Keithley 616 teardown - Page 1
Forum section: electronics / testgear

author: muvideo
author_rank: Frequent Contributor
timestamp: June 25, 2014, 08:12:04 pm
content: Hello,I recently added to my equipment collection an electrometer. I was searching for one just forthe fun of having one, but the prices on ebay were a bit on the high side also for old units,and I really dont need it but when I saw one for cheap I couldnt resist.Here is it, in all his glory, a keithley 616:It is pretty old, probably around late 70' and the unit had some problems, I knew before buying it,since in the pics on the auction the 616 had the display that was not zero, while all the controlswere set for obtaining a zero reading.Once arrived the problem was confirmed: the unit was showing a constant reading offset of180-200 counts. I was hoping that all the input circuitry was intact, thisthing is full of strange and unobtainable devices, and the input stage is where mostof the magic happens.Fast check of the power supply levels, and there is it: -275V rail was indicating -144V withhuge ripple, so that rail was missing it's filtering. I removed the electrolitics and theculprit was evident:Once changed the HV electros the unit was working again:

author: muvideo
author_rank: Frequent Contributor
timestamp: June 25, 2014, 08:13:10 pm
content: An electrometer is 'just' a voltmeter with a high input impedance, and in this case highmeans really high. This enables this kind of instrumets to make measurements otherwiseimpossible. Other than measuring voltage of devices with high output impedance,they are useful for measure very low currents or very high resistances.The working principle of 616 is simple, the first stage is a 1:1 buffer followed by a variablegain amplifier that feeds the integrating ADC of an autoranging voltmeter. Full scale is 2000 counts.The buffer output is routed to the back of the instrument, and the specs tell that it willsettle at less than 20ppm of input voltage, so it could be connected to a better meter.The buffer is reconfigurable to perform voltage, current (shunt or feedback),resistance and charge measurements.And now let's take a look inside:it's very clean, the black box is the input transformer, around it there arefiltering capacitors and voltage regulators. On the bottom there are logic and A/D boardswhile on the top, under the aluminium shielding, there is the input preamplifier andrange selector. The two canned HV transistors between the shielding and the transformerare part of the 1:1 buffer, and are used to generate the X1 output, that is a 1:1buffered representation of the input voltage seen by the instrument, up to +-200V.Another view:This is the logic board, the white chip is the “brain” of the autoranging meter and ADC,therest is the display driving logicThis is the ranging board, fet switches and caddock resistance network perform the variablegain amplifier function, while the socketed device on the bottom is a dual jfetThis is a closeup of the PSU section:

author: muvideo
author_rank: Frequent Contributor
timestamp: June 25, 2014, 08:13:35 pm
content: This part is the range selector and preamplifier, the input goes to two thermallycoupled mosfets, trough teflon standoff mounts, leakage here is to beabsolutely avoided, the bigger can contains two diode connected transistors that protectthe mosfets.The range selector connects to the input one of 11 resistors that go from 9.99ohm to 10^11ohmand 11 capacitors from 5pf to 100nFTeflon insulated side of selectorOn the bottom of the selector can be seen some glass encapsulated high valuesresistances,On the side of the selector there is a reed switch used to short the input for the zero preset.

author: muvideo
author_rank: Frequent Contributor
timestamp: June 25, 2014, 08:14:05 pm
content: Just to put in perspective things I put a charged capacitoron the terminals of the 616 and a keithley 2015 to measure the voltage. The 2015has pretty high input impedance in the order of Gohms, but the 100nF capacitor israpidly discharged by the input leakage current of the instrument:The 616 input shows a very slow decrease in voltage, and this is probably dominatedby internal capacitor leakage, since once disconnected the 100nF cap, the residual inputcapacitance (should be around 20pF) of the 616 is enough to keep the reading nearlast input value.I tried another more quantitative experiment, I connected a voltage source to a 100MOhm resistor string and tried to measure the current trough itThis is the result with 10mV on it, the current will be around I=V/R=.01/100*10^6 =1*10^-10 A,that is 100pAAt 1mV the current is down to 10pA, the 480 picoammeter is at it's limits while 616 is still at full range, readings start become noisyNow a zero check to null out current generated by thermocouplesat 100uV we are around 1pA, 480 is useless, while the electrometer is still a pair of decadesaway from it's limits (theoretically 10^-15A of resolution). At these levels the reading is very noisy,and the sensivity so high that I had to stand still between the readings, otherwise the numberswere moving all around. It's a good result, cosidering that I didnt connect guard or shieldsNow I need some cabling, is there a reasonably cheap solution to build a cable for theelectrometer? It's socket is a triax with 2 180° pins.

author: JuiceKing
author_rank: Regular Contributor
timestamp: June 26, 2014, 12:43:33 pm
content: Great stuff! Thanks so much for posting.

author: tautech
author_rank: Super Contributor
timestamp: June 26, 2014, 02:39:10 pm
content: WOW Not bad for a cheapy.

author: Vgkid
author_rank: Super Contributor
timestamp: June 26, 2014, 05:45:17 pm
content: Thanks for the teardown.Muvideo: How is your 7075 doing?

author: muvideo
author_rank: Frequent Contributor
timestamp: June 26, 2014, 08:08:47 pm
content: Hello,my 7075 is "almost" ok, almost because it works ok for few hours,then suddently stops working, seem thermally related.I didnt had time to troubleshoot it.Now I'm living and working far away from my home lab, so I havevery little time to play with my toys

author: robrenz
author_rank: Super Contributor
timestamp: June 26, 2014, 08:17:14 pm
content: Very nice find and pictures. Try adding ;image after the picture data in your img to have the pictures show.Code:picturedata;image

author: muvideo
author_rank: Frequent Contributor
timestamp: June 27, 2014, 08:52:50 pm
content: Quote from: robrenz on June 26, 2014, 08:17:14 pmThank you robrenz, i tried adding ";image" , what is the difference? On my pc seem the same.

author: free_electron
author_rank: Super Contributor
timestamp: June 27, 2014, 10:59:16 pm
content: ghaaaa . you removed the input buffer ... i hope you didn't touch it with greasy fingers....that dual mos used as protection and the matched pair jfet are pure unobtainium ...

author: muvideo
author_rank: Frequent Contributor
timestamp: June 28, 2014, 07:09:06 am
content: Quote from: free_electron on June 27, 2014, 10:59:16 pmdont worry, no bare fingers were allowed near the preamp or range selector,and the board was extracted by the edges

author: mimmus78
author_rank: Unknown
timestamp: June 29, 2014, 08:17:57 pm
content: Thanks for the nice post(s).

author: anjan
author_rank: Newbie
timestamp: December 15, 2014, 11:55:56 pm
content: Hi,Can you tell me how to measure charge from this electrometer ? I mean the step by step procedure.Thanks

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: May 08, 2018, 01:58:43 pm
content: Wanted to ask if anybody knows the alternative for the 28pin Dip IC used in the Keithley 616. It's a display driver/decoder legend QA408, Keithley part number: LSI-2, it's made by Fairchild SL35155, the ceramic package.I was looking for datasheet for this one and I can't find any, any known replacements, any way to hack it?

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: May 09, 2018, 03:54:11 pm
content: Just wanted to share a quote I've received for ADC for Keithley 616. It's a absolutely obsolete part from Frairchild.I guess they are made out of unobtanium.

author: Joel_S
author_rank: Contributor
timestamp: March 19, 2020, 06:27:58 pm
content: Hello everyone, !Great teardown, thanks !I just got a Keithley 616 which needed some serious repairs, and it is now back to full functionality.As I'm always scared about using unobtainum-based instruments, and after seeing the $799 quote for the LSI-2 chip which is the core of the logic board, I decided to build a replacement.I used a 68HC811 which I happened to have on hand, and it turned out to be an easy task. I now have a neat little PC board which plugs into the LSI-2 socket, and the K616 works exactly as it did with the original chip.If anyone is interested, just let me know !Joel