3A74

Manuals – Specifications – Links – Pictures

The Tektronix 3A74 is a four-channel, 2 MHz vertical plug-in introduced in 1963 for 560-series scopes.

A modified 3A74/MOD730A (with a charge pre-amplifier on channel 1 to accommodate piezoelectric pressure transducers) was part of the Engine Analyzer System.

Key Specifications

Bandwidth	DC to 2 MHz
Input impedance	1 MΩ // 47 pF
Deflection	20 mV/div to 10 V/div, 1–2–5
Features	Invert switch per channel

Links

Documents referencing 3A74

Document	Class	Title	Authors	Year	Links
Tekscope 1969 V1 N2 Apr 1969.pdf	Article	A New Insight Into Reciprocating Machinery	Bill Verhoef	1969	Engine Analyzer System • 3A74 • 561B • 564B

Internals

Each input signal passes through switchable input attenuators and then into a cathode-follower made from a 7586 Nuvistor triode driving a PNP differential amplifier. The output of this differential amplifier, for each of the four channels, passes through a diode-based analog signal switching network (multiplexer). The output of that multiplexer drives the output amplifier, which is part transistor, part tube. The one-hot select line signals for the multiplexer are generated by a transistor-based ring oscillator, a four-state cyclic state machine. This allows each of the input signals to be traced on the screen, one after another.

The 3A74 uses Tek-made 185 Ω potentiometers in for the VAR GAIN controls. These can be a maintenance issue.

The 3A74 contains a total of eight 7586 Nuvistors.

Links

• Tek 3A74 @ amplifier.cd
• Tek 3A74 @ barrytech.com (waybackarchive)
• Tek 3A74 @ radiomuseum.org

Pictures

US-Made Version

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  Front Side

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  Rear Side

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  Left Side

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  Right Side

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  Block Diagram

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  Channel Switching

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  Amplifier

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  VAR GAIN potentiometer, sometimes problematic

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  3A74 Engine Analyzer Amplifier (note CH1 calibrated in PSI/div)

Netherlands-Made Version

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Components

Some Parts Used in the 3A74