7904

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The Tektronix 7904 is a 500 MHz, non-storage 7000-series oscilloscope mainframe with four bays. It accepts two 7000-series vertical plug-ins and two 7000-series horizontal plug-ins. It was introduced in late 1971/early 1972 (Note 1). The successor model 7904A was introduced in 1984. There is also a rack-mount version, the R7903, albeit with only one horizontal bay.

Performance is largely determined by the plug-ins. The 7A19 vertical amplifier and 7B92 time base were introduced along with the 7904 to match the 500 MHz system bandwidth.

Specifications

Bandwidth 500 MHz
Fastest calibrated sweep 500 ps/Div (with 7B92A, other time bases "uncalibrated for first 60 ns")
Acceleration voltage 24 kV (+21 / -3 kV)
Calibrator (7904) 4 mV, 40 mV, 400 mV, 4 V or 40 V (p-p) into 1 MΩ; 2 mV, 20 mV, 200 mV or 400 mV into 50 Ω; or 40 mA; selectable 1 kHz, ½ rep. rate of B Sweep Gate, or DC
Calibrator (7904A) 4 mV to 4 V (p-p) in decade steps, 1 kHz; 40 mV to 400 mV into 50 Ω
Power consumption 190 W
Weight 13.5 kg (30 lbs)
CRT Options

7904A differences

The 7904A re-implements the vertical amplifier using Hypcon-packaged amplifier chips designed for the 7104, such as the 155-0178-00.

The 7904 had an elaborate 5-decade built-in calibrator with variable rate that was dropped from the 7904A. In the 7904A, The Control Illumination switch moved to the rear panel, and the readout system gained a pulsed mode in which it is synchronized with the sweep instead of interrupting the display at random times. This pulsed mode is not compatible with all time base plug-ins (see manual p. 1-12).

Options

  • Opt.2: X-Y delay compensation network
  • Opt.3: Extra EMI shielding
  • Opt.4: Maximum-brightness CRT with reduced area
  • Opt.78: P11 phosphor

Operation

The 7904 has a many modes of operation. One mode is called "Independent-Pairs Display" in the manual, where the input to the left vertical plug-in is displayed using the B horizontal time-base plug-in, and the input to the right vertical plug-in is displayed using the A horizontal time-base plug-in. This resembles the trace-slaving "ALT" mode that is produced by an 1A4 in a 547. Many other modes are available. It is possible to put vertical plug-ins in the horizontal slots for X-Y operation. It is even possible to put a horizontal plug-in in one of the vertical slots to produce a raster scan.

A special plug-in, the 7A21N, provides direct access to the deflection plates up to 1 GHz at an uncalibrated sensitivity of 4 V/Div. This plugin requires modifications to the mainframe to bypass the Y output amplifier (thus disabling readout and the other vertical plugin bay).

Internals

The 7904 uses a switch-mode power supply and has no fan. It incorporates a 65 ns Y delay line. This delay is too short for some older time bases such as 7B53 to display the trigger event.

Thor Hallen designed the main vertical amplifier and the 7A19 plugin (Note 1). It used bridged T coils and the "ft doubler" circuit patented by Carl Battjes to achieve its bandwidth.

The 7904 makes extensive use of custom integrated circuits and hybrid circuits, inter alia, the 155-0064-00 470-ps output amplifier, 155-0022-00 analog multiplexer for channel switching, 155-0059-00 HF amplifier, 155-0095-01 and 155-0095-03 differential HF amplifiers, and 155-0067-02 SMPS controller.

The 7904A includes 155-0173-05, 155-0175-05 and 155-0178-05 amplifiers.

The signal path is completely differential and the internal signal wiring is 50 Ω coax. The CRT (154-0644-05) has distributed vertical deflection plates.

Prices

  Year 1972 1976 1980 1984 1990
Model 7904 7904 7904 7904A 7904A
Mainframe only Catalog price $2,900 $4,100 $6,265 $8,810 $12,750
2016 value $16,500 $17,100 $18,000 $20,100 $23,100
Mainframe with 7A19,
7A19 Opt. 04, 7B92
Catalog price $5,500 $7,765 $12,015 $17,425 $24,190
2016 value $31,200 $32,400 $34,600 $39,800 $43,900

Links

Notes

  1. See 7A11 and John Addis, Good Engineering and Fast Vertical Amplifiers, in Jim Williams (Ed.), Analog Circuit Design: Art, Science and Personalities (1991), p.114, and Tektronix New Products 1972 brochure.

Pictures

7904 non-A

Power Supply

7904A

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