Keyboard Teardown and Alkaline Battery Leakages

Had an external Logitech Keyboard stored away in a box. Alas, I left the batteries on them! It leaked to spring contacts and then the circuit. It has corroded a whole most of the Carbon and Copper pads on the circuit. It’s beyond repair and it’s officially dead.

Coming to the teardown, the internals of most keyboards (Non-mechanical switch type) will contain multi-layer plastic sheets with printed silver contacts acting as switches. When you press down, the 2 layers come in contact and a press is registered. It’s usually wired in a row/column approach and it’s polled many times a second to register a press. These sheets are press fitted on Carbon contacts on the main PCB. Carbon contacts are a cheap and reliable way to avoid physical connectors. In the image, you will see them as black fingers on the edge of the PCB. These are driven by Nordic’s old nRF31504 2.4GHz Wireless IC. It has a PCB antenna which communicates to your PC via a 2.4GHz dongle. Few PCB traces are all corroded by the battery leakage. The metal can is a 16MHz external crystal to maintain timing for the chip. It’s a shame that I couldn’t get this working.

Let’s talk batteries now. Why do Alkaline batteries leak? Usually, when left on a product for a long time, the battery discharges and a chemical reaction causes the generation of hydrogen gas, which can break the seal even in good brand batteries. Once the seal is broken, the alkaline electrolyte Potassium hydroxide leaks onto the contacts and it reacts with Carbon dioxide in the air to form your white powder (Potassium Carbonate) which you find on the metal spring contacts. KOH is highly corrosive and it eats away your copper traces. So never keep batteries in your product if you are not going to use them.

Pro Tip: If you have a problem like this and the leakage hasn’t affected the circuit, you can use any mild acid like vinegar to clean those metal contacts and neutralise the white powder and get your device working.

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Boat Rocker Headphone Teardown and Fault Diagnosis

Boat Headphone Teardown
Boat Headphone Teardown
Boat Headphone Teardown
Boat Headphone Teardown

A friend of mine dropped off a pretty old pair of wireless Bluetooth Boat Rocker headphones which were not working. The power LED turns ON but it’s unresponsive after that. Opened it up and first did a charge test with a USB current meter and found that there was no current taken from the power supply. That’s usually bad news, as that could potentially mean that the battery is bust or the main chip is blown. I opened up the battery compartment and there was no bulging of the battery. The battery was showing voltage, which was slightly low, but nothing too concerning. Then I desoldered the battery and connected a spare battery right on the PCB. Then plugged in the USB meter and voila the new battery is charging. Turned out the culprit was a faulty positive battery wire running all along the headband. It had no continuity and it was broken in between. Replacing that thin wire should get the headphones backup. The device wasn’t turning ON as the main chip couldn’t detect the battery due to the broken wire, even though it was getting external power.

Coming to the circuit analysis, these BLE headphones contain a Bluetooth chip from the Taiwanese manufacturer Airoha (AB1510). It’s a DSP chip with 48MIPS. It has all the bells and whistles for audio input, processing and outputs. It has a BLE PCB antenna. The great part of this chip is a built-in battery charge controller capable of charging at 400mA with protection. This eliminates the need for an external charging circuitry altogether. I couldn’t find a pricing for these chips though. If anyone knows please do comment. The other main IC is 24C128A, which is a 2-wire Serial EEPROM from Microchip to mostly store audio presets I suppose. The LiPo battery has a capacity of 450mAh. All in all, a reasonably well-designed headphone.

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