7A13: Difference between revisions

In the older version, the comparison voltage is generated from a precision zener supplied from either positive or negative 50 V rail, followed by a 10-step switched divider made of 0.1% resistors to select the first digit. For the lower three digits, a ten-turn potentiometer with a mechanical digital readout is used. The V/Div switch changes the decimal point on the readout, which is implemented through small light bulbs above the dial with plastic light guides to make the decimal point appear between the digits. In contrast to the [[back-lit switches]], the decimal point lamps are supplied from logic +5 V so they work also in mainframes without switch light supplies, e.g. the 76x3s.

In the newer version, the zener-derived reference voltage is always positive. A 10-turn pot for coarse selection combines with a single-turn pot for fine control. The resultant voltage is displayed on a 4-digit simplified DVM built around a [[Fairchild 3814]] 4½ digit DVM controller. For negative values of V_C, an opamp polarity inverter circuit is switched in.