Looking at the schematic It either switches in an extra gain stage or bypasses it continuity check all your switch connections
You’d need to work out which position fails boost position would switch volume lug 2 to R16 and R14/ 15 to out jack tip
In the other position volume lug 2 to out jack tip
This is how I think the switch is connected you want to check the middle common row connects to the top row in one position and middle to bottom in the other position but not both at the same time
Here’s a PDF if you’re unsure with continuity checks
https://drive.google.com/file/d/176vVBWeRaSQ0QvZlnozjKHE6AeHsvxFP/view?usp=drivesdk
