Keithley 6517 Electrometer / High-resistance meter repair

Contents

Intro


Image 1: Keithley 6517 electrometer

Keithley is well known brand in low-current/high-sensitivity instrumentation. They invented and designed many related instruments and today part of Tektronix, still providing solutions for science, semiconductor and analog application projects.

When design require measurement of very high resistance, starting from Megaohms or testing sensitive high-impedance circuit regular DMM cannot fullfil the task. Even typical 10 MΩ of typical DMM input impedance may ruin the measurement results, or even overload the circuit. Electrometers are special variant of the meter, designed with main purpose to have very high input impedance. Also being niche application tool, manufactured in far less volumes, they often cost much more than regular DMM, often many thousands $USD. Today we will look at such meter, Keithley Model 6517.

Blabla HP 4339.

Here’s a complete list common electrometers:

Manufacturer Model Type Status
Keithley 6517 EM/HRM Obsolete model
Keithley 6517A EM/HRM Obsolete model
Keithley 6517B EM/HRM Current model
HP 4339 HRM Obsolete model

Table 1: Electrometers DMMs

It does take tremendous amount of time and effort to test, validate and prove stability and performance of such machines, where no detail is insignificant.

Let’s see what we have to do to get unit back to operational state. My plan for this project is to perform full component level repair, and perform overall study of instrument’s internal construction, as repair advances. This means we will not be doing expensive boards swap unless absolutely unavoidable.

As usual, all photos are clickable for high-resolution version.

Specification

Brief specification lalala

Function Range
DC Voltage 100mV 1 V 10 V 100V 1000 V

Table 2: Model 6517 family key specifications

Disclaimer

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All information posted here is hosted just for education purposes and provided AS IS. In no event shall the author, xDevs.com site, or any other 3rd party, including Tektronix/Keithley be liable for any special, direct, indirect, or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortuous action, arising out of or in connection with the use or performance of information published here.

If you willing to contribute or add your experience regarding HP/Agilent/Keysight instruments repairs or provide extra information, you can do so following these simple instructions

Initial inspection and basic disassembly

Damaged parts (and current status):

  • Dead xfrmr Replacement WIP
  • Corroded traces on front-panel Cleaning/soldering pending

Unit covers are bit dirty overall.

Pay attention to mains voltage selection switches, located near mains IEC connector. Using incorrect voltage setting (e.g. 120VAC with 220VAC mains) will likely to destroy transformer, as it is linear type and directly wired to mains! This is likely what happened to my unit by previous owner, and was a root cause of damage. New transformer cost is $650 USD + tax, so was rather pricey mistake!

Manuals references

Original model, covered in this article.

Newer Model 6517A, which is obsolete today and replaced since 2008:

Latest and current Tektronix/Keithley Model 6517B:

Repair workflow

Before starting any repair, good idea to read thru repair manuals, to get idea of board functions and purpose.

Also worth to note requirement of adjustments/recalibration procedures, from repair manual, in case of board replacement/repair:

Ref Designator Agilent Part Number Assembly Description Adjustments Needed
A1 03458-66501 Front panel DC circuitry Offset, DC Gain,Resistance, and DC Current Adjustments
A2 03458-66502 AC converter AC Adjustment
A3 03458-66503 A/D converter and Inguard logic Offset, DC Gain,Resistance, DC Current,and AC Adjustments
A4 03458-66504 Inguard power supply ACAL ALL

Table 3: 3458A Board model numbers and definitions

Boards condition check

Before doing any repairs or applying power, wise to check physical condition of components and boards inside instrument. Often this easy operation can spot obvious failures, such as blown MOSFETs, burnt resistors or missing parts, saving time for further repair.

Have a close look on electrolytic capacitors, especially if instrument’s age is more than 10 years. If you see leaked electrolyte or bulging capacitors, do not apply power. Replace broken parts and clean board thoroughly, to avoid further damage. Often electrolyte soaks into PCB via’s and eat copper tracks, so pay close attention to condition. If any visible discoloration found, use DMM to check connection resistances, and rewire broken nets, if needed.

Good idea to replace old electrolytic aluminum capacitors to fresh ones even if they look okay. It does not cost much, but will save the question regarding if original ones are good or bad, or at borderline. Important to keep same voltage and capacitance rating, and ESR/ESL spec as original caps, unless you really know what you are doing.

Now take a look on PCB and fix issues as we go. Order is

Diagnostics

Now after all initial repairs are complete, it’s time to power it up and test main functions and operation, and likely jump into more complex debug & repair.

Firmware

In many cases an old unit might have been retrofitted with newer firmware, so later firmware revision cannot be used as proof of the date of manufacture.

ftp://ftp.xdevs.com/Keithley/6517/fw/C03/
ftp://ftp.xdevs.com/Keithley/6517/fw/C04/
ftp://ftp.xdevs.com/Keithley/6517/fw/C05/

ROM Revision 2 Revision 4.6 Revision 6 Revision 7 (1992) Revision 8 (1998) Revision 9 (latest)
U110 03458-88820 03458-88840 03458-88860 03458-88870 03458-88880 03458-88890
U111 03458-88821 03458-88841 03458-88861 03458-88871 03458-88881 03458-88891
U112 03458-88822 03458-88842 03458-88862 03458-88872 03458-88882 03458-88892
U113 03458-88823 03458-88843 03458-88863 03458-88873 03458-88883 03458-88893
U114 03458-88824 03458-88844 03458-88864 03458-88874 03458-88884 03458-88894
U115 03458-88825 03458-88845 03458-88865 03458-88875 03458-88885 03458-88895
03458-88887 Single ROM 03458-88897 Single ROM

Table 12: Firmware ROM dumps

To combine 6 ROM images into single one (for digital board with single 27C4002 EEPROM) this Python app was used.

Disabling VFD to save it’s life during remote operation

Vacuum fluorescent displays are prone to age and lose their brightness with age. This is visible by “burned out” pixels, which are constantly lit. To prevent this happening and prolong display’s life, it’s good idea to turn VFD OFF during remote/long-term operation. Simply send GPIB command DISP OFF plus a blank message text, such as:

:DISP:WIND:TEXT:DATA '                '
:DISP:WIND:TEXT:STAT ON

Calibration

Repair of instrument is one thing, but how do we know if it’s good and meets specifications? Well, we don’t, unless we test known standards/voltages and compare readings from meter to actual values. If reading not match actual value more than specification limits, meter need adjustment. Process of comparison measurements to known value (either it need adjustment or not) is called calibration.

Table 15: Calibration test result

Table 16: Initial calibration values readout

Also here’s toolkit to read calibration ROM via GPIB interface:

Performance verification

To be done.

Restoration summary

Item Cost Shipping Supplier
Dead Keithley 6517 ??0$ ??0$ eBay
Transformer Keithley TR 6??$ N/A Tektronix

Table 21: Project cost

Also some generic parts were ordered from DigiKey store, such as DC fan, capacitors for A6 board, suspect resistor arrays for logic on DC board and new IEC connector mains filter. Here’s order list.

Quantity Part Number Description Extended Price USD
3 399-4433-ND CAP CER 2.2UF 50V X7R RADIAL 9.72

Table 22: First Digikey order for parts

Total repair cost in parts as for today: $??. USD

Date Activity Time spent
8/21/2017 Received unit, initial teardown, photos 3 hour

Table 23: Time worklog summary

Total man-hours spent on this project around 6 hours, give or take few. Was it worth doing?

  • Nikon D750 with 105/2.8Micro + AF-S 35/1.8 DX, tripod, light
  • 1L of IPA and alcohol to clean chassis and parts
  • Google – used lots of it!

Thanks for reading. Hope you like the article, and if you do have any questions, jump in comments!

Article revision: 1
Author: Todd M.
Published: Aug. 21, 2017, 12:51 p.m.
Modified: Aug. 31, 2017, 9:39 p.m.

References

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