7A18

Manuals – Specifications – Links – Pictures

The Tektronix 7A18 is a general-purpose 75 MHz dual-trace vertical plug-in for 7000-series scopes. Model 7A18N is identical except it omits the readout circuits.

Option 06 (first offered in 1975 catalog) adds offset voltage controls giving ±200 Div offset range. This option replaces the IDENTIFY buttons with DC offset controls, and adds a fourth "DC offset" position to the input coupling switch.

The 7A18A is mostly identical to the 7A18 predecessor, but replaces the custom low inductance potentiometers for gain and variable gain setting with a FET attenuator and regular potentiometers.

The 7A18(A) uses a discrete front-end followed by M36 channel switch ICs for the vertical and trigger signals.

The 200 MHz 7A26 and 400 MHz 7A24 share the same front panel layout and operation.

Key Specifications

Input impedance	1 MΩ // 20 pF
Deflection	5 mV/div to 5 V/div in 1–2–5 sequence, 2%
Bandwidth	0 – 75 MHz (DC), 10 Hz – 75 MHz (AC)
Noise	< 300 μV in 5 mV/Div
Option 06 offset range	±200 Div equiv. from ±1 V in 5 mV/Div range up to ±200 V in 1 V/Div range (above that limited by maximum input voltage)
Features	Mode: CH1 only, CH2 only, Add CH1+CH2, Alternating, Chopped. CH2 can be inverted. Input sockets with contact ring for probe ×10 / identify information No bandwidth limiter switch
Weight	1.4 kg

Links

• Tektronix 7A18 II Repair and Alignment (With a Deeper Look at the High Frequency Compensation) @ Zenwizard Studios YouTube
• Tektronix 7A18 Repair and Function Generator Alignment @ Zenwizard Studios YouTube

Documents Referencing 7A18

Patents that may apply to 7A18

Prices

Year	1971	1976	1980	1984	1990
Catalog Price	$535	$615	$915	$1,250	$1,790
In 2023 Dollars	$4,100	$3,300	$3,400	$3,700	$4,200

Repair issues

Attenuator solder joints

The pins of the attenuator modules are connected to conductive traces on the ceramic substrate inside by low-temperature soldering, which can go bad. Re-seating the module can temporarily restore the internal contact but will usually not fix the problem permanently. Repair is possible by opening the click-locked lid and carefully resoldering.

Variable gain pots

These pots (R237A,B and R437A,B 315-0241-00) are bonded together - that bond can break which stops the gain push from working. Looking at a few 7A18s, I see that the design was changed at least once.

Pictures

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  7A18 Front

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  7A18 attenuator opened. One of the attenuation R/C modules has been unplugged to reveal the switch contacts.

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  7A18 attenuator, bottom (switch) side

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  A 7603 with two 7A18s

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  7A18 in 7613 mainframe

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  7A18 and 7A18N from service manual 070-1126-01

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  7A18N with "Identify" seen on an auction site - probably cobbled parts

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  7A18N Front

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  7A18N RHS switch cover removed

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  7A18N LHS switch cover removed

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  Broken bond between ganged gain pots

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  Broken bond between ganged gain pots zoomed

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  Intact epoxy between ganged gain pots

Development Prototypes

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  First prototype of 7A18

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  First prototype of 7A18

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  Second prototype of 7A18

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  Second prototype of 7A18

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  Third prototype of 7A18

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  Third prototype of 7A18

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  Third prototype of 7A18

7A18A

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  7A18A Front

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  7A18A left side. The step attenuators can be seen on the left, the amplifier board on the right.

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  7A18A right side. The circuit boards on top carry the readout circuitry.

Option 06

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  7A18A Opt.06 Front. AC/GND/DC switches have an extra rightmost "DC Offset" position enabling the offset controls.

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  7A18A Opt.06 left side (rear). Note the two blue 10T-potentiometers for offset voltage adjustment.

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  7A18A Opt.06 - offset controls take the place of the Identify buttons on the standard model.