Keithley 191 eBay score - lots of photos - Page 1
Forum section: electronics / testgear

author: kj7e
author_rank: Frequent Contributor
timestamp: January 17, 2018, 12:43:20 am
content: I wanted a small, quiet, LED meter (to see across the room or when dark) to sit on a self just to monitor one of my long term voltage reference boards and didn't want to spend much $. Found this Keithley 191 5-1/2 digit 200,000 count meter from 1982 for $45. Very clean, works well and the voltage calibration was nearly spot on with my Keithley DMM7510 (resistance needed a slight tweak here and there). Thought I would post some photos for posterity.Link to manual;http://download.tek.com/manual/191(A-1979)(30183Instruction).pdf

author: lowimpedance
author_rank: Super Contributor
timestamp: January 17, 2018, 12:59:55 am
content: Interesting old meter with 4 wire ohms and I see no AC option, which seemed to be all the rage with Keithley and others at the time. Make you pay for AC and /or current options etc. Now days its software options.At least the circuit and general layout is in the manual if one was motivated to create one!.Any way nice little meter, still many 'years' left on the clock.

author: cncjerry
author_rank: Unknown
timestamp: January 17, 2018, 03:01:00 am
content: The 191 had an A/C option. If you put it in ACV and you get something like "!EEE" then the A/C option isn't installed. I have one with and one without. Great meter for the reason you mention - seeing it across the room. If I remember correctly, it is hard to calibrate as it is a little temperature sensitive so you need to adjust it and then put the covers on and let it warm up again.~Jerry

author: kj7e
author_rank: Frequent Contributor
timestamp: January 17, 2018, 03:36:45 am
content: Quote from: cncjerry on January 17, 2018, 03:01:00 amNo A/C option in this one, but I knew that and didn't need it. Yep, I lifted the cover off and made one adjustment at a time then let it sit with the cover back on for a bit.

author: Terry01
author_rank: Frequent Contributor
timestamp: January 17, 2018, 03:54:55 am
content: A score for sure! Nice one

author: texaspyro
author_rank: Super Contributor
timestamp: January 17, 2018, 07:08:13 am
content: I've been known to drill holes in instrument cases so I can adjust them with the covers on. After adjustment, I cover the holes with aluminum tape.

author: anotherlin
author_rank: Frequent Contributor
timestamp: January 17, 2018, 09:17:15 am
content: The big axial yellow capacitor marked "EC PT1-2674M".Are they "Electro Cube" branded ones?I see them often in high end measurement circuits.For instance, in Tektronix SG5010 low distortion sinus generator, or AA501/AA5001 distortion analyzer.They are pretty expensive.What is the purpose of this one ?

author: ivan747
author_rank: Super Contributor
timestamp: January 17, 2018, 12:50:43 pm
content: Welcome to the club!Mine seems to need recappingSent from my iPhone using Tapatalk

author: kj7e
author_rank: Frequent Contributor
timestamp: January 17, 2018, 10:19:20 pm
content: Quote from: ivan747 on January 17, 2018, 12:50:43 pmOnly a few $ for some new high quality caps from Digikey (part numbers below), measured some excessive ripple on mine, so going to replace them.# Quantity Part Number Manufacturer Part Number Description Customer Reference Backorder Unit Price Extended Price1 1 493-1293-ND UVZ1C153MRD CAP ALUM 15000UF 20% 16V RADIAL 0 2.75000 2.752 3 493-1949-ND UPW1J471MHD3 CAP ALUM 470UF 20% 63V RADIAL 0 1.00000 3.003 2 493-1791-ND UPW1C122MPD CAP ALUM 1200UF 20% 16V RADIAL 0 0.81000 1.62

author: rsjsouza
author_rank: Super Contributor
timestamp: January 18, 2018, 11:57:56 pm
content: Congratulations on your purchase and welcome to the club!One detail: a few years ago I did a high quality scan of its owner's manual and posted in a link in my teardown video at:https://youtu.be/O7fiGPdOiaU(Sorry, in portuguese)I also just uploaded to KO4BB's repo, but it will take a bit longer to be live.

author: kj7e
author_rank: Frequent Contributor
timestamp: January 19, 2018, 12:11:59 am
content: Quote from: rsjsouza on January 18, 2018, 11:57:56 pmThanks! Just downloaded your scanned manual, very nice!

author: hwj-d
author_rank: Frequent Contributor
timestamp: January 20, 2018, 12:07:05 am
content: Quote from: rsjsouza on January 18, 2018, 11:57:56 pmDoesn't matter. The pictures says it all.'nada quebrado...' Thanks a lot

author: rsjsouza
author_rank: Super Contributor
timestamp: January 20, 2018, 01:54:30 pm
content: Quote from: hwj-d on January 20, 2018, 12:07:05 am

author: wagnerlip
author_rank: Newbie
timestamp: November 18, 2019, 12:18:44 am
content: Got two Keythley 191 units, one with AC option is working very nice. The second, without AC option is misbehaving.First the display was flashing erratic.Open the burn smelly unit and found the R159 (10Ω) burn out, it connects the rectifier bridge CR102 negative output to the C114 and 7915 VR102 regulator via power on switch.Replaced resistor and found shorted C113 tantalum 10uF cap at the output of 7915, replaced the 3 tantalum caps (C110, C113 and C116) from +15, -15 and +5V, just in case, by 10uF 25V electrolytics.Power on the unit all voltages are very good now, ripple on all voltages below 25mV.Now display is steady, but measurements are crazy.Used a 20Ω resistor for measuring ohms, scale 200Ω goes into overflow, it should not. The same 20Ω resistor shows 0.089 on 2kΩ scale, what is very wrong.VDC scale is crazy.Measuring 1.5Vdc on input, scale 200mV goes correctly overflow, scale 2V shows 1.01V, 20V scale and up shows 0.00000Measuring 5Vdc on input, scale 2V goes overflow correctly, but 20V, 200 and 1000V scale shows 0.00000.The only gray wire connected to Ohms switch, goes to center of transistors and white tubular cap, is scaling voltage correctly, the 5V on input, 4.66V on that wre, when switching scales, change to 0.466, 0.0466, etc, means the attenuator is working nicely.Can't understand why it shows 0.00000 with 5V input at 20V scale, but overflows on 2V scale.Exchanged the processor board between the working and not working units, not the problem.Exchanged the display board, same thing.I wonder if the switches are not bad contacting.As far as I understand from the schematic, the ADC doesn't play any different role when manutally switching scale for voltages,The ADC selects Vin (for 200mV), Vref, Vsig (for >200mV) and Vzero, as in page 5-3 of the tech manual, the display is 2*(Vsig-Vzero/Vref-Vzero), and that doesn't depend on the input voltage since the scale switches are manually operated to keep Vsig max 200mV. So, the ADC doesn't even know what scale it is operating, considering Vsig is no higher than 200mV, or overflow will show. So, the problem of showing 0.00000 for 20V and up, has nothing to do with ADC or processor, it must be something around the attenuator switches. But the gray wire shows attenuation correctly.One strange visual observation, from time to time, a sequence of 500ms display on, very fast (20ms?) display zero and repeat, like if the processor is trying to indicate something wrong. This is not all the time, this not happen when the processor is showing something different from zero.Ohms scale seems to understand short circuit at input, shows less than 1Ω with leads shorted, but as said before, 20Ω is shown as 0.08 on 2k Scale, overflows on 200Ω scale.The same lower ohms measurement (20Ω as 8Ω on 2kΩ scale) is observed with measuring 1.5V on 2V scale (1.01V), may be ADC Vref is out with other problems.I think I will need to power-on both units side by side and scope (Tek 2235) trace everything.I don't believe the shorted tantalum cap on 7915 and burn resistor as consequence would cause any other problem for the op-amps. May be I could swap the 4 socketed LF351 between the good and bad units, just for diagnostic.======================Okay, scoped and measured all the ADC diagnoses based on the tech manual, Table 6-4, page 6-8.Items 1-10 are okay.Item 11, C112 (the integrator cap) 0 volts to ±15 mV, as "integrator input" is not exactly like that, it is not ground, there is a digital 2V signal there.Item 12, C112 (the other side of the cap), it is said "see waveform per figure 5-7". That also is not correct.Figure 5-7 shows an integration voltage, most analog, ramps, etc, see figure 191-1 attached.What it shows is square waves, and it never reaches zero or negative voltage, figure 191-3 attached.The negative voltage is important to trigger the single slope comparator U07 output pin 7, figure 191-2 attached.The input pin 2 of U07, or Q109e, never goes below ground (see 191-3), so its output never goes down.This is explained at paragraph 5-18 on page 5-6... "The single-slope comparator output is also positive, and it will not switch until the integrator output crosses zero..." Indeed it never does. And that it seems to be the failure here.Any scale of VDC does not produce an image as 191-1 on the integrator capacitor, not even close, it changes a bit according to the selected scale, but it is always like 191-3.Now trying to figure out why the integration over the cap is not linear ramps, but only digital.I removed the C112 integrator cap, measured 3.58nF, pretty close to the 3.6nF specified.Temporarily solder another cap close value to see if integration works, no game.R137F and G measured okay with ohmsmeter.V-F pulses at U104a-2 output shows activity, so the U106+U116 counter/divider is working.Q109 seems to be working.Single Slope trigger is the last to check and could be the culprit.U108-6 has positive pulses from the processor.U108-2 has different pulses.Measuring the transistors inside U108 (CA3086) using Diode function of a multimeter, found Base-Emitter (U108-6 to U108-7) as 2.4V both polarities, it should be 0.7V in way direction. Crude diagnostic defined open NPN U108C, U108-6 being positive must show the same polarity on emitter 108-7, instead, emitter is way negative by the -15 via R137E. Culprid.Temporarily soldered a 2N4401 NPN TO92 bipolar transistor over the pins U108, 6(B), 7(E), 8(C), and badabim badabum,working.Now lets go calibrate all pots for Ω and V.Never give up.

author: fixitanonymous354
author_rank: Newbie
timestamp: November 18, 2019, 05:53:07 am
content: I`ve recently fixed one of those, mine had a bad PIA chip with some pins stuck or intermittent, dead opamp in the ADC, and a shorted out D Flip Flop in the ADC that blew the 5V regulator. It is working fine now but does anyone have a picture of the underside of the PCB ?My unit has a cap soldered across one of the op-amps on the underside and i`m not sure if that is suppose to be there

author: wagnerlip
author_rank: Newbie
timestamp: November 20, 2019, 02:29:27 am
content: Dear Fixiatnonymous354.I will disassemble one unit now, take a picture and post in a minute.I remember see a couple of caps soldered on the components side of one unit, I guess someone thought to be a good think to filter some noise.=====Okay, attached, indeed a ceramic capacitor under an op-amp. I including the fix I did with the 2N4401 transistor solving an open NPN inside the CA3086, also the three capacitors aside the voltage regulators.Cheers,

author: fixitanonymous354
author_rank: Newbie
timestamp: November 20, 2019, 07:02:12 am
content: thank you for the pictures,looks like it is supposed to be there so i`ll leave it alone, also while i was messing with the digital part of the meter i dumped the firmware from ROM chip, maybe it will be useful to someone.