My old cordless phone system had been deteriorating for a while, and was about due for replacement when a totally modern Panasonic system with 5 handsets and all the trimmings turned up on sale a while back.
I think it was on Woot.
Anyway, I bought it, and eventually unpacked and installed it.
A few days ago, the handset in the bedroom started beeping at irregular intervals, then croaked, with all the symptoms of a dead battery.
Oh, well. A new pair of AAA NiMH cells should put it to rights, and won't cost much.
Except that it still failed to charge.
Swapping handsets with the garage points to the charger being the problem. The handset beeps and reports "Charging" when set in the cradle... but never reaches "Fully charged", and slowly drains its battery.
So: have a look at the charger.
Inside the cradle, power from the "5.5V DC" wall wart goes through a fuse, two 1Ω resistors, and a diode, then to the charging contacts.
At the charging contacts, I see 6.6V no-load (eh?) and 2.8V with the phone connected. That would explain why it thinks it's charging, but doesn't actually charge....
Swap wall warts with the garage. Same behavior. The wall wart isn't getting dragged down; the drop is across the diode.
So: remove the diode, and look for a replacement. I have some quite adequate 30V, 2A Schottky parts... but they're in SOD-323 packages, and the original is more SMA-sized. I think I have some SMA-packaged diodes somewhere, but....
Poke the original with a diode tester, to see what's wrong. It looks healthy enough. So....
Re-install the old diode. Now I see 4.5V at the phone when it's charging.
Apparently there was a bad solder joint. (Not visually apparent; those consumer-grade lead-free SMT solder jobs always look defective anyway.)
And the phone now claims to be fully charged, so I guess it's working.
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