Teardown of Keithley Model 3706A switch/scan DMM




System DMMs with integrated switch/scanner relay cards are popular choice for production and research environments and applications. Some of most popular examples of these type are HP/Agilent/Keysight 34970A, Keithley 2700 series and Keithley 200x with scan-card option. These provide few dozen of channels and capable to do all usual benchtop DMM functionality, including 4W-resistance, current and temperature measurements.

If more channels are required, one of options either use standalone switching matrix like Keithley 7001/7002/70x series mainframes or have universal unit with advanced switching capability. Keithley’s (today Tektronix) Model 3706A is a good example of such, allowing up to six large switching cards and having compact full-width 2U rackmount formfactor. It’s a fairly affordable unit for this niche application, with MSRP in range of $2040 – $2940 USD. 3706A Mainframe supports up to 576 input channels in maximum configuration and is designed to work with 3700 series add-on cards.

Image 1: Model 3706A


Below is documentation for both legacy Model 3706 (the unit we review today) and refresh Model 3706A, and compatible cards documentation.

Supported cards documentation


This box was last calibrated in 2007, and fully functional. So no repair is required this time.

This is older non-A version of Model 3706.

Little angry noisy fan.

Standard divider network array from Caddock, Model 1787-53, take care of high voltage range attenuation. Isabellenhutte PBV-Z 0.100Ω 1% shunt in large TO-style package used for opposite spectrum, high current ranges.

Main reference is well known remarked LM399, selected by Keithley. We saw these in many other Keithley instruments, such as Model 2000, Model 2001 and rest. It’s usual to see TF-245 custom resistor network too, but this time there is none in Model 3706. Two BBI networks in SO16 package are located on board near reference instead.

Digital brains of the instrument are fitted on small blue 8-layer PCB module (Keithley P/N 3706-140B1) with tented vias, manufactured in Canada somewhen in 2006. Main processor is based on Freescale (today NXP) MCF5474 series from ColdFire® V4 family. It’s pretty powerful beast with clocks up to 266MHz, MMU, dual FPU, native 10/100 Ethernet MAC and DDR/SDRAM memory controller. Well capable of running Linux. BootROM stored in 32Mbit NOR Flash nearby, Intel GE28F320C3BD70. There is also another BGA package, Xilinx Spartan XC3S500E FPGA as glue logic and interface bridge between processor and rest of the analog and interface side of the meter.

No much else happening on the controller board, rest of components are just few regulators, pair of SDRAM chips from Micron 46V16M16 in TSSOP and I/O connectors. Bottom side just features decoupling capacitors and other passive components.

Keithley Model 3720 Dual 1×30 mux card

Author: Todd M.
Created: Oct. 7, 2016, 4:37 a.m.
Modified: July 22, 2017, 5:33 a.m.