R series PDP-8 board information and R401 Rev D Variable Clock
This board is a Resistor Capacitor (RC) oscillator with output pulse amplifier. This revision D board layout was done in March 1965. Frequency is set by selecting on the backplane one of the 5 on board capacitors to select the frequency range and then using the on board pot to set the desired frequency. The board can generate 30 Hz to 2 MHz. The frequency is stable to 1% with 20% change in power supply. This board cost $45 in 1968.
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