I had a couple of 552D's back in the early 80's. Found a ton of Civil War relics with those machines. If you are hunting in all metal mode you should have a threshold hum, barely audible. Ground balance first, then use the tuner knob to set the threshold. If the threshold drifts just press the button in the handle and it will come back to your setting.
There is of course no ground balance in the disc. mode. When I would coin hunt with the 552D I would set my disc. level enough to knock out foil, lower the coil to approx. 2 inches above the ground and press the button in the handle. Then lower the coil to the ground and do the scrubbing thing. Worked well for me.
As for depth, in all metal mode and properly ground balanced you will get really good depth. In disc. mode not so good until you learn how to use the machine. I found a lot of Minnie balls back then at 8+ inches and I would check the signal in disc. mode by setting the coil on the ground off to one side of the signal, press the button, raise the coil just enough to get a threshold hum, then slowly move the coil over the signal. The 550 series Fishers were great machines. I probably found more with my 555D than any detector I have ever owned, maybe because I actually had time to use it more back then.
