Each channel has a trigger pickoff, a delay line, and a sampler. Specified rise time is 350 ps. The sampling pulses for each channel are generated by a step recovery diode.
The sampling pulses are fed to a sampling bridge made of four GaAs diodes. These diodes are fragile and exact replacements are hard to find. People have improvised replacements using modern integrated Schottky diodes such as those in HP/Agilent's HSMS-28x series.
Dual-trace and X-Y Modes
The 4S1 has a dual-trace mode, where the A waveform and the B waveform are both displayed in the time domain at the same time with a single trigger source and a single equivalent-time sweep rate. Another mode is called "A vert/B horiz". This mode is used in conjunction with an X-Y mode switch on the panel of the 661. The signals enter the 4S1 through GR-874 connectors on the front panel.
The 4S1 uses an unusual trigger amplifier with two trigger signal paths. One path is for low frequencies, the other for high frequencies. (A similar approach was also used in the main vertical signal path of the 7A29 several years later.)
From serial number 101 to 1349, the low frequency path starts with a 25 kΩ resistor that DC-couples the input signal to a 2N1429 PNP emitter-follower stage. After that is a 151-103 NPN common-emitter. The high frequency path starts with a trigger pickoff transformer on the input. The pickoff signal feeds a common-base amplifier that uses a 2N700 PNP transistor. The low frequency and high frequency trigger paths meet at the trigger output of the 4S1. From there, the trigger signal passes through the 661, and into a 5T1 or 5T3. 4S1 units with serial number 1350 and up use a slightly less complex trigger amplifier circuit. The later circuit uses a TA2333 silicon NPN transistor to amplify the entire frequency range.
Comparison with S-1 Sampling Heads
The sampler board in the 4S1 is roughly equivalent to a pair of S-1sampling heads. A short piece of RG/174 coax connects the 661 timing pulse coaxial connector to the SMB connector timing pulse input on the 4S1 sampler board. This signal triggers a blocking oscillator which drives a snap-off diode, D2004, which generates the fast falling edge for the channel A and channel B sampling bridges. The 4S1 uses 7586 triodes as high-impedance buffers between the sampling bridge and the amplification that follows.
The 4S1 has a main chassis and sub-chassis that plug into it. There is a sub-chassis extension, part number 012-069, that allows maintenance of the sub-chassis.