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.
Design Staff of 7904 Project (from July 1971 Tekscope):
Val Garuts was project manager and developed the large signal amplifier circuit used in the horizontal and Z-axis amplifiers. Thor Hallen did the 7A19 plug-in and the vertical amplifier, and John McCormick the horizontal. The trigger and time base (7B92 plug-in) were done by Les Larson and Bill DeVey. Bill Peek worked on the Z-axis amplifier and auto-focus, with Hans Springer doing the mainframe interface and channel switching. Joe Burger's work on the power supply, coupled with Joel Swanno’s efforts in mechanical design, reduced the weight to only 30 pounds. Ken Hawken did the fine job on the CRT.
|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; |
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)|
- A Subnanosecond Realtime Oscilloscope, Tekscope Vol.3 No.4, July 1971
- 7904 Repair with Video
- Tek 7904 / 7904A @ amplifier.cd
- Repairing front panel buttons of Tektronix 7904 oscilloscope
- Tek 7904 / 7904A @ barrytech.com*
- Tektronix 7904 lab check in and LED MOD by Zenwizard Studios @ YouTube
Documents Referencing 7904
|Tekscope 1974 V6 N3.pdf||Article||Servicing the 7904 high-efficiency power supply||1974||7904|
Documents Referencing 7904A
The 7904A (introduced in 1984) borrows heavily from the previously introduced 7104 in terms of front panel layout, mechanical design, and electronics, re-implementing the vertical amplifier using Hypcon-packaged amplifier chips such as the 155-0178-00.
The 7904 had the elaborate 5-decade built-in calibrator with variable rate that was common on all first-generation 4-bay 7000-series scopes, it was dropped from the 7904A. The rear remote control connector unique to early 7000-series scopes was dropped as well. 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).
- Opt.1: deletes read out system
- Opt.2: X-Y delay compensation network
- Opt.3: Extra EMI shielding
- Opt.4: Maximum-brightness CRT with reduced area
- Opt.78: P11 phosphor
The 7904 has 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).
A rear-panel remote control connector has connections for single-sweep ready lamps, a reset button, and readout control.
The 7904(A) 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. 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.
The 7904 uses a switch-mode power supply and has no fan.
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.
Early 7904 have issues with horizontal aberrations for sweep rates of 1 ns/Div or faster. After discovery of the issue modification kits that replace the horizontal amplifier were offered. For more information see the table below and the manual of the 067-0657-00 Normalized Ramp Generator that was used to investigate and rectify this issue.
|Serial Number||A9 Horizontal Amp.||Issue||Modification Kit|
|B010000-B081987||670-1632-00||Fully calibrated sweeps for rates of 1 ns/Div or faster requires a dedicated 7B92 (non A!) and the two units to be calibrated as a system.||050-0636-01|
|B081988-B219999||670-1632-01||Can be calibrated to either work with magnifier timebases (7B70,7B80), or alternatively non-magnifying time bases (7B92A), but not both.||040-0791-00|
|B220000 and above||670-1632-02/03||None - fully compatible with all timebases and all sweep rates.||−|
|Mainframe only||Catalog price||$2,900||$4,100||$6,265||$8,810||$12,750|
|In 2022 Dollars||$19,500||$20,300||$21,400||$23,800||$27,400|
|Mainframe with 7A19,
7A19 Opt. 04, 7B92
|In 2022 Dollars||$37,000||$38,400||$41,000||$47,200||$52,000|
- 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.
7904 (non-A) front
Left side. Left top: Vertical amplifier; left bottom: switch-mode power supply; right center: delay line; right bottom: plug-in bays
Right side. Top left: calibrator; bottom left: plug-in bays; top center: readout board; top right: Z amplifier
Right side detail. Left: calibrator; right: readout.
Plug-in backplane board front (left trigger selector piggyback board removed)
B Trigger selector piggyback board (A6)
Vertical interface piggyback board (A7). The two 155-0095-03 amplifiers (large metal cans) are used to select the signal from one vertical plug-in bay, and drive the delay line.
Plug-in backplane board rear
Backplane board sandwich seen from below
Horizontal (X) amplifier board (late >B220000)
Vertical (Y) amplifier board
Z amplifier board
7904 block diagram
7904 switchmode power supply, covers removed
View from right (as seen from front of instrument)
Primary side circuit board
Primary side capacitors
Detail view of transformer from the side
Secondary side rectifier board
Secondary side post-regulator
HV block, multiplier
7904A Top with Horizontal Amplifier at top right
7904A Right with Z-axis on top right and 2nd Gen. Readout board at bottom right
7904A Left with Vertical Amplifier (top) and Vertical switch board (bottom)
7904A Closeup of Vertical amplifier
7904A Low-voltage Power Supply Schematic
7904A Horizontal and vertical amplifiers
Some Parts Used in the 7904
Some Parts Used in the 7904A