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Sheesh, sure wish I could trade in my 6 AA 'brick' for that nice 9v module!!
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I had that done to my set about 6 months ago. It wasn't a warranty covered modification, but it was worth the money to me. The 9V module includes an auto-off option that saves the battery whenever it detects that the headset is no longer connected to a live plug.
One beef that I had with the old AA brick (I think mine held 4, not 6) was that the cord from the headset to the *brick* was too short. I often had to put the brick in my shirt pocket or clip it to myself so it didn't dangle from the headset. The new setup fixes that problem, plus some. The cord from the headset to the battery is plenty long enough now, and the cord from the battery module to the panel is just as long. I plug into the panel jacks, the battery pack usually just lays on the floor somewhere, and I have full range of head motion without dragging a brick around. The cords are so long that I think I could set in the back row of the plane and still be plugged in up front!
I don't remember the exact cost of the upgrade, but somewhere around $100 rings a bell. Than may sound like a lot, but the extended usuable time of the 9V battery, reduced battery replacement costs, auto-shut-off feature, and longer cord made it worth it to me. Since I originally bought the headset used, I still don't have the total cost of a new one tied up in this unit.
Typical D-C service: when I sent the headset in for rework, they replaced the mic muff and added a cord clip (at no additional cost), and had the unit back in my hands in just over one week door-to-door.