Line Box

The vibrato line box consists of a sequence of 18 (for the console models) or 16 (for the spinet models) second order low-pass passive LC filters. One side effect of low-pass filtering is a time delay, which manifests itself as a phase shift in the output. Audio signals fed into one end are delayed by each stage of the line box, so that at any tap along the line, the phase is retarded relative to the previous tap.

The vibrato Scanner samples the outputs (taps) of the line box from one end to the other and back again, resulting in a frequency modulated signal at the scanner's output due to Doppler shift. Because of the attenuation caused by the low-pass filtering, there is also somewhat of an amplitude modulation.

Like the ToneGenerator, the capacitors making up the filters in the line box can degrade, which results in larger delays which gives more frequency modulation, and also more high frequency attenuation giving more amplitude modulation. Replacing the capacitors will restore the original factory levels of vibrato.

It is also possible to rewire the connections to the line box to change the "shape" of the vibrato effect.

See also: Scanner, VibratoChorus, and VibratoSmall

Overhauling and optionally rewiring the line box is described in this article by StefanVorkoetter http://www.stefanv.com/electronics/hammond_vibrato_mod.html.