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Friden 116x-Series Circuit Board #1
Circuit board #1 in the Friden 116x-Series calculators contains circuitry related to the generation of the CRT display. The mostly analog circuitry on the board makes up the waveform generators that guide the electron beam of the CRT to draw the digits, minus sign and decimal point. There are numerous adjustment potentiometers located across the top edge of the circuit board that provide adjustments to various parameters of the display, including horizontal and vertical position, digit height and width, vertical and horizontal digit spacing, and segment length. Friden provided specifications for the proper digit position, sizing and spacing for optimal display visibility in the service manuals for the 116x calculators, stating that service technicians were expected to adjust the display to match these specifications every time they performed service on one of these calculators, since component values will drift with time, causing the display to go out of optimal adjustment.
This circuit board is the only board that is shared across all models in the 116x-series made for sale outside Europe. There is a slightly different Board #1 used in calculators destined for sale in European countries that adds an additional cross-segment on the leg of the digit 7.
The board generates the standard seven-segment representation for each digit, with some minor modifications. The digit 1 is shifted to the center of the digit cell. The digit 7 is slightly shifted to the left, and the digit 3 has a shorter segment for the middle horizontal. These modifications to the standard seven-segment representation were ostensibly made for better display readability.
There are two integrated circuits on this board, both of which are a custom device fabricated by Texas Instruments for Singer/Friden. The device has part number SN9095, and contains four TTL open collector NAND gates. Apparently Friden needed specific device characteristics for this application, and thus off-the-shelf DTL or TTL devices were not suitable, requiring this custom device. The remainder of the circuitry on the board is composed of discrete components. There are 13 transistors, 16 diodes, three capacitors, and 60 resistors, 12 of which are precision resistors.
The bare circuit board is identified as Part #7002181, Revision 2. The complete assembly is identified as Part #7002180.