Easy peasey, just go to Whites website>manuals>CM 4900/D. There's a note and instructions on the operation of the 4900 "Jimmy Sierra" model. I hope yours is in good working condition because it's a really good detector. I have had my 4900/D Pro Plus now since the late 1980s-early 1990s and I love it, my buddy bought his 4900/SP3 at the same time, we used them side by side while prospecting for gold up in northern New Mexico near Dulce close to the Colorado border, and they each did a really good job, the 4900/SP3 is a bit more specialized for prospecting than the other 4900s, but it's a good detector to use in the field for gen. purpose t-hunting. On any of the old Pro models, the 6000, the 5900, or the 4900, it's very important to follow the directions to the letter to find exact electrical center of those concentric search coils using both a piece of gold as well as a non-conductive mineral sample. The concentric coils on the older Whites machines are, IMHO, much better than any coplanar coil I've ever used (which ALWAYS have two focal points, what a drag that is!) Hip mount that 4900/SP3 and use it for life, you'll love it, just be prepared for a race car of a detector, what I mean is that the PRO series detectors are INFINATELY adjustable, and that's a big problem for most people who just want to turn on and hunt. Good luck with it, and welcome to the 4900 club.
wandering Mike Nesbitt
Whites 4900/D pro plus
flyboy32@juno.com