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Electronics => Repair => Topic started by: sdancer75 on July 17, 2019, 08:39:55 am

Title: MB3773 power supply monitor - Transistor short ?
Post by: sdancer75 on July 17, 2019, 08:39:55 am

Hi,

I am trying to repair a CDI (Capacitor Discharge Igniter) Ignition Unit from a motocycle. I am stuck to diagnose stage since all parts seem to be ok except a PNP SOT23 transistor marked with the code L6 (BSS69 and BSS70) that is connected directly to a power supply monitor MB3773 at pin 2 which is the RESET.

This transistor seems to have short from Base to  Collector. I am not sure if under the L6 SMD is a rail that connects BC but I didn't see anything like that until now.

Also, the Base does not seem to be connected somewhere....

So, do you think that this transistor is faulty? Is the connection between  Base-to-Collector (shorted) normal?

Title: Re: MB3773 power supply monitor - Transistor short ?
Post by: fzabkar on July 18, 2019, 10:40:52 pm

FWIW, the shorted base-collector configuration is often used in current mirrors:

https://physics.stackexchange.com/questions/371360/whats-the-purpose-of-shorting-the-base-and-collector-of-a-transistor-in-current (https://physics.stackexchange.com/questions/371360/whats-the-purpose-of-shorting-the-base-and-collector-of-a-transistor-in-current)

Title: Re: MB3773 power supply monitor - Transistor short ?
Post by: sdancer75 on July 19, 2019, 11:16:46 am

Yes, you have right.... Under the SMD transistor, there is indeed a tray that connects Base-Collector.

Out of the circuit, the transistor works as expected.