Why does the #1 button fail on the KX-TGA400b phone??
The reason the keypads break is because the silicone rubber pad was not baked enough during manufacture at the factory. This leaves behind oily deposits that will seep slowly out of the keypad every time a button is pressed. This is more common than not and very difficult to identify in R&D for product development or in quality control because it takes time (not just force) to move the oil out of the rubber pad.
The kx-tga400b / kx-tga200b models had large thick rubber buttons with plenty of oil… but that’s not the only problem. The circuit board contact pattern requires a 6mm diameter conductive pad but the keypad was made with only 4mm pads. Even when new the conductive contacts were only a hair’s width from not working at all! It was a poor design and I’m not sure how or why it passed unnoticed at Panasonic.
Why the #1 key and not so much the others? Turns out it is the way most people hold the phone and dial using their thumb. They have to reach farther with the finger to dial other buttons but the #1 is right there and maximum pressure is applied to that button. More pressure (even slightly) means more oil over time!
The other Panasonic models since then (such as the KX-TGA450b that replaced the 400b) have used thin rubber pads with a plastic key cap. This reduces the oil problem… but not ALL the buttons used this technique. In the 450b now it’s the TALK and OFF buttons that stop working eventually. But it does take longer than 6 months because the circuit board contacts were designed correctly in the 450b.
I have been repairing Panasonic cordless phones and designing keypad repair kits for many years. I probably personally repaired over 3000 of just the 200b/400b models….. and still counting up, just slower now.
I have a web site with detailed info and help about this problem as well as offering DIY repair kits and repair service for those who do not wish to tackle it on their own. You can find me at http://p1repair.com or at Panasonic KX-TGA450b Repair