7A12

Manuals – Specifications – Links – Pictures

The Tektronix 7A12 is a 120 MHz dual-trace vertical plug-in for 7000-series scopes. This original 7000-series plugin allows large dc offset voltages, however the offset voltage is not available on the front panel to be measured externally as with the 7A11.

Switches on the front panel control miniature relays in the signal path to select input attenuators, through logic circuits.

The 7A12 was designed by Roy Hayes (who also designed the 3A3 and 3A8 before leaving for HP Loveland in Jan 1971). The Evaluation Engineer for the 7A12 was Peter Starič, co-author of the book "Wideband Amplifiers" with Erik Margan.

Key Specifications

Bandwidth	120 MHz (79xx) / 105 MHz (77xx) / 55 MHz (74xx)
Deflection	5 mV/Div to 5 V/Div (1–2–5), variable up to 12.5 V/Div
Input impedance	1 MΩ // 24 pF
Timing	Delay difference between channels < 500 ps
Channel isolation	At least 3000:1 at 75 MHz
Maximum input	350 V DC + Peak AC at 1 kHz or less
DC offset	At least ±500 divisions

Internals

The 7A12 uses the 155-0032-00 as a gain stage and the 155-0022-00 for channel switching.

There were two mistakes, concept errors, that doomed the 7A12 from day one. The first was in deciding to use the 1 GHz F_T Tektronix IC process (called 50/450 for resistivities of the two base layers) for the signal path. This process was too slow for a plugin expected to provide 150 MHz bandwidth in the fastest mainframe, the 7704.

The second day one mistake, later pointed out by Tom Rousseau, was to arrange the two channels side by side. Roy wanted to be able to go from one sensitivity (deflection factor) to another without having to go through all the sensitivities in between as happens with a rotary switch. The vertical row of push buttons was functionally attractive. However, that would require both channels to occupy the same space on a single vertical circuit board.

The solution was two separate, mirror image attenuators and two identical amplifier boards all attached to a main vertically-centered circuit board. The two identical amplifier circuit boards, one mounted on each side of a center board, are cleverly flipped so both face outward from the center board. This is accomplished by having one near the top rail and the other near the bottom rail. There are two pairs of output sockets, only one of which is used per channel. That is expensive and complex.

Tom Rousseau designed a new dual trace plugin with discrete components on a single circuit board that had 80 MHz bandwidth and was much less expensive to manufacture. He did this on his own, not at management's request. Management liked it and it became the 7A18. Later Tom had the advantage of a 3 GHz IC process (SHF2). He used the M84 (155-0078-xx), already in production for the 485, and with circuitry similar to the 7A18. This was the 200 MHz 7A26. It became the best selling Tektronix plugin of all time. Tektronix literally gold plated a 7A26 and gave it to Tom in recognition.

See https://vintagetek.org/100000-7a26-plug-in/

Links

• 7A12 page @ amplifier.cd
• 7A12 page @ barrytech.com

Pictures

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  Front view

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  Front view

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  Attenuator schematic with relays

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  7A12 Amplifier PCB

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  Left side view