Bose X Aviation Headset Repair

Описание: The symptoms were:
1. Power LED would stay on permanently, i.e. no flashing.
2. Power LED would not come on at all.
3. Occasionally it would operate normally.

The problem appeared to be with the power control circuit which is located inside the battery box. The battery box can be disassembled by removing the helical wire wound around the plug side strain relief end and the two screws hidden under the label inside the battery compartment.

The fault was with one of the crystal oscillator loading capacitors on the opposite side to the 4MHz crystal. I was able to make it work by heating the capacitor and cooling it with freezer spray stopped it working completely.

I replaced the faulty capacitor with a 15pF 0603 C0G ceramic capacitor, Farnell pt.no. 1759055 (Multicomp MC0603N150J500CT) and it now appears to start up reliably and operate normally.

If you have similar symptoms then it could be either of the caps connected to the end of the crystal or it could be the crystal itself. It is a 4.0MHz in a HC49SM package.

Watch out for the small metal strips used to retain the battery cover lid. They can easily come off and drop inside where they may potentially short out the circuit.

The power wires from the battery terminals are a very small gauge and are prone to breaking. Make sure that you have a good connection both on the battery terminal and on the PCB. Look out for evidence of battery corrosion and clean it off before it eats away the plating.

As a guide, when the circuit wasn't working correctly, it would either draw 0mA or about 30mA which would not change. When it was working normally, the current would start at about 50mA and after a second or two, drop to about 15mA when the LED started flashing.