Panasonic home telephone making weird sound

[pancake9590]

Hello, I have this home telephone which is around the house for few years already and since nobody was interested in it I thought it is failing and the other members of the house just threw it away but when I shared that there will be good parts in it for scavenging(since I am electronics hobbiest) they said that it works but they just dont use it. Then I said to myself lets connect it and see for myself, but good thing that I am careful I first checked the dc jack and it said plus center and our home phone(current one) uses plus center as most of the trending electronic nowadays, but this Panasonic used minus center. So I bought new jack, wired it and connected the phone to my 12v battery, since it was written that accepts 12v. But now the phone is buzzing, I get it it is charging and the diode for charging is light, but I am afraid if there is internal fail. Can you guide me if there is fail or it is good since it charged for aroudn 10 minutes now? And if there is internal fail, where to look for it, like weak possible spots? Also is the middle pin of all three the minus one (on the charging nest for the phone headset)?

 

[Ylli]

How does it work on the internal battery - without the the external 'charger'?

 

[pancake9590]

How does it work on the internal battery - without the the external 'charger'?

I simulate the external power with my lead acid 12v battery through 2.1mmx5.5mm jack with negative center. I have tried the battery of the phone headset but the led for low power turns on for few seconds so I left it charge for longer.

 

[pancake9590]

I have made pictures of the phone model and serial number, the lead acid battery I used and the type of phone(to show that it has headset which has chargable battery inside and something like a station). I opened the phone too and tried to trace burnt parts or faults on the trace or on the solder joints.(found this interesting...looks like a fuse but I think it is bulb used as fuse or something like that). Sorry if my knowledge is small and I sound stupid, but I really dont know what is causing that buzzing, so please help. One more thing is that it was charging for around an hour, the telephone line cable was plugged into it and it didnt make anything strange except that charge noice and the fact that after an hour of charging the headphone was capable to stay functional for around 10 seconds. Maybe it can not charge good and that sound is some internal fail, under that "fuse"/bulb there was a symbol that looked like -N- whish is not the commong symbol for fuse, but I also cant imagine putting a bulb in a phone. I putted the pictures on a site, because I am new to the forum and it did not gave me to select multiple images. Again thank you for your help and I hope that will be from some guidence for you to give me some ideas, All the best Ivan
Links to the pictures:
https://ibb.co/b6iY5A
https://ibb.co/nNMmQA
https://ibb.co/nHFD5A
https://ibb.co/enbLkA
https://ibb.co/mGXhyV