Colibri Smart Tweezers ST5C (fixed)

[sean0118]

This one is fixed but I thought I would make a post in case it's a common issue. 

I came across some ST5C smart tweezers with a dead battery (3.7V 551430P). It was easy to find a replacement, but even with the battery replaced it would only work intermittently, refusing to turn on at times. It was also heating up and not charging.

I powered it up from a bench supply and found that the current was very low when operating but when in fault mode it would draw ~300mA. There was a three pin 3.3V regulator near the USB port which was getting close to 70C and its output being pulled down.

Eventually I found the analogue supply AVCC was nearly shorted (~1.5 ohms) to the digital gnd DVSS pin on the MSP430G2553 microcontroller. I ruled out all bypass capacitors etc and ended up removing the microcontroller from the board, the resistance between the pins was still ~1.5ohm. I was just about to give up assuming the microcontroller was damaged internally, however there was a lot of residual flux that had been stuck under the IC, it looked particular crusty between the AVCC and DVSS pins (they are next to each other). Cleaning this off fixed the problem, the resistance between the two pins went to ~700kohm!!!

It's back assembled and seems to be working and charging fine now, just shows sometimes the issue is an easier fix than it first appears. It's the fact that it was intermittent that threw me off. 

tldr: 3.3V rail shorted to GND by flux crustiness under microcontroller.