7904

Manuals – Specifications – Links – Pictures

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.

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.

Key Specifications

Bandwidth	500 MHz (with 7A19 or 7A29 amplifier)
Fastest calibrated sweep	500 ps/Div (with 7B92A, other time bases "uncalibrated for first 60 ns")
Trigger Source	Left / Vert Mode / Right
Outputs	Vertical Signal Out (25 mV/Div into 50 Ω, 0.5 V/Div into 1 MΩ); +Sawtooth; +Gate; camera power; probe power
Inputs	Z Axis (15 Vp-p)
CRT	Standard: (8×10 Div @ 1 cm), P31 (green) 154-0644-05 or P11 (blue) 154-0644-09, acceleration voltage 24 kV (+21 / –3 kV) Option 4, high writing speed: (8×10 Div @ 0.5 cm) , P31 (green) 154-0661-05 or P11 (blue) 154-0661-09
Calibrator	DC, square wave (1 kHz or ½ B Sweep gate), 4 mV to 40 V in decade steps (2/20/200/400 mV into 50 Ω); 40 mA current loop
Power	90−132 V or 180−264 V, 48−440 Hz, 190 W max.
Weight	13.5 kg (30 lbs)
Dimensions	34.2 cm × 30.5 cm × 69.5 cm (13.5 in x 12.0 in x 23.8 in) H×W×L
Options	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

Links

• 7904 Repair with Video
• Tek 7904 / 7904A @ amplifier.cd
• Repairing front panel buttons of Tektronix 7904 oscilloscope
• Tektronix 7904A lab check in and LED MOD by Zenwizard Studios @ YouTube

Documents Referencing 7904A

(no results)

Patents that may apply to 7904

7904A differences

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).

Operation

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.

Internals

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.

7904

The 7904 uses a switch-mode power supply and has no fan.

Thor Hallen designed the main vertical amplifier and the 7A19 plugin (Note 1). It used bridged T coils and the "f_t 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.

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.	−

7904A

The 7904A is a complete redesign and uses the 155-0173-05, 155-0175-05 and 155-0178-05 amplifiers developed for the 7104. A fan is added at the rear switch mode power supply.

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
	In 2024 Dollars	$21,800	$22,600	$23,900	$26,600	$30,600
Mainframe with 7A19, 7A19 Opt. 04, 7B92	Catalog price	$5,500	$7,765	$12,015	$17,425	$24,190
	In 2024 Dollars	$41,300	$42,900	$45,800	$52,700	$58,100

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

• 

  7904 (non-A) front

• 

  7904 rear

• 

  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

• 

  front-panel controls

• 

  7904 block diagram

• 

  A 7B92A (B098xxx) displaying a 1.01 GHz sine in HF Sync mode (7904 mainframe, 7A19 amplifier).

Power Supply

• 

  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

• 

  Switching transformer

• 

  Secondary side rectifier board

• 

  Secondary side post-regulator

• 

  HV block, multiplier

7904A

• 

  7904A

• 

  7904A Rear

• 

  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 Topleft

• 

  7904A Closeup of Vertical amplifier

• 

  7904A Low-voltage Power Supply Schematic

• 

  7904A Horizontal and vertical amplifiers

Parts

Some Parts Used in the 7904A

Custom ICs used in the 7000 series readout system

Page	Model	Part nos	Description	Designers	Used in
155-0014-01	M019D	155-0014-00 • 155-0014-01	analog-to-decimal converter	Barrie Gilbert	7000 series readout system • 7854 • 7934 • 7J20 • 7L5 • P7001
155-0015-01	M020F	155-0015-00 • 155-0015-01	analog data switch	Barrie Gilbert	7000 series readout system • 7854 • 7934
155-0017-00	M025	155-0017-00	5 MHz decade counter	Barrie Gilbert	7000 series readout system • 7854 • 7934
155-0018-00	M026	155-0018-00	zeros logic control	Les Larson	7000 series readout system • 7934
155-0019-00	M027	155-0019-00	decimal point and spacing control	Les Larson	7000 series readout system
155-0020-00	M028	155-0020-00	output assembler	Les Larson	7000 series readout system
155-0021-00	M029C	155-0021-00 • 155-0021-01	timing generator	Les Larson	7000 series readout system • 7854 • 7934
155-0023-00	M160	155-0023-00	character generator (0 1 2 3 4 5 6 7 8 9)	Barrie Gilbert	7000 series readout system
155-0024-00	M161	155-0024-00	character generator (↓ < I / + - + C Δ >)	Barrie Gilbert	7000 series readout system
155-0025-00	M162	155-0025-00	character generator (m μ n p X K M G T R)	Barrie Gilbert	7000 series readout system
155-0026-00	M163	155-0026-00	character generator (S V A W H d B c Ω E)	Barrie Gilbert	7000 series readout system
155-0027-00	M164	155-0027-00	character generator (U N L Z Y P F J Q D)	Barrie Gilbert	7000 series readout system