Keithley 6514 Electrometer repair

Contents

Intro

Electrometers are special purpose instruments, which key feature is very high input impedance. They usually have voltage, current, resistance and charge measurement functionality, just like usual DMM, but can do these measurements with minimal loading on the circuit under test.

Modern solid-state electrometers, such as Model 6514 measure input signal by voltage comparator to an internal reference voltage source using an electronic circuit with a very high input impedance, in range of 1014 Ω. A similar circuit used also as a I-to-V converter, to allow electrometer to measure currents down with 10-15 A (femtoamperes) or 0.000000000000001 A resolution. Combined with an internal high voltage source, this current measuring capability also can be easily adapted to measure very high resistances, such as 1017 Ω, which is 100 PΩ(Petaohm).

These instruments usually have special low-leakage shielded input connectors, often with triaxial design with PTFE insulators,


Image 1: Model 6514 front face


Image 2: Model 6514 rear terminals and I/O

Instrument have no missing parts and overall very clean. No surprise, as there are no fans in it.

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 Instruments or 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 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 any test instruments or provide any extra information, such as firmware dumps, internal photographs, you can do so following these simple instructions

Manuals

Here are available manuals from KI so far.

As with all recent Keithley gear, schematics or detailed construction information is not available.

Teardown

Removing cover (which is same as Model 2000, 2000-307 C) is easy task, just few screws and we are in.


Image 3-4: Main PCB, top and bottom view


Image 5: Model 6514


Image 6: Model 6514


Image 7: Model 6514


Image 8: Model 6514


Image 9: Model 6514


Image 10: Model 6514

Front end opamp is Texas Instruments LMC6001, specified and tested for 25 fA bias current. Along with Analog Devices ADA4530-1, AD549 and LTC6268 these opamps are one of lowest current bias spec available on the market.


Image 11: Model 6514


Image 12: Model 6514


Image 13: Model 6514


Image 14: Model 6514


Image 15: Model 6514


Image 16: Model 6514


Image 17: Model 6514 voltage reference

Ovenized 7V Linear LM399 used here as voltage reference, very common in 6½-digit meters like Keithley 2000, HPAK 34401A, and so on. Resistor network TF-245 used to generate secondary voltages and currents. This is same network used in Model 2000 and Model 24xx series SMUs, and could be faulty in some cases.


Image 18: Model 6514

Power supply transformer is TR-330 Rev.A, manufactured by NORTHLAKE Engineering for Keithley.

Bulk capacitors are listed in table below:

Capacitor Value
C74 2200uF 16V
C93, C92, C96, C86 1000uF 50V
C94, C95 47uF 400V
C100 2200uF 16V
C72, C96 100uF 63V
C75 6800uF 16V

Repair

Voltage rail Spec Measured Condition
+15VS analog +15V to SGND +14.963 VDC PASS
-15VS analog -15V to SGND -15.104 VDC PASS
+5VS analog +5V to SGND +4.999 VDC PASS
+25VS analog TP112 +25V to AGND (TP103) +35? VDC PASS
-25VS analog TP111 -25V to AGND -35? VDC PASS
+220VS analog TP113 +220V to AGND (TP103) +250? VDC PASS
-220VS analog TP114 -220V to AGND -250? VDC PASS
-5VB analog (TP116) -5V to BGND (TP105) -5.006 VDC PASS
Reference (TP123) +6.95V to REFLO (TP122) +6.941 VDC PASS
-14V (TP119) -14V to SGND -13.881 VDC PASS
+14V (TP120) +14V to SGND +13.881 VDC PASS
+5VRF (TP118) +5V to SGND +4.963 VDC PASS
+5VD digital +5V to DGND +4.974 VDC PASS
VFD drive voltage +50V to DGND +48.39 VDC PASS

Firmware

Firmware stored in standard JEDEC-compliant memory ROMs which can be UV EPROM or OTP PROMs. Some, but not all, compatible ROM ICs:

  • ST 27C4001-15F1
  • AMD 27C040 (UV EPROM)
Firmware version Combined binary ODD ROM EVEN ROM
Model 6514 A11, 2003 6514-A11 N/A N/A

Calibration ROM dumps:

Calibration ROM Combined CALROM Dump U633 Dump U634

Many other Keithley Instruments firmware dumps are available on xDevs.com documentation site

Calibration

To be done..

Performance test

Conclusion

….

Date Activity Time spent
8/21/2016 Received unit, initial teardown 1 hour

Table 22: Time worklog summary

Total man-hours spent on this project around 1 hours, give or take few. Was it worth doing? That’s up to you, so please leave your comments, so I can do more stuff like this.

Equipment used during restoration project:

  • MiniPro TL866CS programmer to read/write ROMs/RAMs
  • Keithley 2002 DMM (calibration 2007)
  • HP 3458A (calibrated 3/20/2016 to artifact 10V/10KΩ)
  • Nikon D800 with 28-70/2.8D, Sigma 150/2.8 Macro, 50/1.4D lenses, tripod
  • 0.25L of IPA and alcohol to clean chassis and parts
  • Google – used lots of it!

Credits and thanks to Todd, Alan and all our readers to make this all possible. If you find typo or have any questions feel free to leave comment or contact us.

Author: Todd Micaleff
Published: Aug. 21, 2016, 10:20 p.m.
Modified: Dec. 6, 2016, 11:16 a.m.

References

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