Fluke 8050A calibration - Page 1
Forum section: electronics / testgear

author: bob91343
author_rank: Super Contributor
timestamp: July 28, 2020, 03:31:29 am
content: I have one of these that seems accurate except on the 20V dc range. As it happens, there seems to be no adjustment for that.I have searched the diagram and there is no pot for the 20V range. The calibration procedure skips that range.Not that it's terribly far off, maybe around 0.2% at most, but I'd like to trim it.Does anyone know about this?

author: med6753
author_rank: Super Contributor
timestamp: July 28, 2020, 12:49:43 pm
content: First make sure your supply voltages are in spec. There is interaction between R11 (2V cal) and R5 (200V cal) and you may have to adjust for best compromise. Or you can play with the U5 jumpers on table 4-10 on page 4-11 of the manual.Above all, how accurate is your source voltages?

author: bob91343
author_rank: Super Contributor
timestamp: July 28, 2020, 10:26:08 pm
content: Allthe voltage ranges appear to be in spec except the 20 V range.However I will open it again and measure the voltages. There do not seem to be any precise voltages, at least from my schematic perusal. And no adjustments.Can you explain what this is all about? If I can't adjust the power supply I don't know how to correct the error if any. And I will have to look for the tolerances on the supply voltages.

author: med6753
author_rank: Super Contributor
timestamp: July 29, 2020, 02:44:43 am
content: The power supply voltages should be within +/- 10%. If not, investigate the cause.By "source voltages" I am referring to the standards you are using to calibrate it. What is it? Is it a traceable standard? If not then your source voltages are suspect.If the 2V range and the 200V range are properly set then the 20V range will automatically fall into place. If not then there is the possibly, although unlikely, that one of the precision resistor networks (U1 - U5) has drifted.

author: Fungus
author_rank: Super Contributor
timestamp: July 29, 2020, 05:13:48 am
content: Quote from: bob91343 on July 28, 2020, 03:31:29 amYou need to calibrate it on the the 200V range, the 20V is derived from that with precision resistors.Feed it a voltage while it's on the 20V range and twiddle R5. The reading on screen should change.For me the 20V range is the most useful so I'd "Lego" two 9V batteries together and use the 18V output to match it against my best meter while the 8050A is on the 20V range...

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: July 29, 2020, 06:44:55 am
content: I agree with med6753 that having a calibration standard with wide enough of a calibration voltage range would be a good start. The 8050a DC calibration step 3 and 4 requires 190V and 1000V DC source, a stable calibration grade source at these voltages are not common in most labs. Low end bench supplies are simply not precise or stable enough for most calibration work.It would be beneficial to share with others on how your unit was calibrated, and with what equipment. When you said other ranges other than the 20V were within spec, what equipment did you use to verify them, and what are these test points.If I were you, I would do the proper calibration first, then test each range at its respective test point, before trying to troubleshoot a specific range.Good luck.

author: bob91343
author_rank: Super Contributor
timestamp: July 29, 2020, 07:53:31 pm
content: I am verifying the unit with an HP 3456A which is almost overkill for accuracy. I like the idea of miscalibrating the 200V range to bring the 20V range in; most of my measurements are on the 20V range. I will also look into a possible resistor out of tolerance to make the two ranges exactly 10 to 1.First I have to clean up the workbench; there is a spectrum analyzer all apart that has an intermittent.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: July 29, 2020, 10:48:58 pm
content: That's exactly what I do for handheld DMM that has a non-linear reading and only 1-2 adjustment pots. Adjust it based on the range you use most rather than the entire range.

author: bob91343
author_rank: Super Contributor
timestamp: July 30, 2020, 12:23:37 am
content: Can someone point me to the resistor divider that makes the 200V and 20V ranges in the proper 10:1 ratio? If I find it I can trim the offending one and have what I want.

author: Martian Tech
author_rank: Regular Contributor
timestamp: July 30, 2020, 02:02:42 am
content: You can find the schematics online. Looks like it is not a voltage divider, but the gain of the buffer stage is either x1 or x10. A 2.2K resistor in U34 (which appears to be a custom part with multiple precision resistors in a SIP package) - pins 4 & 5 - is switched into the feedback loop or not by the CMOS switches U19C/U19D based on a control signal from the CPU. And just to make things more confusing, the RMS converter has settable gain as well for AC signals.

author: Fungus
author_rank: Super Contributor
timestamp: July 30, 2020, 05:16:12 am
content: Quote from: bob91343 on July 30, 2020, 12:23:37 amLook in the manual, it has all the schematics, etc:http://manuals.chudov.com/Fluke-8050A.pdf

author: bob91343
author_rank: Super Contributor
timestamp: July 30, 2020, 05:27:29 am
content: It appears that the ac ranges do not suffer the problem, just the dc. So maybe I would be better off leaving well enough alone.A man with a watch knows what time it is. A man with two watches is never sure. A man with 20 or so voltmeters (me) scratches his head in confusion.Anyway, while this unit has many nice features, it also has down sides. No autorange and no readout illumination. That's offset by the nice conductance ranges for very high resistances. I think it also calculates dB and some other stuff.

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: July 30, 2020, 05:58:26 am
content: I feel your pain with DMM alternative truth, it was bad.I cured it by sending four bench meters back to Keysight each year for calibration. I calibrate the less precise bench meters and handheld DMMs myself with a Krohn-Hite calibrator, now the pain is gone and every meter agrees with each other :-)You should at least get one of your meter through the NIST calibration so you know what the truth is. Perhaps the 3456A is a good candidate for it.

author: Fungus
author_rank: Super Contributor
timestamp: July 30, 2020, 07:27:13 am
content: Quote from: bob91343 on July 30, 2020, 05:27:29 amIt's a different circuit.Quote from: bob91343 on July 30, 2020, 05:27:29 amThere's no way you can break anything*. Those things aredesignedfor people to go in and twiddle manually - it's how they did it at the 8050A factory.(and those people were working in a hurry...)All you need is a stable voltage source that's somewhere near the top end of the range you're adjusting and a better meter for comparison.(*) Famous last words.

author: bob91343
author_rank: Super Contributor
timestamp: July 30, 2020, 08:13:10 pm
content: matiantech has convinced me not to meddle in this thing with a soldering iron. I will do the compromise tweak and stop agonizing. (yeah)I also see some drift in readings for the first few minutes. I presume that's endemic to the unit.

author: med6753
author_rank: Super Contributor
timestamp: July 30, 2020, 11:25:58 pm
content: As I've stated before you can play with the resistor values as stated in table 4-10. Then do a complete DCV re-cal and see where it's at. If it's unacceptable you can reverse it.Some drift up to a half hour is normal for any 4.5 digit DMM.

author: Martian Tech
author_rank: Regular Contributor
timestamp: July 31, 2020, 01:22:49 am
content: Assuming I'm reading the schematic correctly, here's what I don't understand: If it's accurate on the 200mV scale, it should be accurate on the 20V scale as well. They both use the x10 buffer gain, and on the 20V and 200V scales, the input signal is divided by 100 by U1(1,2), R5, U1(3,4). So on the 200mV and 20V scales, 0-200mV is presented to the buffer and multiplied by 10 to present 0-2V to the integrator. On the 2V and 200V scales, 0-2V is presented to the buffer and multiplied by 1 to again present 0-2V to the integrator.If the divide by 100 is inaccurate, both the 20V and 200V ranges should be off. If the x10 multiplier is off, both the 200mV and 20V ranges should be off.Could it be something as simple as oxidation on the switch (S7) contacts in the 20V range?

author: bob91343
author_rank: Super Contributor
timestamp: July 31, 2020, 03:52:21 am
content: Well I never checked the 200 mV or 200 V ranges so that's my next step. It may be that a full adjustment will fix everything.

author: med6753
author_rank: Super Contributor
timestamp: July 31, 2020, 08:55:08 am
content: Quote from: bob91343 on July 31, 2020, 03:52:21 amWell, that would explain your issue and you should have stated right from the get go that you did not attempt to calibrate to other ranges. Follow the DCV calibration procedure as stated in the manual in the order given. But obviously you may not have access to an accurate 1000V source so that range can be left alone.

author: Fungus
author_rank: Super Contributor
timestamp: July 31, 2020, 09:04:28 am
content: Quote from: med6753 on July 31, 2020, 08:55:08 amAll you need is enough 9V batteries...

author: bob91343
author_rank: Super Contributor
timestamp: August 01, 2020, 02:44:20 am
content: They are not adjustable. Well the combination is adjustable in 9V increments. To get 200 Volts one needs about 22 of them. I think they cost around $3 so that makes the source a bit pricey.I have planty of sources of voltage in that range anyway. If there is one thing I have plenty of, it's power supplies.

author: Fungus
author_rank: Super Contributor
timestamp: August 01, 2020, 06:00:30 am
content: Quote from: bob91343 on August 01, 2020, 02:44:20 amHere's 40 of them for under $20, shipped from the USA:https://www.aliexpress.com/item/32324041879.html

author: CDaniel
author_rank: Frequent Contributor
timestamp: August 01, 2020, 07:57:37 am
content: The calibration procedure was made to be standard , but if you know what are you doing any voltage close enough in that range is good when you have another calibrated voltmeter .

author: coromonadalix
author_rank: Super Contributor
timestamp: August 01, 2020, 12:46:33 pm
content: Maybe tons of 9v cells from a dollar store ??

author: Fungus
author_rank: Super Contributor
timestamp: August 01, 2020, 03:17:20 pm
content: Quote from: CDaniel on August 01, 2020, 07:57:37 amYep. If you have a second meter with better precision than the one you're calibrating then there's absolutely no need to generate a precision voltage, just a voltage somewhere near the top of the range being calibrated. The OP says he has a 6.5 digit HP 3456A, that's easily good enough to lick a Fluke 8050A into shape.The only important thing is to calibrate somewhere near the top of the range being calibrated. In theory you can calibrate a 20V range with a 1.5V battery but I'd be much happier working with 18V from two 9V batteries. It will be much easier to do and the error will be 12x smaller (18/1.5=12).

author: CDaniel
author_rank: Frequent Contributor
timestamp: August 01, 2020, 07:58:04 pm
content: Half range should be fine , and if for some reason it's not perfectly linear here is a good point for compromise

author: maginnovision
author_rank: Super Contributor
timestamp: August 01, 2020, 09:30:10 pm
content: If it weren't for COVID I'd offer to let you use my calibrator... Perhaps later. Assuming 91343 is your zip we're practically neighbors.

author: bob91343
author_rank: Super Contributor
timestamp: August 01, 2020, 10:02:28 pm
content: Yes that is my zip. Where are you?

author: maginnovision
author_rank: Super Contributor
timestamp: August 01, 2020, 10:16:10 pm
content: Near the mission police station.

author: JimS
author_rank: Contributor
timestamp: August 02, 2020, 11:40:28 am
content: I was working on a 8050 about 2 weeks ago.It had been on a shelf unused for around the last 3 years if not more.The ohm's with a shorting bar was off on every other ohms range.A good spray cleaning of the switches and it was ok after that.

author: AudioGuy
author_rank: Newbie
timestamp: March 06, 2024, 10:44:55 pm
content: Quote from: Fungus on July 29, 2020, 05:13:48 amI have the exact same issue with my 8050A, so I resurrected this thread. Everything stated in this thread about the 200V range and the 20V range (DC only) are off on my meter. I went a step farther and looked at the voltage divider network for the DC path. Everything is spot on using my  meter. I swear there is something after the A/D process. I'll explain why and look for some feedback.I use a 19.00 Volt DC source and select the meter to 20V DC range. The 8050A reads 19.00. When I select the 200V range, the meter shifts the decimal point, but reads 18.60. If I select the 2000 range, the meter shifts the decimal and reads 19V. I look at TP6, which is the output of the divider circuit. On the 20V range and the 200V range, the divider is spot on and the reading is .190V. This is with the meter powered on or off. No supply offset. So this tells me the issue is with the 'digital' side of the meter. But for the life of me, I can't see the difference on the digital side for 20V or 200V.If I calibrate the meter for the 200V scale, the 20V scale is off. This is linear through the 20V scale (and if I calibrate for the 20V scale, the 200V is off linear in its range).For what it is worth, the 2000V scale seems fine. I can only go to 500V DC, but it is just about spot on. AC and Ohms measurements are fine.What am I not seeing here with the 20 / 200 ranges. I have the original manual and copied the schematic and color-coded paths, but don't see anything different.Thanks

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2024, 05:10:13 am
content: Quote from: AudioGuy on March 06, 2024, 10:44:55 pmRead the theory of operation part in chapter 3 of the manual. The ADC itself has two ranges, 200mV and 2V. The 20V and 200V ranges both use the same 100x divider and then the lower ADC range for 20V, the higher for 200V. You need to do the DC range calibrations in order as specified--2V, 200mV, 200V, 1000V. Then it all should work.

author: AudioGuy
author_rank: Newbie
timestamp: March 07, 2024, 01:04:33 pm
content: Quote from: bdunham7 on March 07, 2024, 05:10:13 amMy  200mV, 2V, 20V and 2000V are spot on. I read the theory of operation. I also understand the diagram at the top of page 3-7 showing the function and range switches. 200mV & 2V are together, 20V & 200V are divide by 100 and 2000 is divide by 1000.If the DC divided voltage value at TP6 doesn't change between the mechanical selection of 20V or 200V (Switch 7 & Switch, then it is not a resistive divider problem. The only difference from the switches at this point is changing the microprocessor via the S7 or S8 to change the state of RNG B or RNG A (pins 3 & pins 4). So then I assume this means the microprocessor selects AZ+INT+LO or AZ+INT+__LO. So my issue is the path from U19C or U19D? CMOS switch going bad or C8 or C33 flaky?

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2024, 02:13:34 pm
content: Quote from: AudioGuy on March 07, 2024, 01:04:33 pmCan you try measuring a 190mV signal on the 2V range?

author: AudioGuy
author_rank: Newbie
timestamp: March 07, 2024, 02:51:07 pm
content: Quote from: bdunham7 on March 07, 2024, 02:13:34 pm@bdunham7 That was it! There was just a small difference in with my 190mV calibration. 190mV was 188.5 on the 200mV scale, but 190.0 on the 2V scale. I went through and recalibrated 190mV, 2V scale was still fine and then 190V needed a small adjustment. That brought the 20V scale perfectly spot on when I went back and checked it. So now, 19.00 reads on the 20V and 200V scales.Did I bump the 190mV pot? I don't know. But a second look and input from someone else helps. I appreciate the support.I hope this thread helps others in the future.