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==Internals== | ==Internals== | ||
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. | 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. | 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<sub>C</sub>, an opamp polarity inverter circuit is switched in. | |||
===Repair issues=== | ===Repair issues=== |