Fluke 8840A stuck in measurement - Page 1
Forum section: electronics / repair

author: ulix
author_rank: Regular Contributor
timestamp: July 23, 2014, 05:43:03 pm
content: Hello Guys,I bought last week a Fluke 8840A-AF on ebay. The Problem is that the meter is stuck in a measurement. I can't change anything by switching into A, V or Ohms as well as making the self test. When I opened the lid, I discovered a inflated cap, it's a 6800mF cap (C601), the first for the 7805. But this didn't change the problem, I'm at this point I can't find a error.Can anyone give some tipps?cheers

author: ulix
author_rank: Regular Contributor
timestamp: July 24, 2014, 09:25:01 pm
content: is there no one which has any idea?cheers

author: xwarp
author_rank: Frequent Contributor
timestamp: July 24, 2014, 10:02:21 pm
content: So what does the meter do upon first power up?Also, make sure the switch for rear inputs is OUT.

author: ulix
author_rank: Regular Contributor
timestamp: July 25, 2014, 12:25:08 pm
content: Hi,here is the start up.Rear side:cheers

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: July 25, 2014, 02:39:02 pm
content: I have seen something similar to this with a 3458a on the AC range. It was a bad op-amp. It wasn't stuck it just had a constant output. I am not familiar with this model but I hope you can trace through the ranging and A/D sections.

author: xwarp
author_rank: Frequent Contributor
timestamp: July 26, 2014, 02:27:01 am
content: Hold the "SRQ" button down and see if it runs through the self tests. If it faults, you will know it.Looks to me, and this is just a guess, that the calibration is bad.

author: ulix
author_rank: Regular Contributor
timestamp: July 26, 2014, 09:18:51 am
content: Self test runs, and each test fails!!! From 1 to 21 eache with ERROR

author: xwarp
author_rank: Frequent Contributor
timestamp: July 26, 2014, 01:27:12 pm
content: Do you have the service manual?If you have it, starting on page 138 of the pdf, it talks about the calibration of the A/D converter and the precision voltage reference.It appears that you may be having an issue with one of the two, and probably the reference being that ALL tests fail.

author: ulix
author_rank: Regular Contributor
timestamp: July 27, 2014, 09:29:29 am
content: I have a printed Manual. Which page do you mean, 6-33?

author: xwarp
author_rank: Frequent Contributor
timestamp: July 27, 2014, 03:29:45 pm
content: Sorry,page 6-29.I must have been looking at the service manual for the 8842.

author: ulix
author_rank: Regular Contributor
timestamp: January 13, 2025, 06:49:31 pm
content: Hi Everyone,sorry for digging up this thread. At the time I didn't get the problem solved and the instrument still sits here. Hope someone can lend a helping hand to solve the problem.At the time I remember I had found something odd but did not pursue this further at an OPAMP. Currently I'm trying to solve the problem and found some thing odd located at the prescision voltage reference. At the Dual Opamp U702 (LF412). The first image summarizes the mearuemnts on the U702 pins.I'm wondering, shouldn't I measure foltages on Pins 2,3,5 and 6, I find it odd that I cannot meaure anything with my multimeter.Second image is a section of the mentioned schematic.Any help is much appreciated. thanks

author: bdunham7
author_rank: Super Contributor
timestamp: January 13, 2025, 06:59:34 pm
content: First question--do you have the manual for the 8840A/AF or the 8840A?Second--can you take off the covers, completely remove the AC and GPIB boards and then take a clear photo of the main PCB? I have several 8840A but not an A/AF, so if I follow along and post voltages at test points and so forth it would be handy to have the photo and correct manual to reference.Third--does shorting the inputs make any difference in the numbers displayed on the display?Last, I don't remember the specifics of the #1 vs #2 commons on these, but if you have the negative lead of your DMM on something that is connected to the #1 common then you should measure about 0V on those pins because two of them are connected directly to #1 and the others are the opposite pin of an op amp. If the op amp is operating within its normal range its basic function is to make those two input leads equal. If your +7V and -7V test points are correct--or close to it--then your problem is not in this area.

author: ulix
author_rank: Regular Contributor
timestamp: January 13, 2025, 07:38:14 pm
content: Hi bdunham7,thanks a lot for your reply.1) Yes, I have the manual for the 8840A.2) Please see attatchments. If more are needed please let me know. (Not sure if there is a difference between the 8840A and 8840A/AF).For measureing the voltages on U702, I had disconnected the GPIO board, the AC Board was still connected. Can I power the device without the AC board savely?thanks

author: ulix
author_rank: Regular Contributor
timestamp: January 13, 2025, 07:40:19 pm
content: Place holder images.

author: bdunham7
author_rank: Super Contributor
timestamp: January 13, 2025, 08:37:12 pm
content: Quote from: ulix on January 13, 2025, 07:38:14 pmI was asking whether you had the 8840A or the 8840A/AF manual as they are different instruments to a degree. AFAIK the 8840A/AF manual isn't available so everyone makes do with what they have, but if you or someone has an actual 8840A/AF manual that would be even better.Yes, you can and should power it up with the AC board completely removed.If you could, back up a step and perform some basic checks just so I know we're on the same page. Check the power supply voltages, TP601-607 (refer to the PSU schematic page on the manual) and post the results. Also, after you power it up select ACV and AC current ranges and see if you get an Error 30 or if something else happens.I have an old 8840A I can disassemble and poke at for you, but it is in storage waiting for me to get to it--it's on the "someday list". It will be a few days before I will retrieve it but when I do I can do some comparisons for you as you probe your way through the circuits.Also, what other equipment (DMMs especially) do you have available?

author: ulix
author_rank: Regular Contributor
timestamp: January 13, 2025, 09:28:02 pm
content: As I understand it, I have the 8840A manual, not the 8840A/AF. However, I’m not entirely certain, as the manual does not specify which model it pertains to (I couldn’t find a reference to the model, and the front page is missing).1) Test Points (TP601 to TP607):These test points are not labeled on my board. However, I found test points that reference voltages matching those specified for TP601–TP607. Based on this, I’m using these points for testing. Please see the attached image for the measured voltages.2) Shorting the Inputs:Does this make any difference? No, it doesn’t appear to have any effect.3) Current Ranges:Pressing the mA DC (second-to-last white button at the top) twice results in no number being displayed. (Only blinking S at the top of the display). Pressing mA AC (the last white button at the top), the display shows 30 (error).Available equipment:- Digital oscilloscopes (100 MHz) and analog- Measurement was done with a Gossen Metrawatt Metraline DM62. I additionally have stowed away a Metrahit World, Metrahit Xtra, and a Fluke 87, as well as a Fluke current clamp meter.- DC power supplies and frequency generator (up to 100s of kHz, I think, stowed away as well)

author: bdunham7
author_rank: Super Contributor
timestamp: January 13, 2025, 09:46:45 pm
content: OK, one more set of tests if you can before I have to run off and do something.Set up your other DMM to measure DC milliamps if you can and connect it to the INPUT leads (not SENSE) on the 8840A/AF. Press the "kΩ-2W" button and then the "2" button and read the mA shown on the other meter. It should be about 1mA, if it isn't then try working the Front/Rear switch repeatedly and see if you can get a reading. After that, set your DMM up to measure volts again and set the 8840A/AF to the DCV function and press the "20" button, insuring the the "AUTO" indicatior is off. Check and post the voltages at TP301-303 as shown on the DC Scaling/Input schematic page. Also check and post the voltages at TP701-702 on the precision reference schematic you were working on earlier.

author: ulix
author_rank: Regular Contributor
timestamp: January 13, 2025, 09:57:35 pm
content: Just tried the first step, not gettingany currentonly 3uA reading at Input for "kΩ-2W" and "2". Yet when removing the multimeter, the display changes to "1" and OVER on the 8840A/AF.Could you let me know what you mean with working the front/rear switch? Do you mean the on/off switch?

author: bdunham7
author_rank: Super Contributor
timestamp: January 13, 2025, 10:24:24 pm
content: The front/rear switch is the input selection switch near the front input jacks. It should be in the out position. Sometimes the contacts get funky and excercising the switch a few dozen times will clear it.What did the 8840A/AF read in 2WΩ before you disconnected the other meter?Regardless of how that turns out, go ahead and continue with the rest of the tests I suggested. I forgot a step in that--when you set it to the 20VDC range, please short the input jacks as well.

author: ulix
author_rank: Regular Contributor
timestamp: January 15, 2025, 08:47:57 pm
content: 1) I have tested the front switch previously. There is no change now either.2) What happens when the input is shorted in 2WΩ mode? Just tried it again with a bridge, it reads 4.3kOhm and "1"+Over when bridge removed.Please find the voltages of TP301–TP303 and TP701–TP702 listed below.One odd observation: the capacitor C309 shows a short between its two pins.

author: bdunham7
author_rank: Super Contributor
timestamp: January 16, 2025, 02:22:44 am
content: Quote from: ulix on January 15, 2025, 08:47:57 pmIf you haven't already, try setting the switch to rear and using the rear inputs instead of the front.A high resistance reading when shorted is usually high resistance in the input circuit somewhere. So check R309 and then measure from R309 all the way to the positive input connector.Those voltages look good and you can rule out problems with most of the input and reference circuitry, at least as far as causing the major problems and self-test failures. One more thing--when the 20VDC range is shorted and you read ~0V at TP 302, what does the meter's display show?How did you come across the short on C309? Did you test it out-of-circuit? A short would cause the final stage of the T&H amp would go to unity gain instead of open loop, but IDK what the overall effect would be. Maybe the T&H amp is oscillating? If C309 is truly shorted, you can probably just remove it and put an oscilloscope on it to see what it is doing.

author: ulix
author_rank: Regular Contributor
timestamp: January 16, 2025, 08:56:18 pm
content: Quote from: bdunham7 on January 16, 2025, 02:22:44 am=> Switching to the rear does indicate other values. Yet these values seem to be "stuck" as well.Quote from: bdunham7 on January 16, 2025, 02:22:44 am=> I'm intimidated by the sheer schematic. I have indicated the values measured. Please let me know if this is okay. All values where measured in the circuit, and the fluke powered down + inputs shorted.Quote from: bdunham7 on January 16, 2025, 02:22:44 am=> (for the front) -4.3192V (for V-DC); -430.89 mA DC (for mA-DC)=> (for rear) it shows +0.9999 DC (yes the "V" is missing)Quote from: bdunham7 on January 16, 2025, 02:22:44 am=> Just check this again in the circuit and not able to measure this short again (when powered down and powered up) - it was definitly there when I noticed it.

author: bdunham7
author_rank: Super Contributor
timestamp: January 16, 2025, 10:07:48 pm
content: Quote from: ulix on January 16, 2025, 08:56:18 pmI'm sorry--I wasn't thinking too clearly. Your A/AF uses the rear for ratio, not regular inputs. The display and missing "V" are completely normal. Your measurements look normal. Barring something extra-funky, the input circuitry is not the issue, at least not the immediate issue.We're getting closer to the problem. Set the meter to the 20VDC range and short the inputs. If it still reads -4.3V or so, then measure the voltage at TP103 on the A/D converter schematic or if you can't find that, at the R318/C311 junction near U307. If you see about -4.3V then your problem is to the left with the T&H amplifer; if you see about 0V then your problem is to the right in the A/D converter. If the former, re-verify that you get about 0V at TP302.

author: ulix
author_rank: Regular Contributor
timestamp: January 17, 2025, 08:16:09 pm
content: Quote from: bdunham7 on January 16, 2025, 10:07:48 pmNo worries, I'm extremly appreciative for helping me!Quote from: bdunham7 on January 16, 2025, 10:07:48 pm=> I measured at TP103 it is 0V (to reference GND1) or -1.120V (to reference GND2).I guess this means that the fault seems to be at the A/D converter?

author: bdunham7
author_rank: Super Contributor
timestamp: January 17, 2025, 10:08:29 pm
content: Quote from: ulix on January 17, 2025, 08:16:09 pmYes, I think so. Additionally, the service manual describes desoldering and lifting one end of R318 (at the junction of R318 and U307) and then shorting TP103 to REF LOW (GND 1), this should result in a near-zero reading on the meter. You can then also put a test voltage of no more than +/- 2V between TP103 and REF LOW (make sure R318 is disconnected) and the display should be the inverse of this voltage in the 2V range. I'm pretty sure the result will be the same, but it might be interesting to see if anything happens when you disconnect R318. I'm not sure there is supposed to be that much of a discrepancy between GND 1 and GND 2, so that might be a clue. There might be a fault current running somewhere in the system. I'll probably retrieve my spare meter today or tommorrow morning and I'll check this. It's been a while since I poked at one of these and I'm not sure how GND 1 and GND 2 are interconnected. I don't see anything about it in the manual, but if someone knows perhaps they'll chime in.

author: bdunham7
author_rank: Super Contributor
timestamp: January 18, 2025, 02:21:56 am
content: So I retrieved my test unit. It's an 8842A but similar enough for our purposes. I don't have a TP306 that I can see, but for GND 2 you can use the INPUT LOW terminal on the front (provided the F/R switch is out) or the rear lead of CR308 or the front lead of CR310. For GND 1 the L-shaped metal shield between U202 and U101 seems to be the recommended spot. I measured basically a dead short between GND 1 and GND 2 with the unit powered off. With it on, I measured zero voltage except that I was able to see a few hundred microvolts when the 2V range was overloaded. I also showed the exact same voltage at TP103 regardless of which reference point was used. So can you specify exactly where you are connecting to GND 1 and GND 2 to make those measurements of TP103? If you truly have a difference between them, then that needs to be explored. If not, as in there was some error in selecting the test point, then you can proceed onwards into ADC troubleshooting.

author: ulix
author_rank: Regular Contributor
timestamp: January 18, 2025, 09:57:33 pm
content: At the time, I measured on Pin22&Pin21 of U101.=> Checked, and I'm getting now: GND1 & GND2: Short(off) and 0V(on);=> at TP103 (between Pin 22 of U101 and Low Input/Pin21) 0.015V to 0.018V (Input shorted).Had a quick look at some of the inputs/outputs of U101:- PIN23: 1.23V (Ref GND2)- PIN20: 1.138V (Ref GND2)- PIN19: 1.275V to 1.825V (Ref GND2)- PIN18: 0.027V to 0.031V (Ref GND2)What do you mean by "proceed onwards into ADC troubleshooting"? Do I understand it correctly that U101 is the ADC? Can this even be troubleshot further?- Does it make sense to remove U101 (it is seated in a socket)?- Should I remove R318?BTW thanks a lot for your effort, much appreciated

author: bdunham7
author_rank: Super Contributor
timestamp: January 18, 2025, 11:33:55 pm
content: Quote from: ulix on January 18, 2025, 09:57:33 pmI would not remove any parts yet. For further measurements in the reference/ADC area, I'd recommend using the L-shaped shield between U101 and U202 as your ground reference point so that our readings match if we compare. Yes, U101 is the main ADC controller and switch, but there are important external parts adjacent to it that might be the issue. We should be able to determine what the issue is eventually. It is very possible that the issue is one or more of the analog switches internal to U101, but it's worth investigating.The measurement points you mentioned are mostly not static and would have either be looked at with a scope or with the ADC stopped. You can stop the ADC by pressing the EXT TRIG button on the front, the display will show no reading as it is waiting for that external trigger signal that will never come. I have not delved deeply into ADC troubleshooting on these because I haven't had any with those problems, so I'll be exploring this with you for the first time. I'd recommend you read the ADC troubleshooting section in the manual as it is pretty detailed and does explain what to look for.Perhaps you could go through that part of the manual and post readings of all the pins of U102, U103 with the ADC stopped and whatever else you test. You can also have a go at using your scope to look at TP101, etc as suggested in the manual. It will be a few days before I can do extensive comparisons because I have a large Bang & Olufsen turntable strewn all over my bench and it is needing more and more as I go along.

author: ulix
author_rank: Regular Contributor
timestamp: February 04, 2025, 11:02:37 pm
content: Hi bdunham7,just had a few minutes to look into the manual. For now I connected an Oszilloscope to TP101 (GND to L-shaped shield), as told in the manual I have fed in 1.019V (could not adjust it better).1) When I tested the feed in of the 1V into the High/Low on the front, I noticed that the display reading (in 2VDC) changes from -0.43191V to +0.38155. (See first image).2) I have captured four waveforms. This post shows:a) First Waveform is when the the 1V is fed in (overall view).b) Second and third is the first part of the waveform - still 1V on.c) later part of the signal when fed is still 1V on.d) Fourth is the waveform then the supply on the input is off.I can't really compare this to the manual, because is not that destinguisabel - do you get something similar?Next step will be checking the "readings of all the pins of U102, U103 with the ADC stopped".

author: bdunham7
author_rank: Super Contributor
timestamp: February 04, 2025, 11:25:47 pm
content: I checked TP101 on the 2VDC range with the input shorted (reading -0.00001) and with exactly 1V input (reading +0.99999).I got waveforms with 5 distinct periods and in both cases the first three were steady but different for the two values and the last two were constantly changing. #194 is the 1st (0V) and #195 is the second (1V). I couldn't tell you right now precisely what the various sections represent.

author: bdunham7
author_rank: Super Contributor
timestamp: February 05, 2025, 02:38:50 pm
content: I went ahead and had a look at TP102 since I had the meter out and open. This is a video of the display (100mV/dvi and 500µs/div) with exactly 1V in and the display flickering between 0.99999 and 0.99998. The same negative pattern that shows steadily in the third period and flickers in the 5th also appears occasionally in the fourth. I think these two represent something like a zero count and a negative 1 count for each nibble, but IDK for sure. The manual doesn't get very specific about decoding TP102 but they do give some hints and show some pictures. Perhaps I'll reread the theory section...https://youtu.be/VjeNcCLoUN8

author: Paceguy
author_rank: Regular Contributor
timestamp: February 05, 2025, 03:12:56 pm
content: I'm following this interesting post since I have a 8840A that is stuck in the DC volt function andwon't switch over to any other function. When powered on it appears to be ok for a minute or two but becomes stuck afterwards.

author: ulix
author_rank: Regular Contributor
timestamp: February 07, 2025, 08:28:48 pm
content: Ok, for now I have checked U102 and U103 with EXT Trigger on => When turning on EXT Trigger there is no number on the display shown.U102, Voltages VPin_1: 1.205VPin_2: 1.205VPin_3: 1.205VPin_4: -15.42VPin_5: 0VPin_6: 0VPin_7: 0.001VPin_8: 14.82VU103, Voltages VPin_1: -4.039VPin_2: 1.208VPin_3: 1.209VPin_4: -4.167VPin_5: -4.045VPin_6: 1.209VPin_7: 7.86VPin_8: 0VDo these values refect the values which should normally be present?

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2025, 09:09:42 pm
content: Quote from: ulix on February 07, 2025, 08:28:48 pmI will check when I can. What input voltage (if any) was there? If you know, what was the voltage at TP103? Can you put your scope on TP102 and compare what you see to what I have? Getting that triggered and displayed smoothly might be a big tricky, try about 7ms of holdoff if you have trouble.

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2025, 09:11:10 pm
content: Quote from: Paceguy on February 05, 2025, 03:12:56 pmCan you clarify the sequence of events? When you first turn it on do any of the keys work? SRQ? And by "stuck" do you mean the keys don't work or that the display freezes? Does the DCV function work correctly?

author: ulix
author_rank: Regular Contributor
timestamp: February 07, 2025, 09:34:37 pm
content: Quote from: bdunham7 on February 07, 2025, 09:09:42 pmJust checked again. Initally these values were measured with nothing connected. Now I have measured the same again (with 1V at Hi/Lo), and they are the same values - except Pin_8 changes slightly to 0.017V. TP102 is always at 0.001V (to L-shaped shield).

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2025, 09:47:47 pm
content: Quote from: ulix on February 07, 2025, 09:34:37 pmI meant for you to scope it with a 1V input and the meter operating normally, not frozen by the EXT TRIG button.

author: ulix
author_rank: Regular Contributor
timestamp: February 07, 2025, 10:30:07 pm
content: Quote from: bdunham7 on February 07, 2025, 09:47:47 pmSorry, here is with 1V fed into the Hi/Lo probed at TP102 (signal from left to right, first image and second image are the full signal)

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2025, 10:33:05 pm
content: OK, here are what mine reads, input shorted and EXT TRIG selected. Measuring from the shield with a 10M DMM.U1031: -6.592: .0003: .0004: -6.735: -6.606: .0007: 6.518: .000U1021: .0002: .0003: .0004: -15.155: .0006: .0007: .0038: 15.01TP101: .000TP102: .003TP103: .000

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2025, 10:55:25 pm
content: Could you check pins 21, 22 and 23 of U101 and then TP101, TP102 and TP103 with the input shorted and the ADC stopped (EXT TRIG selected)?I can't go through the entire logic of it right now (I'm multitasking....) but it looks to me like the three op-amps and the bootstrap circuit are operating as intended and the problem is that there is the 1.208V on pin 3 of U103 when it should be 0.000V in the AZ mode.

author: ulix
author_rank: Regular Contributor
timestamp: February 07, 2025, 11:08:44 pm
content: Just checked again with the inputs shorted, like your list; in VDC+2V+EX_Trig. All my values are in Italic, the first values are from you. (was just typing this up when your new message came in, have added this at the bottom).U1031: -6.59;-4.0432: .000;1.2053: .000;1.2054: -6.73;-4.0475: -6.60;-4.0466: .000;1.2087: 6.51;7.868: .000;0.000U1021: .000;1.2102: .000;1.2103: .000;1.2104: -15.15;-15.425: .000;0.0006: .000;0.0007: .003;0.0018: 15.01;14.82TP101: .000;1.121TP102: .003;0.001TP103: .000;-0.004(new) U10121:0.00022:-0.00423:1.216

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2025, 11:53:14 pm
content: OK, can you also check pins 24 and 25 of U101?What I haven't figured out yet is when and which if any of S1/S2/S3/S4 are on during the AZ cycle and what the "POL" refers to.Other than those issues, it looks like the nearly-zero volts from pin 21 U101 isn't making it to pin 23 through the AZ analog switch.Edit: C102 and C103 are for storing and reading successive remainders, S1 and S2 to store and S3 and S4 to read. "POL" apparently refers to polarity, this goes to timing and data control within U101 and is how the ADC reads the results of the comparisons the ADC is making. So I'd expect S1-S4 to all be closed during AZ. IDK what voltages might appear on C102 and C103. Other than some major leakage to the output circuit of U103, I think this looks like a bad U101. You could try removing CR105 and CR106 just on the off chance one of those are bad. I suppose U103 could have an issue as well but it really appears to be working.

author: ulix
author_rank: Regular Contributor
timestamp: February 08, 2025, 11:04:08 am
content: First, I have checked both pins 24 and 25 of U101. Both Pins show a voltage of arround 3.5V and then are pulled to zero when measuring.I followed this with desoldering one leg of CR105 and CR106. This did not change anything, all measurments are the exact same. So guess it really is U101

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2025, 11:00:33 pm
content: The manual does mention the possibility of digital control issues being mistaken for ADC or other component failure. I don't think this is too likely but there are some pretty good methods for checking the integrity of the data bus lines in the manual. See sections 6-40 and 6-41 and see if you want to try that. I didn't see any details as to how you can verify with certainty that U101 has been properly commanded into the AZ configuration but seeing all 6 bus lines switching properly would make that a near certainty. As I see it, the three possibilties at this point are:1. U101 is bad2. There is a problem with the µC or the data lines going to U101 so as to not control it properly3. Some unknown issue that I haven't thought of is putting that voltage on the non-inverting input of U103If anyone else has any suggestions please chime in. I'm out of ideas at this point. IMO the likelihood of the above options is 1-66%, 3-33% and 2-1%.

author: ulix
author_rank: Regular Contributor
timestamp: February 10, 2025, 06:48:25 pm
content: Hi bdunham7,I have tried to ask a seller on ebay if the would send me a replacement for the U101 - yet it seems that the are not interessted in selling sending to me.More than happy to check the bus lines, but I must say I find it hard to get any information out of Figure 6-16. Just to check how they want to troubleshoot the six bus lines. Is it okay to check each bus line with the scope, and when below 3Vpp then is line is "bad". I'm not getting what and how they mean to use the 1kO resistor to drive the data lines...

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2025, 09:25:51 pm
content: Quote from: ulix on February 10, 2025, 06:48:25 pmAt this point I would just plug in a different U101, but I have the luxury of having more than one of these. I'm not sure how accurate the unit will be after replacing U101.With the bus lines, I'd just do the procedure in 6-41 and 6-42 and scope the relevant pins of U101 to make sure there was a clean 1kHz signal. If so, then no need to go further. The test they refer to with a 1k resistor and all of that refers to testing the traces on the board itself with both U101 and the µC removed, or at least that's the way I interpret it. You are very unlikely to arrive at the point where that test is relevant and there are probably better ways to verify trace integrity.

author: ulix
author_rank: Regular Contributor
timestamp: February 11, 2025, 05:54:48 pm
content: @bdunham7 thanks again for your input. I've now probed all the (six) datalines from U101 and they seem all to do just fine. On every bus line there is communication noticable. Sadly, I can't find any replacement for U101...

author: bdunham7
author_rank: Super Contributor
timestamp: February 11, 2025, 06:41:44 pm
content: Quote from: ulix on February 11, 2025, 05:54:48 pmWell, it can take time for those to show up. I guess you just put it on a shelf and check eBay and so forth once in a while. You might also find a trashed complete meter for cheap.Can you post a photo and/or the part number on U101? I think they're all interchangeable but I've never checked before on the A/AF version.

author: ulix
author_rank: Regular Contributor
timestamp: February 11, 2025, 07:28:16 pm
content: Quote from: bdunham7 on February 11, 2025, 06:41:44 pm@bdunham7, I greatly appreciate your time! The chip is labeld: Fluke; 715680; 1986; 33;

author: bdunham7
author_rank: Super Contributor
timestamp: February 11, 2025, 08:19:52 pm
content: Quote from: ulix on February 11, 2025, 07:28:16 pmThat is the usual part number for these. There are two on eBay here, one for $7 + shipping, the other $475. Interesting fact--they apparently use this same ADC ASIC in all of the 8840/8842 series and the 5700/5720 series calibrators. Keep an eye out, one will show up sooner or later.