Hi and welcome to the site, I’m Louie. Sorry you had to wait awhile but I’ll do my best to help.
Initially, might it be for a VTech i5871?
To my understanding, there were two (2) adapters that came with this model; a base power adapter and the second handset dock charger adapter.
Which of the 2 is needed please?
Unfortunately, it would appear that the VTECH SX8-70-1100-US is no longer readily available.
Is the old base power adapter even if defective still available?
Bear with me; post back with clear pictures of the base power adapter. Please click HERE.
A hopeful workaround is to modify a power supply / AC adapter for another product.