Identify BBH transistor

Thread Starter


Joined Mar 9, 2022
I just spent the day diagnosing this board and found an atypical issue.

Symptoms: Turns on, belt doesn't move.

Process: First I checked the mosfet. As IamJatinah stated, the relay coil clicks and the Red LED goes full brightness informing you the motor power rail is now under full (lethal!) power. After a failsafe timeout period with nothing coming back from the encoder the relay clicked off. I carefully (CAREFULLY) gave the mosfet 15V from a nearby cap (Soldered a stud onto the pin to clip an aligator lead to) to its gate and the motor gave a jerk as the capacitors quickly discharged through it. I highly do not recommend doing this unless you're very good. And please be aware of the Dunning–Kruger effect when thinking "Yeah, I am very good so I'll give it a go" You may think "What's the big deal, 15V is safe!" Well, it's floating 15V, meaning ground could be 95V and the positive "15V" could be 110V relative to you!

After that I checked the signal at the testpoint marked "PWM" with my oscilloscope that was hooked up via an isolation transformer for differential probing. No signal.

Followed the circuit back to what drives the pwm signal back to the opto that separates the high from low voltage. There's a white one (TLP521 [marked just p521]) that tells the mcu when the circuit is safe to give full power to the motor rail via the relay. Thanks to IamJatinah I realized this was to signal that the rail had the 18V precharge and was ready for full power. Shorting this will make the MCU believe the precharge is still being applied and will never turn the motor on, so this is another potential failure mode to consider if one of your symptoms is that the relay doesn't click on at all.

The black opto right next to it supplies the inverted PWM motor driving signal via pin 6 (the center pin that isn't cut off.) That had the (inverted) signal when probed.

Followed that trace to pin 6 of the first st lm339 chip and was an inverting input of one of the comparators. Pin 1 on that 14 pin chip now has the un-inverted PWM motor drive signal and when probed was present.

This makes its way over to the second lm339 to pin 7 (non inverting input) and the signal once again is outputted on pin 1 (on the second chip) However! Probing this yielded a very weak PWM signal that was only about 1V and the rails were at 0 and 15V so there was definitely something wrong with this part of the circuit.

Conclusion: It was a SOT23 PNP Transistor (marked BBH) labeled Q7 on the PCB; It had no physical sign of damage. It was shorting the pwm motor drive signal (from the LM339) to ground via the base pin to the collector pin (GND.) Probing the bjt for resistance from the base (probe+) to the collector[GND] (probe-) was netting about 100ohms whereas a fresh pnp salvaged off an old board was in the multi kOhm range. The function of this small bjt is to drive the large power mosfet for the motor, and as such it is located right next to the mosfet.

Just thought I'd add this failure mode to the tread since this is a top result when searching for this board model number on google.
Would you know the equivalent of SOT23 PNP Transistor (marked BBH) labeled Q7 and Q8 on the PCB.
I cannot find any info on this BBH transistor. I have attached the schematic if this is any help.
Any help would be welcome.

Thank you.

Moderator edit: New thread created from this. Read post #48 in that thread.

motor control circuit-partial.jpg