- Initial inspection and basic disassembly
- Manuals references
- Repair workflow
- Chassis repair
- Triaxial cables and fixtures
- Performance verification
- Noise evaluation at shorted inputs
- Restoration 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, 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:
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.
Brief specification lalala
|DC Voltage||100mV||1 V||10 V||100V||1000 V|
Table 2: Model 6517 family key specifications
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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!
Original model, covered in this article.
Newer Model 6517A, which is obsolete today and replaced since 2008:
Latest and current Tektronix/Keithley Model 6517B:
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
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.
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.
|ROM||Revision 2||Revision 4.6||Revision 6||Revision 7 (1992)||Revision 8 (1998)||Revision 9 (latest)|
|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
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:
To be done.
|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
|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
Modified: Aug. 31, 2017, 9:39 p.m.