Hi @msh1919 ,
No schematic but perhaps the [https://thinkpads.com/support/hmm/hmm_pdf/t420_t420i_hmm.pdf|service manual] will be of some help or at least a start.
On p.106 it details the necessary pre-requisite steps and then the procedure to remove the DC-In jack.
You could start from this point. With the power and battery totally removed from the laptop, use an Ohmmeter to measure that the jack itself is OK
Here is the part number for the DC-In jack, in case it is faulty and you have to order a new one, 04W1635 (found on p.140)
Just search online using the number ''only'' to get results for suppliers of the part.