B&K 707 DCV issue

Having issues with a B&K 707 tube tester that I purchased 2-3 years ago it was supposedly replaced caps soldered #55 bulbs and calibrated it recently stared giving me erratic readings . DC bias voltage was off , I recapped and replaced all the carbon comp resisters . Able to calibrate everything except for the dc bias . Spec is -2.5vdc when I adjust pot voltage jumps from 3v to 0v back and forth. I have triple checked my work but have been unable to find any errors.
Tried both SS rectifier and #83 tube , swapped out 6BN8 no change.
Any thoughts on what to look for .
Thank you