how does a substantial payment on a 6 year old delinquent account affect Your credit if its the only bad ref.?

Answer 1

If this is for a credit card or loan, the reporting SOL would be 7 years.
If you make a payment on it, without requesting deletion upon payment, you would not be re-setting the reporting SOL - but, it would update the debt to make it look newer than it actually is (which will hurt your scores)
A paid negative is still a negative.

It is also possible that you are out of the collecting SOL if it has been 6 years. If you are out of the collecting SOL, you have a legal right to not pay.

If you do want to pay, be sure to request total deletion. If you pay a portion of the debt as payment in full, be sure that you have their agreement to that, and the deletion, in writing

You might click on my profile and do some reading in the links I have listed. To the FDCPA, FCRA, etc., there is also a link where you can check the collecting SOL in your state.

Answer 2

Speaking as a nationally known credit score and lending expert (book, radio shows, newspaper columns, etc.)...

IF, IF the "date of last activity" is a few years old or even a year old, making a payment on a debt that old will drop scores AND re-set the 7-year clock before it falls off your credit reports.