512

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The Tektronix Type 512 is a 2 MHz oscilloscope with differential inputs, introduced in 1948. Tektronix engineer Logan Belleville did most of the design work for the 512. Frequency response extending all the way down to DC was a distinguishing feature at the time the 512 introduced.

Specifications

Bandwidth DC to 2 MHz for sensitivity of 0.15 V/cm or lower. DC to 1 MHz for sensitivity between 5 mV/cm to 0.15 V/cm
Rise time 200 ns for for sensitivity of .15 V/cm or lower. 400 ns for for sensitivity between 5 mV/cm to 0.15 V/cm
Input Impedance 1 MΩ // 45 pF
Maximum sensitivity 5 mV/div
Sweep 0.3 s/cm to 3 μs/cm ±5%, 20% magnification with magnifier in
Time Marker Input 8 V Minimum
Output Signals Sweep, Delayed Trigger, Gate
Calibrator ~1 kHz, 50 mV to 50 V , ±5%, square wave
CRT 5CPA Curved face or 5ABP Flat face, 6(V) × 10(H) divisions
Line Voltage 105-125 V or 210-250 V AC 50-60 Hz
Power Consumption 280 W
Dimensions (H/W/D, 15.5" × 12.5" × 21.5")
Weight 54 lbs


Links

Internals

Vertical Amplifier

The vertical signal path, fully differential and DC coupled from the front panel A and B inputs to the vertical deflection plates, is as follows:

  • Stage 1: differential, 5879 pentodes
  • Stage 2: differential, 12AU6 pentodes
  • Stage 3: differential, 12AU6 pentodes
  • Stage 4: differential, 12AU6 pentodes
  • Stage 5: differential, 6AG7 pentodes

The 512 has two inputs, "A" and "B", each having its own UHF connector. The scope can be used with a single input or it can used in "A-B" mode, where the displayed signal is the difference between the voltage at the A input and the voltage at the B input. Built-in true differential inputs are also seen in later scopes such as the 502, 503, and 504.

The vertical pr-amplifier stage consisting of stage 1 and 2 is mounted in a sub-frame directly beneath the Vertical Deflection Sensitivity switch. This stage is switched out of the signal path for higher input signal levels.

A cathode follower voltage regulator V11 - 12AU7 - is used to supply the reduced and regulated plate voltages to stage 1 and 2. Similarly another 12AU7 - V12 - is used for both screen and plate supply to stage 3 and 4.

Matched pairs of 12AU6 and 5879 are kept inside the instrument as spare. They are mounted in a bracket behind CRT base.

Sweep/Horizontal Amplifier

Tek 512 uses a Phantastron oscillator with miller integrator for sweep generation. The final DC output level of the horizontal amplifier is dropped using a neon divider before driving the deflection plates. This is done to achieve better beam focus by approximately matching deflection plate DC level to that of 2nd anode of the CRT.

CRT

Up to serial number 2525, the 512 used the 5CPA CRT. After that, the 5ABP CRT was used.

The Tektronix 512 normally came with P7 phosphor. P1 and P11 were optional, at no charge. Options were also available for different ranges of sweep rates.

Total 3 kV of acceleration voltage, −1500 V for the cathode, +1500 V for the anode.

Timing Capacitors

The Tek 512 manual says,

Since 1954, we have manufactured our own timing capacitors with the characteristics needed to maintain sweep-time accuracy and linearity. The capacitance ratio between the capacitors used is accurate within half of one per cent so that the time-base calibrations will be right at all speeds. Most capacitors change value with voltage, temperature, and age. Variation of capacitance with voltage is particularly undesirable because it causes nonlinearity of the time-base sawtooth. Our timing capacitors are especially free from this voltage effect. They also have minimum temperature and aging variations.

HV Power Supply

Tektronix 512 oscilloscopes with serial numbers 101 through 2146 have an unregulated HV power supply that uses a 2 kHz oscillation frequency. Starting at serial number 2147, the HV power supply runs at 70 kHz and has regulation in form that is typical of Tek scopes of the 1950s and 1960s − the CRT cathode voltage is divided, compared with the −150 V supply and an error signal is produced, which controls the screen voltage on the HV oscillator pentode 6AQ5.

In both the regulated and unregulated versions of the Tek 512 HV supply, the CRT cathode and anode voltages are produced by half-wave rectification of a single secondary on the HV transformer. Later Tek scopes, e.g., the 531, have two secondaries on the HV transformer, one to produce the CRT grid voltage and the other to produce the CRT cathode and anode voltages. The later design allows the blanking signal from the sweep circuit to control the DC voltage of the "grounded" end of the secondary that produces the CRT grid voltage. When the grounded end is shifted up and down by 50 V, the CRT grid voltage is shifted up and down by 50 V relative to the constant cathode voltage, thereby controlling CRT beam current.

Trace Blanking

The blanking circuit in the 512 is somewhat unusual. Since the 512 is designed to work all the way down to DC and support slow sweeps with long waiting time between trigger events, the blanking circuit needs to be able to keep the beam on or off for unlimited time, which precludes simple AC-coupled blanking. The approach taken in the 512 was to modulate the blanking signal on a carrier, much like a CW transmitter sending morse code (or any other on-off signal). The signal passes through a transformer which has approximately 0 VDC on the primary side and approximately −1500 VDC on the secondary side for grid bias. The signal from the secondary is demodulated using a 6AL5 diode detector (rectifier), recovering the blanking signal, which controls the grid bias voltage on the CRT. The 6AL5 blanking detector and its filament is elevated to -1500V.

Power Supply

The power supply is stacked design with a 5651 voltage reference tube as the reference, 6AU6 as error amplifier, and 12AU7 and 6AS7 as series pass tubes. The power rectifiers in the 512 are all tubes, 6X4 and 6W4. Instead of using a long tail comparator, the 5651 is used to bias the cathode of 6AU6 error amplifier in the -150 V reference regulator. The rest of the design is very similar to what evolved and was later used in the 547, 549, and most of rest of the 500 series scopes. It has no thermal cutoff.

Pictures