Fluke 8050A is showing "OL" always - Page 1
Forum section: electronics / repair

author: HummingBird
author_rank: Contributor
timestamp: May 11, 2020, 06:12:28 pm
content: Hello everyone,I got an 8050A which is, after POR, showing shortly 000 and then 1___ as "overload". I found another topic saying that with same problem exchange of U7 & U9 has helped but not in my case. I see an nice charge/discharge at pin 2 of U7, an ramp at pin 7 (picture 2) which is output of U7 and an (quiet) short negative impulse on output of U9 (comparator, oicture 3) which is received by EXT_INT of main uC. The volate at TP6 (picture 1) is changing accordingly to Vin, also the charge/discharge curve does...but no change at pin7 of U7...and thats strange I think?Any specialist here who might help me to understand why the analog ADC ist not working an the uC does not see an PWM (if you can call it like that with ~2Hz) to measure the voltage?I just want to understand what the issue is and "rescue" that piece of measurement history (and maybe use it if one of the others are not at hand)Thx, Klaus.

author: HummingBird
author_rank: Contributor
timestamp: May 15, 2020, 10:23:42 am
content: ...no, nothing, not any ideas? There are so many collectors of vintage DMMs and I hoped to get some "hand on" experiences here?Klaus.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 15, 2020, 05:26:20 pm
content: You should be patient here, it wont help you if you put somekind of pressure ... when someone can answer you may get some help.

author: HummingBird
author_rank: Contributor
timestamp: May 15, 2020, 06:31:33 pm
content: Ok, thx...maybe I am used to the reaction time in other ee-blogs / "forums"...but ill be patient and wait now, lets see what happens!Klaus.

author: retiredcaps
author_rank: Super Contributor
timestamp: May 15, 2020, 07:21:32 pm
content: Quote from: HummingBird on May 11, 2020, 06:12:28 pmWhat is POR?Does it show 1 or overload in all modes? Does it show overload in DCV? ACV? resistance?What works? DCV? ACV? resistance?

author: HummingBird
author_rank: Contributor
timestamp: May 15, 2020, 07:57:18 pm
content: POR = power on reset, so if I switch it on first time. On TP6 I see Vin, after U7 is see a charge/discharge curve depending on Vin but after comp Ü9 and input pin38 of uC I only see the short spike from 3rd picture. So, "somehow" the changing charge/discharge curve is not affecting the pulse/pause at pin38 - I assume that it should so? I was thinking about testing the uC beside by using a duty cycle control of a 2Hz PWM and a optokoppler instead of U9... (just to see IF it WOULD work, if the ADC is up again).EDIT: Yes, it shows "1 . " over all modes and ranges, so at kleast nothing works but that seems OK without the "bootleneck" of the ADC running?Klaus.

author: Lifeborne
author_rank: Contributor
timestamp: May 20, 2020, 12:50:13 am
content: Have you checked the input power rails for the comparator?

author: HummingBird
author_rank: Contributor
timestamp: May 22, 2020, 05:58:25 pm
content: " input power rails" means the power supply of the LM393 itself? Yes, its ok. Anyway ist works as you see due to the short spikes on its output...but even the input to the comp is not "dependend from Vin", so how shall it compareBut to be honest I do not yet understand what happens bewteen TP7 and input of comp...somewhere between "sh*t happens".Klaus.

author: HummingBird
author_rank: Contributor
timestamp: May 26, 2020, 07:50:48 pm
content: Today i made some more tests...and the error is occuring not so often any more (means: only after 5...15mins):IF it is OK, then all ranges & modes work (tested DC V / mA / R, not AC yet)...even if the readings are not very precise (yet, I have to check how to adjust).IF the ERROR occures, some "voltage is running away" and the readings rise and rise and rise (also the oszi readings are changing strangly)...until "1 _ _ _ _" - switch off and on agian, works again. Hmpf...what the &%$*#. Voltages of all ICs are OK and all other volatages I could find in schematics are OK also (no difference between OK and ERROR). But I guess there is some problem with any "reference or so".Problem can not reproduced with "bending" the bord or parts (a little bit), also not with ice spray.-> Any ideas?Klaus.

author: HummingBird
author_rank: Contributor
timestamp: May 27, 2020, 07:27:35 am
content: OK, it IS an thermic issue...I was wondering that the "ramping up" of the reading till 1. _ _ _ _ was slower, the the longer the device is on. So I heatet up "the whole board" with an hotair and the error ist gone...using ice spray "all over" brings back the error. OK, so maybe a bad solder point or sth...the next rainy day I have to test in areas to localize it step by step.EDIT 1: So after 2 more tests I think its in the area of U20 (without having it localized to one specific component).EDIT 2: That was it - 4 solder points of U20 lock a little bit "grey" and at least they produced the issue. I guess its an problem of some acid fumes from the batteries at least...but why only these 4 pins? Anyway, it works again and is more or less "in spec".Klaus.