basic electronics question

I'm sure Dave will know this, I'm not up on some of this. But look at the note at far top right "Adjust AP1 for 5mv across R53"

This is all DC coupled which means a change/ failure in a "far removed" part of the circuit can affect things down the line. also there are some TP readings that might be of value?

I did a brief search on this and found some reference to a service manual, but most links went to "phake" pay to download places