Back-lit switches
Tek introduced back-lit pushbutton switches at the end of the 1960s in the 7000 scope family.
There is one miniature 5 V incandescent lamp per interlocking group of buttons, with clear plastic light guides reflecting the light to the backs of the transparent plastic buttons. In the Off state, a small metal flap covers the un-lit button end, in the On state it is pushed open.
The lamps are not supplied from the standard +5 V (logic supply) rail but from a separate lamp supply rail which is not present in all mainframes (e.g. missing from 7xx3 series scopes - see modification 040-0686-01 adding this supply). In mainframes that do supply the lamps, the lamp voltage (and therefore lamp intensity) can often be varied, e.g. through a 3-way (off/mid/high) switch in the back of the instrument. In the 7904 (non-A), this is a front panel control.
