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The Tektronix 661 is a sampling oscilloscope that was introduced in 1961. It accepts two plug-ins, a sampling unit and a timing unit.

Four sampling units were made:

Three timing units were made:

Inter-module signals

The timing units use tunnel diode triggering. Two 50 Ω coaxial cables in the scope connect the sampling unit to the timing unit. One of these cables sends the "internal trigger signal" from the sampling unit to the timing unit. The other cable sends the "start sample signal" from the timing unit to the sampling unit, telling it when to sample.

The connectors used for the inter-module coaxial connections were made by Gremar. The connector on the plug-in side is a Gremar 8212A. The connector on the scope side is a Gremar 8205A. When operating one or both of the plug-ins outside of the 661, the sampling unit still needs the sampling pulse from the timing unit. The Gremar extension cable, part number 012-070 (shown below) enables that.

Triggering modes

A 661 can be triggered in at least four distinct modes:

  • The 4S1 uses a trigger pickoff transformer to produce the internal trigger signal that can trigger the timing unit. The 4S1 is the only 661 sampling unit that produces an internal trigger signal.
  • An external trigger signal can be fed to the timing unit via its front panel.
  • The timing unit can be operated in in free-running mode and the resulting pulse signal can be the stimulus for the device under test. This mode is similar to a TDR.
  • The calibration signal generator in the 661 can be used as trigger source, as described below.

Subsystems of the 661

Other than the two plug-ins, the 661 mainframe essentially consists of four subsystems:

  • power supply
  • indicator
  • amplitude/time calibration signal generator
  • delayed pulse generator

Power Supply

The power supply is typical of Tektronix scopes of early 1960s. It is linear. All power rectifiers are silicon diodes. An OG3 tube is used as a voltage reference for the +300 V supply. The other supply voltages use the +300 V supply as their reference. The +19 V and -19 V supplies use BJT-based regulators. The other regulators are tube-based. A 45 second delay tube is used so that plate voltage isn't applied to any tube in the 661 until the cathodes are hot. The 661 has a 137°F/58 °C thermal cutoff. In practice, it doesn't run hot.


The indicator is a conventional X-Y indicator. The total CRT accelerating voltage is 3 kV. The 661 uses a T5030 CRT with P2 phosphor. The vertical and horizontal amplifiers are essentially the same, each consisting of a two-stage differential amplifier. The first differential stage is made of a pair of OC170 germanium PNP bipolar junction transistors driven single-endedly with emitters connected directly together (maximum voltage gain). The second differential stage is made of both triodes of a 6DJ8 tube with cathodes connected directly together (maximum voltage gain). The vertical and horizontal amplifiers have feedback loops around them that determine their gain.


The amplitude/time calibrator is a Colpitts oscillator that uses a 7119 tube. It produces clippped sine waves at frequencies from 100 kHz to 100 MHz and amplitudes from 1 mV to 1 V. The output is 50 Ω GR-874 connector. The signal from the calibration generator is available on the front panel and is also sent to the timing generator through the multi-pin plug-in connector. This allows the timing plug-ins to select "CAL" as a trigger source. In this mode, the calibration generator can be used as the stimulus for the device under test. In many situations, this eliminates the need for external triggering.

Delayed Pulse Generator

The delayed pulse generator is a tunnel diode circuit that produces a negative-going 250 mV pulse with a risetime of about 150 ps and a pulse width of about 400 ns. The output is a 50 Ω GR-874 connector. When a timing unit (e.g., a 5T1) triggers, it sends a pulse through pin 10 of the J4 interconnect to the delayed pulse generator, which regenerates the pulse. There are three versions of the 661 delayed pulse generator (serial numbers 101 through 2829, 2830 through 3459, 3460 and up). All three versions use a 50 mA, 6 pF germanium tunnel diode to generate the actual output pulse. In early 661 production, a 1N3130 tunnel diode was used. Then it was replaced by a TD1081. The circuit versions also differ in how they bias and trip the output tunnel diode.

Based on the available schematics, the 661 appears to have been designed in 1961.

During what years was it manufactured?
Why is it that the 661 has a dedicated high-speed coaxial interface between the sampling unit the timing unit

while later 560-series sampling systems (3S2, 3T77A, etc.) simply use the regular plug-in connector and mainframe wiring harness for routing trigger and timing signals between the two units?

Some 661s have a 41 pin Bendix connector, J5, on the rear panel, perhaps to allow the 661 to be interfaced to low speed data acquisition equipment or a computer. This is essentially a pass-through from J2 & J3, the secondary multi-pin connectors on the vertical & horizontal plug-ins. These connectors carry switch position information (number, magnitude & units) and clock & gate pulses. Not all plug-ins had this 2nd connector; the 5T1A does, while the 5T3 does not.

The Tektronix 012-064 is a plug-in extension cable for the 661.


please add


The 661 is constructed similarly to late-model 500-series scopes. The chassis is made of sheet aluminum. Most wiring is on ceramic strips. The side panels come off like those of a 545B or 547. The plug-ins of a 661 are incompatible with any other Tek scope, but the construction style is similar. The 661, like the 500-series scopes, uses Amphenol 26-series connectors for the electrical interface between plug-in and mainframe. The sides and rear of the 661 painted identically to late-model 500-series scopes, i.e., Tek-blue wrinkle.



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