For those too paranoid to open a .PDF from the NSA, several solid parts in the typewriter were replaced by solid-looking parts with embedded circuits and altered magnetic characteristics.
These circuits magnetically detected the motion of the arms that carried and moved the character ball. So non-printing keystrokes, like space, tab, and backspace, and the character that did not rotate the ball, '-', were not detectable. The circuits had a tiny buffer, so would transmit the detected characters as they were being typed. Early models used batteries. Later bugs diverted some of the AC mains power from the typewriter.
It is likely that a relay antenna was employed to increase the detection range of the transmitted signals, which used the same frequencies as local television broadcasts.
That's really about it. The bugs were largely undetectable without x-raying every single part in the typewriter, which is how they were eventually found.
These circuits magnetically detected the motion of the arms that carried and moved the character ball. So non-printing keystrokes, like space, tab, and backspace, and the character that did not rotate the ball, '-', were not detectable. The circuits had a tiny buffer, so would transmit the detected characters as they were being typed. Early models used batteries. Later bugs diverted some of the AC mains power from the typewriter.
It is likely that a relay antenna was employed to increase the detection range of the transmitted signals, which used the same frequencies as local television broadcasts.
That's really about it. The bugs were largely undetectable without x-raying every single part in the typewriter, which is how they were eventually found.