Fluke 8808a Won't Auto-Range - Page 1
Forum section: electronics / repair

author: GAD
author_rank: Regular Contributor
timestamp: January 09, 2016, 07:22:18 am
content: Hey all,I started a thread in the Test Equipment forum that's evolved into a troubleshooting exercise. If anyone has any experience or insight with the Fluke 8808a or rebranded models of the same ilk, I'd appreciate any help you might be able to provide.Here is the original thread:https://www.eevblog.com/forum/testgear/fluke-8808a-question/Here is a summary of the behavior:With a 9V battery fresh out of the box, here are the readings in each range (meter set to "slow"):200mV: ~139.75mV2V: 1.568V20V: 4.59V200V: 8.245V750V: 8.25VMy Fluke 175 reads 9.15V from the same battery using the same probes.The same deal for AC (Ranges are the same) with the probes mashed into an AC outlet:200mV: ~55.18mV2V: 0.48744V20V: 0L (yay!)200V: ~107.695V750V: 109.52My Fluke 175 reads 118.2V in the same outlet using the same probes.I think this meter isn't measuring right at all. What started this all is the fact that it won't auto-range, which is apparent when I choose DCV, probe a 9V meter and it reads ~139mV without clicking or reporting 0L.Voltage readings using my Fluke 175 while measuring a 9.17V battery:I have the option of just returning it for a full refund, but now I feel like I'm on a quest. I'm sure you all know what that's like.

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: January 09, 2016, 03:03:17 pm
content: I would recheck the 2v & 200mv with the proper voltages for that range instead of using higher voltage ..Do you have a schematic for the meter cause it looks like to me the 200mv,2v,and 20v range are tie together some how and the 200v & 750v ranges are pretty close..

author: GAD
author_rank: Regular Contributor
timestamp: January 09, 2016, 05:48:47 pm
content: Quote from: TorqueRanger on January 09, 2016, 03:03:17 pmI'll do that. I cannot find a schematic or service manual for the meter. I do have the calibration manual, but without a good calibration source it doesn't do me much good.

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: January 09, 2016, 06:20:37 pm
content: Quote from: GAD on January 09, 2016, 05:48:47 pmDo you have another meter ??You could try using a lm317 and cal with another meter .. The lm317 is pretty accurate reference in a tight spot..

author: Macbeth
author_rank: Super Contributor
timestamp: January 09, 2016, 07:13:09 pm
content: I admire you wanting to fix it, it's what I would do - but the very least you want is to negotiate a fair value for the meter considering it was sold as working with a guarantee. Also the seller has to bear the cost of returning it to him (and ebay insist this has to be tracked post - expensive) as it is clearly faulty.Make the seller aware of this - you are 100% protected by ebay - and the fact it would probably be next to worthless to them when returned as they will either have to fix it (which is commercially not feasible when time + knowhow + parts is money, unlike your hobby repair in your own time) or they will have to sell it on as a parts unit. The return postage costs would probably kill its value as a parts unit so not worth the bother.

author: TheSteve
author_rank: Unknown
timestamp: January 09, 2016, 07:24:32 pm
content: The seller has already posted in the original thread that he is willing to negotiate or pay full return shipping - seems like a good guy.Shame there isn't a little more info such as a service manual.Stuck range perhaps? Maybe a relay that never toggles.

author: GAD
author_rank: Regular Contributor
timestamp: January 10, 2016, 08:12:12 am
content: Got the shields off and took some detailed pics of the boards.

author: GAD
author_rank: Regular Contributor
timestamp: January 10, 2016, 09:14:00 am
content: I've done more detailed testing of the input protection resisters. Data is as follows:All voltages measured on Fluke 175.R26 and R27 are tied to the current inputs, and are always zero. Thus they are not included in the interest of brevity.DCV in: 0.015Code:Resistor Readings:R28:0.015 / ~0.015R29:~0.015 / ~0.015R30:0.015/ 0.015R8:4.56 / 4.568808a Displays:200mv:2.025mV2: 0.0164020:0.0050200:0.007750:??DCV in: 1.499Code:Resistor Readings:R28:1.499 / 1.360R29:1.3725 / 1.350R30:1.3725 / 1.3725R8:~4.5 / ~4.58808a Displays:200mv:168.1982:~1.372xx20:0.7517200:1.348750:1.35DCV in: 10.0Code:Resistor Readings:R28:10.0 / 9.9R29:9.9 / 9.89R30:10.0 / 9.99R8:4.6 / 4.68808a Displays:200mv:~1302:~1.520:5.0292200:9.029750:9.04DCV in: 30.0Code:Resistor Readings:R28:30.0 / 29.72R29:29.72 / 29.43R30:30.0 / 30.0R8:4.57 / 4.68808a Displays:200mv:~45mV2:~1.413720:15.0823200:27.082750:27.10All of the readings with ~ in front of them are moving constantly. Here's the scope looking at 5.0VDC in on the outbound side of R28 (The first resister in the path). This behavior is on 200mV and 2V ranges only. As soon as the relay kicks in for 20V, the line is flat. This behavior is variable. If I reboot the 8808a it may only do it for some voltages. When this occurs, the inbound side of R28 is flat.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 10, 2016, 10:05:59 pm
content: Since there is no service manual, I'm going make some guesstimates based on how other handheld Fluke meters works.1) Most Flukes have a precision network resistor that acts as a voltage divider when the meter is in volts mode and as a reference resistor when in ohms mode. This network is usually marked Z1. On the 8808A, Z1 is next to the LM399 as this picture showshttps://doc.xdevs.com/doc/Fluke/8808A/photo/ourdev_634637W7XGTB.jpghttps://doc.xdevs.com/doc/Fluke/8808A/photo/Fluke-Vref.jpghttps://doc.xdevs.com/doc/Fluke/8808A/photo/DMM4020-PCB-Annotated.jpg2) The precision network resistor, depending on the selected range, lets the voltage pass through or divides the input by 1/10, 1/100, 1/1000. So if you are sourcing a 9.15VDC and assuming the 8808A full scale reading is 2V, I would expect to see 915mV on one of the pins of the precision network resistor. The COM jack is ground.3) If you see 915mV on one of these pins, that implies the analog portion of the meter is likely working. If you see something like 800mV and the 8808A is showing 8.00V DC for a 9.15V battery, then there is something wrong with the analog part.4) Since you mentioned the 8808A can be put into Fluke 45 mode, there is a service manual for the Fluke 45 and the theory of operation describes the above in more detail.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 10, 2016, 10:12:19 pm
content: The Fluke 45 service manual also has a section (2-21) on how the relays work which may provide clues to the 8808A relay problem?

author: retiredcaps
author_rank: Super Contributor
timestamp: January 10, 2016, 10:21:41 pm
content: BTW, Dave's teardown on the Fluke 45 might also provide some education/insights/clues ...

author: retiredcaps
author_rank: Super Contributor
timestamp: January 10, 2016, 10:30:00 pm
content: Quote from: GAD on January 10, 2016, 09:14:00 amAssuming the mV and 2V range don't require any part of Z1, the above readings are obviously wrong. Where it is failing is the question.

author: Macbeth
author_rank: Super Contributor
timestamp: January 10, 2016, 10:37:08 pm
content: Board looks like its only 2 layer - take some hi res pics of it with a light behind, always good to help deconstruction / reverse engineering.

author: GAD
author_rank: Regular Contributor
timestamp: January 11, 2016, 02:18:07 am
content: Quote from: retiredcaps on January 10, 2016, 10:05:59 pmMan, do I ever wish there was a service manual for the 8808a! That doc is fantastic!I took 10.00VDC in and measured all three pins on Z1 through all ranges on the meter. Here are the results:Code:Range P1 P2 P3 8808a Display200mV ~4 ~1.3 ~-3 ~130mV2V ~4 ~1.3 ~-3 ~1.52V20V ~4.4 ~1.74 ~-2.45 5.0363V200V ~4.39 ~1.725 ~-2.45 9.041V750V ~4.386 ~1.720 ~-2.422 9.05VP1 was *never* stable, though the differences get smaller as the range increases. Here are the scope traces. Note that at 200mV and 2V the vertical scale is 1V, while on the rest the scale is 100mV.All at 10.00VDC in. Probe on P1 of Z1200mV:2V:20V (note the smaller vertical scale)200V750V

author: retiredcaps
author_rank: Super Contributor
timestamp: January 11, 2016, 03:15:16 am
content: Quote from: GAD on January 11, 2016, 02:18:07 amHmm, none of the above readings makes sense to me. It is possible that I'm completely wrong about the function of Z1, but I see no other precision resistor network in the photographs?While I think of possibilities, I mentioned in the other thread that the input impedance for all the ranges look good.For the sake of completeness, let's ensure the input protection is okay despite the input impedance looking okay. The MOVs (circled in blue) should read 0L ohms on the Fluke 175. The green fusible resistor (circled in green) should measure 1k ohm. The black PTC (circled in black) should read about 1.1k ohm.I can't quite make out the pcb lettering, but the two components circled in red might be some sort of spark gap? If I'm correct, they should also read 0L ohms.All these components can be measured "in circuit" with power off. Report your readings for all of them.

author: GAD
author_rank: Regular Contributor
timestamp: January 11, 2016, 03:29:29 am
content: Quote from: retiredcaps on January 11, 2016, 03:15:16 amI see no other precision resistor networks, either.My readings as requested:Green: 1.012KRed: Unable to probe since the parts cover the connetions. Mine are different parts and are labeled TAIYO AG20 which are indeed spark gaps.Blue: Both 0LBlack: .981KAll seem within what you expected where measured.

author: retiredcaps
author_rank: Super Contributor
timestamp: January 11, 2016, 03:34:16 am
content: Quote from: GAD on January 11, 2016, 03:29:29 amCan you measure the resistance of the spark gaps on the backside of the pcb?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 11, 2016, 03:54:51 am
content: I am a little late with this. I decided to record the analog switch address lines in DC and AC Volts modes similar to how Keithley did it in their service manuals. They are recorded in the attached spreadsheet. All readings are 5V logic except U60, which appears to use 3.3V logic on pin 9.

author: GAD
author_rank: Regular Contributor
timestamp: January 11, 2016, 04:16:46 am
content: Quote from: retiredcaps on January 11, 2016, 03:34:16 am0L on both.

author: GAD
author_rank: Regular Contributor
timestamp: January 11, 2016, 11:42:52 pm
content: Quote from: ManateeMafia on January 11, 2016, 03:54:51 amHmm... I set out to test all of these. Every pin 9/10 I tested from the spreadsheet in 200mV is doing something like this, though this is not from this test. The range of the fluctuations got smaller as I stepped up the 8808a's range just like it did on the Z1 tests.I set out to test the power supply and managed to have a probe slip and blew the AC fuse. More testing will have to wait until I can score another. My home office is all torn apart and I can't find anything so I'll have to go buy another if I can't source one close in value from something else.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 12, 2016, 01:55:49 am
content: Quote from: GAD on January 11, 2016, 11:42:52 pmPretty much sums up my home office. I had to take over another room for my home lab.I plan on doing as many of the comparators tonight as possible. I have had a few of them go bad in the Keithleys.

author: GAD
author_rank: Regular Contributor
timestamp: January 13, 2016, 12:47:19 am
content: Manually tested all the relays and they all function. Got a fuse, so that's a good thing.

author: GAD
author_rank: Regular Contributor
timestamp: January 13, 2016, 09:19:50 am
content: Today I measured the transformer output. Could this be the issue? Is it possible that it worked within spec at 220V but doesn't at 120V? Never mind - no sleepCode:Wire Spec MeasuredYel 10V 4.05VOrn 0V 1.82VYel 10V 7.42VBlu 13V 8.62VRed 0V 1.035VBlu 13V 6.66V

author: deadlylover
author_rank: Frequent Contributor
timestamp: January 13, 2016, 09:57:45 am
content: How are you probing the transformer? For the first tap for example, you should be probing from Orange to Yellow.Are those TO-220 devices 3 terminal regulators? (I can't make them out from the photos) Check their output voltages.Make sure it's not something silly, you've set it to 120V right?edit: oops I meant 120v

author: GAD
author_rank: Regular Contributor
timestamp: January 13, 2016, 04:59:42 pm
content: Quote from: deadlylover on January 13, 2016, 09:57:45 amGah! This is why I shouldn't try to think when I can't sleep at 4AM. The voltages are 9.8V and 12.4V when measured properly.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 13, 2016, 05:06:27 pm
content: I will confirm my voltages later this afternoon. Been working late hours and trying to make time for more data has been difficult.

author: GAD
author_rank: Regular Contributor
timestamp: January 15, 2016, 06:07:20 am
content: Thanks for all the help, but I've decided to return it. As much as I liked the meter (love the dual display), I was spending too much time on it without any real results. The seller is being very cool about it and it's going back from whence it came.I'm an Agilent man now.