Bluetooth watch repair

[Vytautas]

Hi, guys,

Could anyone have any ideas to help me with troubleshooting?
Me and my wife have Kronaby watches. These are bluetooth. We like them, as no need to charge, the battery should last for two years. Mine is running already for more then a year, no problems. But my wife’s watch, when it came, the battery was flat dead. I thought, its just battery. Replaced it. Two months later, her Kronaby app in phone informs, that battery is almost depleted. Hmmm... Something wrong. The watch works fine, but the power drain is excessive. The reseller would not accept it back, saying I already opened the cover to replace the battery. (You know, they do not like customers, they like their money.)

So, I disassembled it, to compare the current draw with that of my unit. Checked for shorts, with a thermal camera and with multimeter, checked the caps, etc. No problems. Then compared the current draw. Measured it with a scope across a 10 Ohm resistor wired in series. Here is the picture of voltage drop in my watch:



Now my wife’s watch. There is a constant voltage drop on the resistor of around 7.6mV. You can see the cursors showing it:



So, differently from my watch, my wife’s watch is constantly drawing about 760 uA of current.

I looked the datasheet for the Bluetooth module used in it. (Have not disassembled my watch to access the module, but it may be different as the movement of my watch is different from that of my wife). It says in the datasheet that the 16MHz crystal should draw only around 25 uA in standby:



Where do you think I should look for problem? Could it be that this T160 crystal is sucking current constantly? Any ideas? Here is the photo of internals:



Thanks a lot.

[RoGeorge]

Apart from possibly different hardware, maybe it's a different firmware version.  Try a firmware update for the watch that is draining its battery.

If all else fails, there are rechargeable CR2032-compatible batteries, so at least you won't have to buy single-use Panasonic CR2032 every two months.

[Vytautas]

These use not CR2032, but CR3032. Unfortunately, no rechargeable version was able to locate.

[MathWizard]

Whats all the analog looking stuff with the big caps in the upper left? Some power supply or drive section ? What are those coils, are they electromagnets driving gears/hands ? Or do they ever wrap coils around piezo-materials ?

Is there any sort of step-up voltage regulator ? Like charge pump's, or switching regulator ?

So what was the time base on your 1st pic with about 3mA spikes ? Are those spikes for moving the second hand, or maybe the MCU and/or BT chip doing it's routine ?

[RoGeorge]

These use not CR2032, but CR3032. Unfortunately, no rechargeable version was able to locate.

Doh, I didn't even know there is such thing as CR3032, sorry. 

Well, if CR2032 and CR3032 are the same thickness, maybe a rechargeable CR2032 might still be sited inside the watch, eventually with some paper napkin squeezed one side to fill the diameter gap.  Not a very professional fix, but at least you don't throw away the watch.

[Vytautas]

This watch is different from usual Bluetooth watches. Kronaby is a quartz watch, and when the Bluetooth connection is lost, it continues working as a quartz watch. So, the module with the electromagnets is for literally driving gears and hands.

The time base in the 1st picture was 20mS, so the spikes occur every 40+ mS. They are for routine, not for moving a hand (there is no third hand for seconds in the watch, only minutes and hours).

Here is a detailed picture:



Concerning the power supply conversion, I do not see any regulators on-board. The datasheet for the Bluetooth chip shows it has its own DC/DC converter:



The schematic for the Bluetooth module implementation in this case is this one from the ones shown in the datasheet:



You can see the big 10uH inductor to the left from the Bluetooth module and a small 15nH bellow it in the picture of the board.

[Vytautas]

I was wrong. It seems the YH7 is some kind of voltage regulator. It has 1.62V on its pin 5. Also, a cap from that pin to ground (above the YH7), probably to ensure stability of the supply.