Keithley 6517A electrometer restoration project

Intro

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, these devices are manufactured in far less volumes, require careful labor work for assembly. As result they often cost much more than regular DMM, often over $5000 USD. Current Model 6517B is listed now for $8270 USD on Tektronix site. This is $1400 more than top of the line, 8½-digit Keithley Model 2002 DMM.

Today we will attempt to service electrometer after unsuccessful 3rd-party repair, Model 6517A.


Image 1: Keithley 6517A face, as is received for service

Disclaimer

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  • Redistributions of article must retain the above copyright notice, this list of conditions, link to this page (https://xdevs.com/fix/k6517a_u2/) and the following 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 Keithley/Tektronix 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 instrument repairs or provide extra information, you can do so following these simple instructions

Manuals references

Exterior


Image 2-3: As received, dirty case and unit parts.


Image 4: Rear connectors

This electrometer is 100% silent, as it does not have any cooling fans.

Design and internal design review


-1200 and +1200VDC voltages present inside of this electrometer when operation are dangerous and even lethal. Pay attention of what you are doing, and keep all connections fixed and robust. Do not use hands during probing unless checked and absolutely safe. Use tools rated for CAT III operation while servicing high-power and high-voltage circuits in this instrument.


Image 5: Instrument overall block diagram, courtesy Keithley service manual.


Image 5: Power rails and supplies, courtesy Keithley service manual.


Image 5: Analog isolated domain block diagram, courtesy Keithley service manual.

Damaged parts (and current status):

  • Destroyed analog PCB. Replacement WIP
  • Fake FETs in voltage source regulator TBD
  • Bad DAC TBD
  • Bad capacitors TBD
  • Mains input CMF assembly not original TBD
  • Bad SMT resistors and ruined pads TBD
  • Broken VFD glass, no vacuum TBD
  • Damaged banana jacks TBD
  • Dirty yellow front panel TBD
  • Unit out of calibration TBD


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side


Image 5: Digital board on bottom side and analog assembly on top side

ADC here is very same module that used in 7½-digit Keithley 2001 DMM. Module have ALTERA EPM7128 in PLCC64 package, configured to 2001-801-A01 RTL clocked from 7.68MHz XO. SPI-like interfaces are used as data interface to digital board. This chip also act as bridge to all analog switches and control muxes between isolated analog and earth-referenced digital domain.


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge

Repair workflow

Review soldering instructions prior to any reworks or soldering on the instrument boards. Failure to apply care during rework may and will cause damages to sensitive circuits and components!

  • Use an OA-based non-acid flux, and take care not to spread the flux over unrelated areas of the circuit board.
  • Carefully remove the flux from the work areas when the repair has been completed. Use IPA and pure distilled water along with clean foam-tipped swabs or a clean soft brush to remove the flux. Be sure not to wash contaminated flux or water over other PC board areas.
  • Once the flux has been removed, swab only the repaired area with methanol, then blow dry the board with dry nitrogen gas.
  • After cleaning, the board should be allowed to dry in a +50 °C low-humidity environment for several hours. There should be no residues or foreign particles visible on any surface.

Digital board

Let’s try with easy stuff first, digital board.


Image 5: Testing digital board operation and power consumption, measured by Keysight E36312A PSU.

Front panel

Analog board

AC power is applied to the AC receptacle J1001 through the fuse F101, line switch S101, and line voltage selection switch S103 to the power transformer T1. The power transformer has a total of 10 secondary windings for the various DC supplies in the instrument.


Image 5: A/D Converter and interface bridge

The input preamplifier stage provides the high input impedance necessary for the voltage function, as well as the low input bias current and current- or charge-to-voltage conversion for the amps, ohms, and coulombs functions. The input preamplifier is essentially made up of three sections: the input stage, range/function switching, and the output stage.

The input stage is made up of U405 and associated components. This IC has the required low input bias current, and it also provides the overall gain for the input preamplifier.

The output stage includes Q402-Q413 and associated components. This stage supplies the necessary voltage and current drive capability for the input preamplifier.


Image 5: A/D Converter and interface bridge


Image 5: High-impedance shielded circuit section, top and bottom view with shields removed.


Image 5: High-impedance shielded circuit section, top and bottom view with shields removed.


Image 5: High-impedance shielded circuit section, top and bottom view with shields removed.


Image 5: High-impedance shielded circuit section, top and bottom view with shields removed.


Image 5: High-impedance shielded circuit section, top and bottom view with shields removed.


Image 5: High-impedance shielded circuit section, top and bottom view with shields removed.


Image 5: Ultra-low input bias current TI LMC6001 opamp in hermetic package.


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge


Image 5: A/D Converter and interface bridge

Check power supply voltages

Section under test Expected procedure and range Reference for measurements
F101 line fuse Check continuity Remove to check
Line switch 115V/230V as required. Line voltage selection switch
Line power Plugged into live receptacle, power on Check for correct power up sequence
Q305, pin 1 +5V, ±5% Referenced to Common K (U301 pin 2)
U301, pin 1 +12V, ±5% Referenced to Common K
U302, pin 3 -12V, ±5% Referenced to Common K
+120VK (VR301 anode) 110V to 130V Referenced to Common K
-120VK (VR302 cathode) -110V to -130V Referenced to Common K
+1200VK (VR303 anode) 1100V to 1300V Referenced to Common K
-1200VK (VR304 cathode) -1100V to -1300V Referenced to Common K
+5VB (U408 pin 3) +5V, ±5% Referenced to Common B (BGND, U408 pin 2)
-5VB (Q401 pin 3) -5V, ±5% Referenced to Common B
+25VS (C461 +) 22V to 28V Referenced to Common S (SGND, U207 pin 2)
-25VS (C460 -) -22V to -28V Referenced to Common S
+5VS (U207 pin 3) +5V, ±5% Referenced to Common S
+220VS (C401 +) 200V to 240V Referenced to Common S
-220VS (C403 -) -200V to -240V Referenced to Common S
+15VS (U206 pin 3) +15V, ±5% Referenced to Common S
-15VS (U205 pin 3) -15V, ±5% Referenced to Common S

Test results

Supply Rectifier Filter Regulator Measured Result
+5VK CR306 C305, C308 Q305 NOK
+12VK CR307 C307, C309 U301 NOK
-12VK CR307 C306, C310 U302 NOK
+120VK CR301 C302 N/A NOK
-120VK CR301 C301 N/A NOK
+1200VK CR304 C311-C313 N/A NOK
-1200VK CR305 C315-C317 N/A NOK
+5VB CR403 C407, C408 U408 NOK
-5VB CR403 C422 Q401 NOK
+25VS CR402 C404, C461 N/A NOK
-25VS CR402 C402, C460 N/A NOK
+5VS CR201 C201-C203 U207 NOK
+220VS CR401 C401 N/A NOK
-220VS CR401 C403 N/A NOK
+15VS CR202 C211, C217 U206 NOK
-15VS CR202 C209, C218 U205 NOK

Conclusion

Cost involved in this project:

Item Cost Shipping Supplier
Dead Keithley 6517A N/A N/A Customer
Dead Keithley 6517 for donor parts $1000 ?0$ eBay
Transformer Keithley TR $600 N/A Tektronix, replaced by customer
30 pcs CHV2010-FX-1005ELFCT-ND Resistors 10 MΩ 2010 size $10 0$ Digikey USA

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

Date Activity Time spent
2/6/2018 Received unit, initial teardown, photos 3 hours

Discussion about this article and related stuff is welcome in comment section or at our own IRC chat server: irc.xdevs.com (standard port 6667, channel: #xDevs.com). Web-interface for access mirrored on this page.

Author: Illya Tsemenko
Published: Feb. 7, 2018, 9:03 a.m.
Modified: Feb. 27, 2018, 5 a.m.

References

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