off. Can you identify the model number on the chip?

No beep on any leg. Baterry is disconnected. Number is not very visible
But it could be
TC4056 or TC4050
And then below it
A037944 or A057944

4056 seems to be an IC for battery charging. Then this is not the right one. Do the same beep test from pin 1 of the transistor to the large chip with 14 legs on the other side of the board, and if there is also no connection, then test from transistor pin 1 to the pins of the three smaller transistors on the other side of the board (Q2, Q3 and another one).

The purpose of this is to find out which device on the board is supposed to send the control voltage to the transistor. If possible, try to find out the model number of the large chip as well.

One chip leg beeps when testing with Pin1 on the transistor. Unfortunately, seems there is no any marks on it.
Three smaller transistors do not beep with Pin1 on the transistor

That's going to be not so easy without knowing what the pins do. Things we can test for now:

The large chip is likely a microcontroller. The pin of the microcontroller that leads to pin 1 of the transistor, test its resistance to the negative battery terminal (should be very high).

Test if any of the two diodes near the microcontroller (D1, D2) is connected to pin 1 of the transistor. If yes, test it using diode mode of your multimeter.

The on/off switch of the device, is it a momentary switch, a toggle switch or something else (a dial to change speed or so)?

With the power switch on, test if the microcontroller is getting power (any pin has voltage > 0V).

Test the resistors near the transistor: R9, R14, R12 (should be 1000, 1000, 0.01 ohm).

Test the transistor's body diode: Multimeter in diode mode, red lead on pin 2, black lead on pin 1. Reading should be around 0.6V. Reverse the red and black leads, reading should be nothing.

One thing that was off about your earlier measurements was the gate-source leakage of the transistor (100kohm). This is quite high. What could be happening is that the controller tries to initially turn on the transistor at a low duty cycle, but the gate voltage is leaking away so fast that the transistor never turns on, and the process just stops.

The best thing would be to desolder the transistor because this will allow you to properly test it. With everything soldered on, you can't properly test the controller and the transistor's behavior in the above scenario. If you don't have a heat gun to desolder, it's enough to desolder pin 1. You can melt the solder with your iron and slide a razor blade under the pin while it's molten, remove soldering iron, slowly pull out the blade. Wrap the blade with a few layers of paper on one side to not burn your fingers while you're holding it.

asdf53 wrote on 2025-10-21, 18:02:

The large chip is likely a microcontroller. The pin of the microcontroller that leads to pin 1 of the transistor, test its resistance to the negative battery terminal (should be very high).

~ 140 kOhm

asdf53 wrote on 2025-10-21, 18:02:

Test if any of the two diodes near the microcontroller (D1, D2) is connected to pin 1 of the transistor. If yes, test it using diode mode of your multimeter.

They are not connected

asdf53 wrote on 2025-10-21, 18:02:

The on/off switch of the device, is it a momentary switch, a toggle switch or something else (a dial to change speed or so)?

It's a basic switch wit ON (press) and OFF (release function)...first image bellow

asdf53 wrote on 2025-10-21, 18:02:

With the power switch on, test if the microcontroller is getting power (any pin has voltage > 0V).

Switch OFF Switch ON
Pin1: 4.11V | 0.06V
Pin3: 2.03V | same
Pin9: 4.14V | 3.94
Pin10: 4.09V | 3.94

Pins are as on image 3 bellow

asdf53 wrote on 2025-10-21, 18:02:

Test the resistors near the transistor: R9, R14, R12 (should be 1000, 1000, 0.01 ohm).

R9: 1 kOhm
R14: 1 kOhm
R12: 0.05

asdf53 wrote on 2025-10-21, 18:02:

Test the transistor's body diode: Multimeter in diode mode, red lead on pin 2, black lead on pin 1. Reading should be around 0.6V. Reverse the red and black leads, reading should be nothing.

Not sure that I have diode mode. My meter is on second image bellow

asdf53 wrote on 2025-10-21, 18:02:

The best thing would be to desolder the transistor because this will allow you to properly test it. With everything soldered on, you can't properly test the controller and the transistor's behavior in the above scenario. If you don't have a heat gun to desolder, it's enough to desolder pin 1. You can melt the solder with your iron and slide a razor blade under the pin while it's molten, remove soldering iron, slowly pull out the blade. Wrap the blade with a few layers of paper on one side to not burn your fingers while you're holding it.

I have a heat gun, but there is a quite a lot solder on Pin 3 on the other side of PCB. What would be the samfe temp for heat gun and how long?

Okay, so the controller is getting power, and the power switch is working - the controller sees pin 1 drop to 0V, this is the sign that it should turn the motor on. The only problem is the path between the controller and the transistor - either the controller doesn't send a signal, or the signal is lost at the transistor due to leakage. It's unlikely that the controller is damaged, more likely it's the transistor.

Your multimeter seems to have a diode mode, it's at the top left of the settings dial.

Write down the pin of the microcontroller that is connected to pin 1 of the transistor, this could be important later.

For desoldering the transistor with a heat gun, remember you have a plastic connector near it, and also the motor wires. It would be easier if you just try to desolder pin 1 of the transistor, this has the same effect. Put your soldering iron on top of pin 1 and when the solder melts, slide a piece of paper or a razor blade under it.

If you use the heat gun, you could try to shield the connector - wrap a bit of paper towel around it and then cover it with aluminum foil. Same with the motor wires. I'd recommend preheating the area to 120C with the gun and then desoldering at 380C for 10-15 seconds. If it doesn't come off in that time, repeat with 400 to 420C.

Can I simply then cut the pin1 leg, and later if needed I can solder the gap? It is much easier for me

boby wrote on 2025-10-22, 21:23:

Can I simply then cut the pin1 leg, and later if needed I can solder the gap? It is much easier for me

Yes, of course. That's also a great idea. Just be very careful not to twist or pull the leg while cutting it, or you'll rip off its solder pad from the board.

Once you've disconnected the pin, the next measurements would be as follows, all with the battery disconnected:

- Resistance from pin 1 to 2, 1 to 3, and 2 to 3. The first two should be > 1 megaohm. A reading of 0-1000 kiloohm means the transistor is likely damaged.
- Switch the meter to capacitance mode (dial set to 20n at the bottom). Measure from pin 1 to pin 2. Reading should be around 1-5nF. If it's 0 or very high, it's damaged.
- Switch your multimeter to diode mode, then put the red probe on pin 1 and the black probe on pin 2 for one second. This will charge pin 1 to a small voltage. Switch the multimeter to resistance mode and measure the pin 3 to 2 resistance. It should be low (0-500 ohm). After that, discharge pin 1 again by short-circuiting pin 1 to pin 2 for a second.
- In diode mode, measure from pin 3 to 2, then from pin 2 to 3. First should be 0.6V and second a high voltage or OL.

With the battery connected and the switch turned on, measure the voltage at the solder pad where transistor pin 1 was connected to. If there is a voltage, it means that the microcontroller is working correctly and sending the signal to turn on the motor.

If the transistor is damaged, you need to do one further test: Desolder one wire of the motor (doesn't matter which) and use diode mode to test the diode next to the motor, again in both directions. If this diode is not working, it can cause damage to the transistor - its purpose is to shield the transistor from voltage spikes generated by the motor.

asdf53 wrote on 2025-10-22, 21:44:

- Resistance from pin 1 to 2, 1 to 3, and 2 to 3. The first two should be > 1 megaohm. A reading of 0-1000 kiloohm means the transistor is likely damaged.

1 to 2 ~ 100 kOhm
1 to 3 ~ 1 MOhm
2 to 3 ~ 1 MOhm

asdf53 wrote on 2025-10-22, 21:44:

- Switch the meter to capacitance mode (dial set to 20n at the bottom). Measure from pin 1 to pin 2. Reading should be around 1-5nF. If it's 0 or very high, it's damaged.

show 0.00

asdf53 wrote on 2025-10-22, 21:44:

- Switch your multimeter to diode mode, then put the red probe on pin 1 and the black probe on pin 2 for one second. This will charge pin 1 to a small voltage. Switch the multimeter to resistance mode and measure the pin 3 to 2 resistance. It should be low (0-500 ohm). After that, discharge pin 1 again by short-circuiting pin 1 to pin 2 for a second.

~ 2 MOhm

asdf53 wrote on 2025-10-22, 21:44:

- In diode mode, measure from pin 3 to 2, then from pin 2 to 3. First should be 0.6V and second a high voltage or OL.

3 to 2 shows 1153, 2 to 3 shows 271. So it shows voltage even it is in diode mode? Before both of those, my multi meter beeps for short

asdf53 wrote on 2025-10-22, 21:44:

With the battery connected and the switch turned on, measure the voltage at the solder pad where transistor pin 1 was connected to. If there is a voltage, it means that the microcontroller is working correctly and sending the signal to turn on the motor.

Battery black and pin1 show 0V. However, baterry black and pin3 shows 4.17V

asdf53 wrote on 2025-10-22, 21:44:

If the transistor is damaged, you need to do one further test: Desolder one wire of the motor (doesn't matter which) and use diode mode to test the diode next to the motor, again in both directions. If this diode is not working, it can cause damage to the transistor - its purpose is to shield the transistor from voltage spikes generated by the motor.

Is it the black one next to wires? Shows nothing (1) in correct direction. Negative probe on left, positive on the right.
Also show 175 in opposite direction

The gate of the transistor is damaged, meaning that the transistor can't be switched on and off. You have to replace the transistor. To determine which one you need, it would be helpful to measure the activation voltage that the microcontroller is sending (see below). If not possible, and there is no diode at the output pin of the microcontroller, we can assume it's using the default battery voltage (4V).

3 to 2 shows 1153, 2 to 3 shows 271. So it shows voltage even it is in diode mode? Before both of those, my multi meter beeps for short

When you say that the diode mode shows 1153, 271, 175 - is that millivolts? Usually, the reading is in volts, such as 1.5, 0.6 and so on.

Battery black and pin1 show 0V. However, baterry black and pin3 shows 4.17V

You put the red probe onto the pin that's connected to the PCB, right? Don't put it on the pin that's still connected to the transistor. There is also another possibility that your multimeter is not sensitive enough to pick up voltage here. The controller could be switching the voltage at high frequency and low duty cycle, and to the meter it looks like 0V. If you have an oscilloscope, you could probe this spot to verify.

Is it the black one next to wires? Shows nothing (1) in correct direction. Negative probe on left, positive on the right.
Also show 175 in opposite direction

Yes, that's the right one. Again, please clarify what you mean by 175, you should be getting a voltage. The "correct" direction would be red probe on left pin (where the black motor wire is), and black probe on right pin. In this direction, you should be getting 0.2-0.7V, and with the probes reversed, you should get >2V or no reading. If one or both values are off, the diode is also damaged and needs to be replaced.

asdf53 wrote on 2025-10-23, 14:06:

The gate of the transistor is damaged, meaning that the transistor can't be switched on and off. You have to replace the transis […]
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The gate of the transistor is damaged, meaning that the transistor can't be switched on and off. You have to replace the transistor. To determine which one you need, it would be helpful to measure the activation voltage that the microcontroller is sending (see below). If not possible, and there is no diode at the output pin of the microcontroller, we can assume it's using the default battery voltage (4V).

3 to 2 shows 1153, 2 to 3 shows 271. So it shows voltage even it is in diode mode? Before both of those, my multi meter beeps for short

When you say that the diode mode shows 1153, 271, 175 - is that millivolts? Usually, the reading is in volts, such as 1.5, 0.6 and so on.

Battery black and pin1 show 0V. However, baterry black and pin3 shows 4.17V

You put the red probe onto the pin that's connected to the PCB, right? Don't put it on the pin that's still connected to the transistor. There is also another possibility that your multimeter is not sensitive enough to pick up voltage here. The controller could be switching the voltage at high frequency and low duty cycle, and to the meter it looks like 0V. If you have an oscilloscope, you could probe this spot to verify.

Is it the black one next to wires? Shows nothing (1) in correct direction. Negative probe on left, positive on the right.
Also show 175 in opposite direction

Yes, that's the right one. Again, please clarify what you mean by 175, you should be getting a voltage. The "correct" direction would be red probe on left pin (where the black motor wire is), and black probe on right pin. In this direction, you should be getting 0.2-0.7V, and with the probes reversed, you should get >2V or no reading. If one or both values are off, the diode is also damaged and needs to be replaced.

That is the thing, I don't know what it is. I just see those numbers on the screen when the multi meter is in diode mode. There is nothing else on the screen

It's probably millivolts then. Millivolts would make sense for the diode readings that you got for the transistor. So this one is almost 0.2V, which is okay. If it gives you no reading in the other direction, then it's good.

So I would need a new transistor then? How to find a proper one?

For finding a replacement transistor, if you can't read the original marking, try to photograph it with flash enabled. This often helps to make the letters visible. If it's not possible, you have to find a similar one with appropriate values.

Measure the width of the transistor (the black rectangle across the large metal tab) to determine what size you need: TO-252 (10mm) or TO-263 (15mm), then we can search for a replacement.

I have possible replacement by size, from one old motherboard. So how can I determinate the value for those:

The left one (85U03GH) should work fine:

https://www.alldatasheet.com/datasheet-pdf/do … /AP85U03GH.html

If you want to know if a transistor will work on your PCB, look up the datasheet by googling its model number. It must be an n-channel MOSFET. Then look for one of these graphs:

Gate-to-source voltage vs. Rds(on)
Gate-to-source voltage vs. Drain-to-source current.

At a gate voltage of 4V, you need at least 4A of current, if the resistance of your motor is 1 ohm in the off state. The 85U03GH can do that, and at 4A, it would generate around 0.2W of heat, which is fine. There is a small chance that the switching current of your microcontroller is not enough for this transistor, but I don't think it will be a problem.

If your microcontroller switches the transistor at a high frequency, the heat generated can be more than 0.2W (can be double of that or more). But it's difficult to calculate when we don't know so many variables. You should test the transistor for possible overheating when the motor is running - put your finger on it or use a thermometer.