Fig. 1 — Diagram courtesy of Tom Arnold / offset.guitars.com
This assembly hinges on a pivot plate, which is attached to the face of the unit by three screws. When the bar is depressed, this pushes the anchor plate down toward the spring, which, in turn, pushes back (Fig. 1).
The most interesting feature may be the unassuming little button on the forward edge, known as the trem-lock (Photo 9). Many players erroneously believe that engaging this button effectively hard-tails the vibrato, but that’s not at all correct. In fact, with it engaged, you can still depress the bar—you just can’t pull up. And there’s good reason for that.
The actual intended purpose of the trem-lock button is as a sort of mechanical memory for when a string breaks. When a break occurs, the decreased string tension causes the unit to pull sharp. Sliding that button back (Fig. 2), the vibrato “recalls” the tension of the remaining strings, returning them to pitch automatically. It’s all about balance.
Fig. 2 — Diagram courtesy of Tom Arnold / offset.guitars
To get the trem-lock working properly, make sure your strings are tuned, then depress the vibrato arm until you can slide the button all the way back. If you release the arm and the pitch is lower than it should be, you’ll need to loosen the spring tension by turning the screw counterclockwise. Return your strings to pitch, and repeat until you’ve achieved balance. If you can easily slide the trem-lock button all the way back but still pull up on the bar, then you’ll need to tighten the spring’s tension by turning the screw clockwise.
Incidentally, setting up the trem-lock button is also useful as a guide for the overall feel and play of the vibrato. In my view, doing so brings out the best in the system, with plenty of upward and downward pitch variation. Of course, I recommend that players adjust their instruments according to their needs, so if what you want is maximum downward travel, then, by all means, tighten that spring.
If you break strings frequently on your Jazzmaster or Jaguar, it’s probably happening in one of two areas of the vibrato itself. To properly diagnose these issues, it’s important to pay attention to how the string is breaking.
If the string breaks at the anchor plate, this could mean that a burr has developed where the ball end rests. A visual inspection is usually all you’ll need to confirm this. The best way to fix the problem is to use a small file to gently round off any sharp edges on the perimeter of the string-through hole. Mitchell’s Abrasive Cord, sold by StewMac and most woodworking retailers, is a godsend in this regard—it’s the Soap on a Rope of the sandpaper world! Thread it through the hole in question and then use it to “floss” the plate. It’ll take care of this issue in no time flat.