The Tektronix Type 6R1A is a plug-in for the 567 oscilloscope. It replaced the 6R1. The 6R1A provides digital readout of time and voltage measurements on waveforms. The 6R1A (and the 567 in general) is oriented toward sampling measurements. The 6R1A is a counter and a comparator. To digitize analog voltages, the counter and comparator are used as a ramp-compare ADC. Card Q (Voltmeter) contains a 1 MHz crystal oscillator. The counters (Card A) are made of flip-flops made of 2N1754 Germanium PNP transistors.
The 6R1A differs sightly from the 6R1, mainly in what determines the 0% zone. In the 6R1, the 0% zone is always the leftmost major division of the trace. In the 6R1A, the horizontal position of the 0% zone can be set arbitrarily.
Internally, the 6R1A differs from the 6R1 in how its functions are implemented. The 6R1 uses about seven tunnel diodes while the 6R1A uses only two tunnel diodes. The 6R1 uses several Nuvistor tubes. In the 6R1A many tubes were replaced by transistors. Take for example the front end of the SIGNAL COMPARATOR board. In the 6R1 it uses two 7586 Nuvistor triodes. In the 6R1A it is two NPN transistors.
There is an extender plug-in, the 067-0505-00, that allows a 6R1 or 6R1A to be operated outside the plug-in bay of the 567 for maintenance purposes. There are extension cards, the 012-067 and 012-068 (shown below), that allow the circuit cards in the 6R1 to be accessed for maintenance while the instrument is powered up.