Tektronix 3A1 10 MHz Dual-Trace Amplifier Plug-In (1968)

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  • Bandwidth : DC to 10 MHz
  • Sensitivity : 10 mV/div to 10 V/div
  • Risetime : 35 ns
  • Input Impedance : 1 MegΩ ,47 pf

    The Type 3A1 Dual-Trace Amplifier plug-in unit is designed to operate in Tektronix 560-Series Oscilloscopes. The unit has two identical amplifier channels with a switching circuit that can select independent operation of either channel, switch back and forth between channels either at an internally set chopping rate or on alternate sweeps, or select the algebraic sum of the two inputs.

    Operating Characteristics

    Each channel has ten sensitivities from 0.01 volts/div to 10 volts/div. in a 1, 2, 5 sequence. Accuracy is ±3% of indicted sensitivity. Each channel has a variable attenuator which may be used to introduce a continuously variable attenuation to a maximum of 2.5 X the indicated sensitivity. A warning indicator lights when the variable attenuator is not in the calibrated position.

    Frequency Response
    DC to 10 mc

    35 nanoseconds

    Input Impedance
    1 megohm, paralleled by 47 pf
    Input Coupling
    Ac or dc coupling of signals may be selected for either channel. A 0.1 µf capacitor limits low-frequency response to about 2 cps on ac coupling.

    Operating Modes
    Channel 1 only: Channel 2 will not operate.
    Channel 2 only: Channel 1 will not operate.
    Alternate: Electronic switching between channels at the end of each sweep.
    Chopped: Electronic switching between channels at a rate of about 200 kc, with a 5 µsec sample of each channel. Blanking occurs while switching is taking place.
    Added: The algebraic sum of both signals is displayed. When the channel 1 signal is inverted, the difference is displayed.

    An internal signal for triggering is fed from the Type 3A1 through the indicator unit to the time-base plug-in unit. Trigger signals may be supplied from channel 1 only, or from channel 1 and channel 2, depending on which channel is displayed.

    Mechanical Features
    Construction: Aluminum alloy chassis
    Finish: Photo-etched anodized from panel
    Weight: Approx. 5 pounds

    Above excerpt from Textronix Type 3A1 Plug-In Instruction Manual


    The Type 3A1 is normally used with a 560-Series Oscilloscope to provide vertical deflection signals with respect to horizontal units of time provided by a time-base plug-in unit such as the Tektronix Type 2B67. In some applications, an ampifier plug-in unit instead of a time-base plug-in unit can be used to provide horizontal deflection.

    The following instructions are based on using a time-base plug-in unit to provide the horizontal deflection.


    First-Time Operation

    The following procedure will help you become familiar with the Type 3A1 operation.

    1.   Preset channel 1 and channel 2 controls as follows:
       AC-DC-GND SwitchDC
    VOLTS/DIV. switch.2
    POSITION controlMid-range
    MODE switchCH 1
    INV. (CH 1) NORM. switchNORM.
    TRIGGERPushed In
    2. Connect a jumper from the oscilloscope calibrator to both plug-in input connectors and set the calibrator voltage to 1 volt. Adjust the time-base sweep rate for 5 msec/cm and the triggering controls for a stable 5-division square wave display. Adjust the CALIB. control if necessary. Turn the channel 1 POSITION control to move the display above the center graticule line.
    3. Turn the MODE switch to CH 2; a similar 2-division square wave should be seen. Turn the channel 2 POSITION control to move the display below the center graticule line.
    4. Turn the MODE switch to ALTER.; the upper and lower square-wave display should be seen (The flicker depends on the setting of the time-base plug-in sweep rate).
    5. Turn the MODE switch to CHOP.; the upper and lower square wave display will have less flicker than in the ALTER. position. Set the time-base plug-in for 5 μsec/div. The individual chopped portions of the waveform will be seen. Set both Type 3A1 VOLTS/DIV. controls to .5.
    6. Set the time-base plug-in for a 5 msec sweep rate and the Type 3A1 MODE switch to ADDED. The display should be one square wave, 4 divisions in amplitude. This is the channel 1 and 2 waveforms added together (2 divisions each). Notice that either Type 3A1 POSITION control will move the trace vertically.
    7. Turn the INV. (CH 1) NORM. switch to INV. The display will be a straight line if the gain of both channels is exactly equal. Since both signals are 2 divisions, the difference between them is zero or a straight line.

    Front Panel Controls

    AC-DC-GND Switch
    With this switch in the DC position, both the ac and dc components of the signal will be displayed. To remove the dc component, set the switch to the AC position. In the AC position, the dc component is blocked by a capacitor in the input circuit. The low frequency limit (3-db point) in the AC position is about 2 Hz (about 0.2 Hz when 10X probe is used).

    In the GND position, this switch grounds only the Type 3A1 input; not the input connector.

    VOLTS/DIV Switch

    A ten position switch that sets the vertical sensitivity of the Type 3A1. The VARIABLE VOLTS/DIV. control must be full clockwise with neon UNCAL. light extinguished for calibrated sensitivity. Range is from 0.01 volts to 10 volts/div. (100 volts/div. when 10X probe is used).


    The red knob on the VOLTS/DIV. switch is a variable resistor that provides continuously variable attenuation (uncalibrated) up to about 2.5 to 1. For example, if the VOLTS/DIV. switch is in the 1 volt position, the VARIABLE VOLTS/DIV. control will vary the sensitivity between 1 volt and 2.5 volts/division (uncalibrated). When used in conjunction with the oscilloscope calibrator voltage, this control can set any specific sensitivity within the range of the instrument.

    UNCAL. Light

    A neon lamp that lights when the VARIABLE VOLTS/DIV. control is not in the CALIB. position.

    POSITION Control
    There are two POSITION controls; one for channel 1 and the other for channel 2. They both shift their respective channel displays in a vertical direction.

    MODE Switch

    A five position switch that sets the mode of operation. The positions are as follows:
       CH 1Connects the internal circuits to operate channel 1 only. Channel 2 will not operate.
    CH 2Connects the internal circuits to operate channel 2 only. Channel 1 will not operate.
    ALTER.Sets the amplifier channels to display on alternate sweeps. For example, the first sweep whould be the channel 1 signal, the second sweep the channel 2 signal. The flicker between channels will depend on the sweep rate of the time-base plug-in.
    CHOP.Electronically switches between channels at a 200 Hz. rate with 5 μsec samples of each input shown on the display. The switching transients are removed by dual-trace blanking of the crt (See oscilloscope instruction manual).
    ADDED   The algebraic sum of the channel 1 and channel 2 signals will be displayed with the MODE switch in this position. To measure the difference between these signals, turn the INV. (CH 1) NORM. switch to INV.

    INV. (CH 1) NORM. switch

    A two position switch that presents the channel 1 signal in normal or inverted polarity.


    A push-pull switch that connects the channel or channels in use to the time-base trigger circuits when pushed in, and connects channel 1 only to the trigger circuits when pulled out. It is especially useful in Alternate and Chopped modes where triggering becomes difficult with certain waveforms.


    A screwdriver adjustment that sets the gain of the output amplifier. When the Type 3A1 is moved from one oscilloscope to another, adjust the gain to compensate for differences in crt sensitivities.

    DC BAL.

    This control is a screwdriver adjustment of the dual-range or "back lash" type. If the dc balance of a channel is not properly adjusted, the position of a no-signal trace will shift on the crt as the VARIABLE VOLTS/DIV. control of that channel is turned.

    excerpts from Textronix Type 3A1 Instruction Manual
    3366-148Knob (Position)
    5366-189Knob, Small Inner (Inv/Normal)
    5366-175Knob, Large Outer (MODE)
    10366-031Knob, Small Inner (Variable)
    10366-160Knob, Large Outer (Volts/Div)
    8384-245Rod w/ Knob (Trigger-Ch 1)
    R153,R253311-1142 x 250K (POSITION)
    SW155260-442Rotary (concentric w/ SW290) INV/NORM
    SW290260-442Rotary MODE
    SW150,SW250311-319Ganged w/ R139/239 - CAL
    R139,R239311-319375ohm Variable (gang w/ SW150/250) (VARIABLE)
    SW110260-441Rotary Switch (VOLTS/DIV)
    SW101260-448Slide (AC-GND-DC)
    SW390260-212Slide (TRIGGER)