Schlumberger 7081 up to 15V OK - Page 1
Forum section: electronics / repair

author: Ypsi
author_rank: Contributor
timestamp: August 13, 2019, 09:54:06 pm
content: Hi friends!I bought to my lab old-new Schlumberger 7081. Nice look, but problematic measuring.. All selftest passed, no power-up problems. Both batteries are OK.Ohms tested, OK, but DC voltage is OK only up to 15V, then i hear click by relays inside, and i have nonsense dates on display [618-979 and so on...]Had someone this problem with that multimeter?Schematic is for me too much chaotic.Thanks for any help

author: coromonadalix
author_rank: Super Contributor
timestamp: August 13, 2019, 10:46:15 pm
content: first thing i would do is check all the supply lines for noise / bad filtering ... do you have a scope ?I know the tests passes but i would start there first, you dont know the meter history ....I would check all connectors / if you see bad soldersAnd the annoying thing is to check all the input paths up to the dc range section .... maybe some oxydized relay contacts or a defective relay, transistors switching etc ...

author: Ypsi
author_rank: Contributor
timestamp: August 14, 2019, 06:59:01 pm
content: I did that test, it is what i do, correct voltage, no ripples.. Oddly is, when i reach voltage upper 15V, i don't have any OVERFLOW message, only nonsense value...I don't understand principle of divider and input amplifier with corresponding circuits..I hope that calibrations data is OK..

author: Kleinstein
author_rank: Super Contributor
timestamp: August 14, 2019, 07:21:53 pm
content: Does the meter read >15 V correct, when manually choosing 100 V or 1000 V range ?If not chances are there is something wrong with the relays. One should be able to check the ones at the input in circuit.If the manual mode works, the problem seems to be with detecting overflow and maybe doing auto ranging. This could be something like IC102 , IC103 driving the test point TP106/107.

author: Ypsi
author_rank: Contributor
timestamp: August 15, 2019, 06:22:19 pm
content: Hm, i tried that, i have something interesant. Last two ranges (1000V, 100V), manually switched, has problem. I did this test with 1V DC, and values on those upper ranges were really unreal...Something in divider?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 15, 2019, 06:59:24 pm
content: I would more suspect the relays than the divider itself. A first test would be measuring the input resistance, when in 100 V / 1000 V range. This likely is 10 M - if so would confirm RL6 and the divider resistors are OK.My main suspect would be RL4, either the contracts or the coil / driver.

author: Ypsi
author_rank: Contributor
timestamp: August 15, 2019, 08:33:38 pm
content: Kleinstein, thanks for tips, weekend coming, i will have time for experiments, than i write some new findigs

author: Ypsi
author_rank: Contributor
timestamp: August 17, 2019, 06:51:23 pm
content: Hi!Back from investigation. I did the test with 1V DC on ranges 100 and 1000V, switched manually. On 1000V was displayed 46V... Then i tried the exchange RL5 to the RL4 (only for testing...). Then i rise up wit input voltage and then i got to 26V on display.. No more. Relays back, switch on the machine...... NVM failNow, i'm not sure, is the calibration constants located on those NVM memory, or not? (i think yes.) But, to this moment, i checked the voltage on TP101 and 102, and on 5sec intervals the voltage junps to -17V....I'm confused..

author: Kleinstein
author_rank: Super Contributor
timestamp: August 17, 2019, 07:40:35 pm
content: The jumps to -17 V at TP101 /102 could be a transient think when switching (e.g. AZ modus). If it does move this is a good sign. To me it more looks like a relay problem ( as the manual 100 V range does not work). The suspicion is still RL4 not closing.If possible one could check this at pins 1 and 5 of RL4 directly with a 2nd DMM in Ohms / diode test, while switching to the 100 V range.The NVM fail could be a consequence from soldering, maybe some ESD or similar disturbance to the meter.

author: Ypsi
author_rank: Contributor
timestamp: August 17, 2019, 08:24:50 pm
content: I think so that the NVM fail can be caused by ESD disturbance, because that problem was found when i bought the machine by original owner. After two weeks on calm was the machine OKTommorow i do the test RL4 and i write the result.

author: Ypsi
author_rank: Contributor
timestamp: August 18, 2019, 09:05:44 pm
content: Some new facts. I found leaked voltage trough RL5 contacts, thats affects voltage level on 100 and 1000V ranges. For testing i replace this relay to other one, 1-10V ok, 100 and 1000V are better but still wrong, after applying 24V on input, i read only 16V.. however, on HI testpoint is 0.24V, thats OK.Some problem with gain???During the test was damged IC102, was replaced to LM741, wit IC103 as well.. And the message NVM Fail is gone, fortunelly.Another oroblem is with indication of Overrange, no reaction, only the display freeze, and no response from relays, i have to switch the ranges manually..Magic?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 19, 2019, 04:38:09 pm
content: If the relay RL5 was bad, this could have caused a faulty calibration of the 100 and 1000 V ranges, kind of trying to compensate for the error.Using LM741 for IC102/IC103 could be borderline with the supply voltage.

author: Ypsi
author_rank: Contributor
timestamp: August 19, 2019, 07:24:24 pm
content: I hope that the calibration data are OK, the problem will be somewhere in "hardware"...Now i would like check the ranging circuit (TR101-103), because the AUTO function working bad.