Fluke 8050A Repair help: Always reads OL (Overload) - Page 1
Forum section: electronics / repair

author: bsgd
author_rank: Regular Contributor
timestamp: February 19, 2021, 01:18:02 am
content: Hi all,I have been given a Fluke 8050A bench DMM in non working state and I need some help to fix it, after trying to do it by myself with no success. I am not very familiar with how DMMs really work so this is also an opportunity for me to learn something new!The issue is that in the OHMS function, the display always shows OL (actually on this unit OL is represented by a '1' on screen). No matter which range I select, it always shows OL even with the probes shorted.After first opening it up I noticed it was in perfect shape and no parts had been previously replaced. I found 4 leaking electrolytics and decided to just replace all electrolitycs in there. After that I did some research on forums and found that many reported problems with U7 and U9, so i just decided to replace them too. I also cleaned the board where the caps leaked and checked continuity on every pcb trace nearby to make sure they were ok. Nothing helped, sitll get OL.If I try to measure DC volts with the unit, it kinda works. I say kinda because on some ranges it reads perfect, while in other ranges it reads around half the correct voltage. Right now Im focusing on trying to fix the OHMS problem first, one step at a time, so Im not yet worried about the huge error on the voltage readings...I do have the service manual, but to be honest those schematics are too comple for me right now. Im reading about the principles of operation so I can get used to how these units operate, but really need some help to further investigate and solve this issue.Thanks guys

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2021, 01:25:34 am
content: Do you have another DMM? The first thing to try is to measure the ohms test current (put it in the lowest range and measure the current output with your other DMM) and the ohms test voltage.Edit: I missed thisQuoteSo the first thing you should know is that the OHMS range works by the meter putting out a constant current and then measuring the voltage across the outputs. R = V/I, so if it puts out 1 milliampere of test current, the meter will read 1 volt for every 1000 ohms, and so on. The circuit that puts out the constant current has a voltage limit, this is the open-circuit test voltage. The meter will display the overload sign if the voltage exceeds the max for that range, but that will happen at a lower point than the open-circuit voltage.EDIT: It actually doesn't work that way, see comments below and Fig. 3-5 in the manual. It does happen to put out 1mA in the 2K range as I state here, but not for the reason I stated.I happen to have an 8050A right in front of me, so I tested it and found that on the 2K OHMS range, it put out a current of 1.019mA and had an open circuit test voltage of 3.24 volts. Max range of 2000.0 ohms (actually 1999.9) would be at 2.0000 volts.If your meter is reading overload, one possibility is that there is an open circuit somewhere in the output, so the meter thinks the leads are always open. Others have mentioned switches as an issue, but my heap of broken 8050A parts tells me it could be almost anything. Testing the output will tell you where to start. And start staring at that schematic, post it if you need more help.

author: Martian Tech
author_rank: Regular Contributor
timestamp: February 19, 2021, 03:56:05 am
content: It could be as simple as a bad or dirty switch (S4, the kOhm selector). I'd check that first.

author: bob91343
author_rank: Super Contributor
timestamp: February 19, 2021, 05:25:43 am
content: I have that model, or something very close. And the switches do get cranky. Sometimes I have to push them fairly hard and then push them repeatedly. Hold one button down to release them all so you can push the others many times.

author: bsgd
author_rank: Regular Contributor
timestamp: February 19, 2021, 12:18:41 pm
content: Hey guys, thanks for the help.I forgot to mention that I did exercise the contacts A LOT yesterday and sprayed some isopropyl alcohol to try to clean them. It mande no difference, but I also do not know how to test them and make sure they are doing what they should...So I do have another DMM and tried to measure the current. 0mA and I confirmed it, theres absolutely no current in Ohms mode...One interesting thing is that this morning, after leaving the Fluke off all night, it also showed OL in all other functions, volts and mA. After leaving it on for 20 minutes and doing a few power cycles, Volts and mA went beck to normal, but Ohms is still showing OL. Im not sure what could explain this 'warm up' condition for it to fail. I tried gently bending the PCB just to check if this was not a case of bad solder or contacts but no matter how I bend it it makes no difference.I did inspect most solder joints with a magnifier and found them to be in good condition, even though I did reflow a few suspect ones. No difference either.So I guess my main issue here is that theres no current flowing in Ohms mode, and theres something failing that needs some warm up and a few power cycles to work. How do I start investigating the lack of test current and what components are the most suspect?I'll keep on reading the schematics and service manual and see if I can at least figure out something from those complex schematics

author: bsgd
author_rank: Regular Contributor
timestamp: February 19, 2021, 01:16:11 pm
content: Quick update: I left the DMM off for an hour and turned it on in the V function.With probes shorted, the display shows a quickly increasing nevative voltage till the reading goes OL.... after 20 minutes and a few power cycles, V function goes back to normal and Ohms reading still shows OL.Not sure what this means in terms of what could be failing.

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2021, 03:26:17 pm
content: The schematic does seem a bit hard to follow, I wasn't able to identify all of the specific areas of operation. I will try to get a chance to look at it later or in a few days. The processor in the right lower corner is a variant of the Fairchild F8 and the analog front end is on the top to the right about 2/3 (they don't even have a letter/number grid on this schematic) starting right below where it says "+13". The dual-slope A/D converter is below that and the ohms current source appears to be over near the inputs. You should read the "Theory of Operation" part of the manual to try and figure out what's what.

author: Martian Tech
author_rank: Regular Contributor
timestamp: February 19, 2021, 05:22:10 pm
content: Fluke 8050A doesn't use a current source for measuring resistance. It has a reference resistor (somewhere) that gets put in series with the unknown resistance. Then it puts a voltage across the string and compares the two. It's been a while since I looked at the schematics, but iirc, everything to the right of the analog switches (U18) is simply measuring a voltage in the range of 0-200mV, and everything to the left conditions the input to a voltage in that range. (Follow the switches: the 200mV range should pass straight through). So if it works OK reading DC volts, you can pretty much assume that U18 and everything to the right of it is working properly.Note that when reading resistance, RT1 is also in series with the unknown and reference resistor, so if it is bad it can mess up the results. It's bypassed when reading voltage...

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2021, 05:54:50 pm
content: Quote from: Martian Tech on February 19, 2021, 05:22:10 pmRight you are and that explains why I didn't understand the current source part. I misread the description in the manual and I thought that the ratio method was for the higher resistance ranges and not 200R/2K. So R3 in the schematic would be the VΩ REF, RT1 and R2 are as shown.I think I read that the A/D converter can do either 200mV or 2V directly, so that the signal conditioning only needs to use X1 and X100 ratios.

author: bsgd
author_rank: Regular Contributor
timestamp: February 19, 2021, 08:26:18 pm
content: Hi all,Yes, I have been reading the theory of operation part of the SM, but to me who never ever read a multimeter schematics, all is too new so Im still trying to digest all the information. Not easy for a newbieOk, I see theres no current source, but there should be a reference voltage and I get 0V at the Fluke leads in Ohms mode, testing with another multimeter, so I know something is not right. in the schematic, is U8B generating this voltage reference? It is the +3V ilustrated in the schematic at U8B output? The SM does not describe in details who generates the Reference Voltage and how the circuit around it works, ate least, not detailed enough for someone like me... so Im trying to figure it all out.I did test RT1 and it read around 1K, so I guess its fine.

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2021, 08:41:01 pm
content: Quote from: bsgd on February 19, 2021, 08:26:18 pmI don't find that schematic easy to read either, so don't feel badly about it seeming difficult. It doesn't have aconstantcurrent source, but still works more or less like other ohmmeters.U8B pin 7 should have the 3 volts. On the 2K range, you should track the 3 volts through the two switches below that in the diagram, which should be in the position drawn in the 2K range. From there, up to the switch pin marked "S4F" and then through the maze of switches (good luck!) to R3. Let us know where you lose it, if there is 3V at U8B pin 7 in the first place.

author: bsgd
author_rank: Regular Contributor
timestamp: February 19, 2021, 09:03:09 pm
content: Thanks for the kind words bdunham7, I was really feeling dumb trying to figure out this schematics. Following anything through that maze of switches seem almost impossible.The good news is that I made some progress!! I got 0V on pin 7 OF U8B, so I went ahead and measured voltage at pins 5 and 6. Pin 5 had 2.6V (kinda low but OK) and pin 6 had 0V as its tied to 7. So I replaced U8 and now I do get an Ohms reading. Not perfect but OK, it somehow works now!Reading Ohms with leads shorted results in around -0.5 Ohms (Yes, negative 0.5 Ohms). I am also still getting some annoying 'warm up' problem. When I leave the meter off for some time and then turn it on in Ohms range with leads shorted, it first shows something like -60.0 Ohms and the reading slowly starts to move towards 0 Ohms, and settles at around -0.5Ohms after a few minutes.What could cause this strange behavior? I will test the Fluke a little more now to see what other problems it might have.