- Manuals and service information
- Design and internal design review
- Repair workflow
- Conclusion and summary
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.
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If you willing to contribute or add your experience regarding instrument repairs or provide extra information, you can do so following these simple instructions
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
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.
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.
Let’s try with easy stuff first, digital 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.
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: Ultra-low input bias current TI LMC6001 opamp in hermetic package.
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|
Cost involved in this project:
|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
|2/6/2018||Received unit, initial teardown, photos||3 hours|
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Modified: March 15, 2020, 7:46 p.m.