The Surfmaster II is the same as White's Classic II. I bought one off E bay where the owner said it is intermittent. It does in fact work but has no volume threshold.
It is full on or full off. I adjusted the sense and it does see a coin waved in front of the coil but there is no lower volume as you approach it. I've been trying to
work out the schematic. White's has used one schematic for 3 or 4 generations of detectors and no one has showed complete ones of each model.
Here is the best I could do but I'm wondering what VR4 is doing? Also how to adjust the pots on the board? In the last 20 years no one has explained that...

VR1 is Ground Balance should that be a fixed 5 degrees? VR7 is probably for Salt reject. Take it to the beach and adjust over salt water? And VR4 is the magic pot, adjust that one correctly and you can see 3 feet deep!
Here's some other data I have on the Surfmaster II;

It is full on or full off. I adjusted the sense and it does see a coin waved in front of the coil but there is no lower volume as you approach it. I've been trying to
work out the schematic. White's has used one schematic for 3 or 4 generations of detectors and no one has showed complete ones of each model.
Here is the best I could do but I'm wondering what VR4 is doing? Also how to adjust the pots on the board? In the last 20 years no one has explained that...
VR1 is Ground Balance should that be a fixed 5 degrees? VR7 is probably for Salt reject. Take it to the beach and adjust over salt water? And VR4 is the magic pot, adjust that one correctly and you can see 3 feet deep!
Here's some other data I have on the Surfmaster II;