7A18

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.

Manuals – Specifications – Links – Pictures

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

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

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

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.

Components