Knob and tube wiring

Hi, I recently bought a 3 family home in mass for rental property. One of the tennants called today that lives on the 2nd floor,and said thier door bell isn't working all the time.Only sometimes.... It's an older home in excellant cond.What should I check and how? Iam handy,but haven't worked on doorbells before.The voltage isn't dangerous is it? Thanks for tips to check for?

I recently installed a 220 V line to accomodate a Kenmore brand dryer for rental property. When I ran the dryer, it turned on and the motor was running and was working in all modes with the exception of it heating up. The tenant bought it used and apparently it used to heat up at some point. I understand that their are many possibilities for a no heat situation. To my understanding, the person who hooked up the exhaust (not sure if it was a repair technician) mentioned that the dryer was only getting 110 V and that was the reason it was not heating up. My question is could the connections either in the panel or outlet be miswired somehow to cause the no heat problem or the 110 V suppossed problem? And wouldn't the motor be working if there was only 110V? This 220 line is a 30 amp 3 prong outlet. I measured with a volt meter across the two hot on the outlet and measured 240 V. In the panel I have a double pole 30 amp breaker and I used 10/3 wire. Any suggestions? I'am going there to insure and verify the correct voltages by meter.