Troubles calibrating Meratronik E317 RLC meter - Page 1
Forum section: electronics / testgear

author: bborisov567
author_rank: Regular Contributor
timestamp: May 17, 2024, 11:17:21 am
content: Hello, i am trying to calibrate an old Meratronik E317 RLC meter. I follow the calibration steps describe in chapter 9 of the service manual. I have done a rough translation at the end of the post since it is only in german and polish. Everything goes as described until 9.7. where i can't adjust the measured R value. For a connected 4k7 resistor i get a reading around 30k (i had switch to the upper range to see that). R177 adjust only few hundred ohms. I tried compensating with R183 but then i get wrong reading in the 100 ohms range. I have checked the power supplied an they are ok. Do you have an idea where to start troubleshooting? At the end of the post i have attached the polish and german service manuals and the schematic for the analog front end. There are some missing designators in the german manual so cross referencing by the chapter numbers with the polish manual is recommended.Link to the german manual:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Polskie%20mierniki/E317.pdfLink to polish manual:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Polskie%20mierniki/Moje/E317.pdfTranslated calibration instructions:Quote

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 17, 2024, 10:00:01 pm
content: The adjustment range of a few hundred ohms seems plausible. That should be enough for calibration if the instrument works properly. So I think it's clear there is more to do than calibration.Possible next steps:With the 4k7 or any other known resistor at the input, check the resistance with your DMM (RLC meter off) at the input nodes of the circuit on the PCB. That way, you get to know whether there are contact/cable problems on the input section.The instrument is about 50y old, right? Check the electrolytic capacitors, visually and electrical. Even if they are not needed for resistance, it is difficult to rule out that they had a bad influence on your settings in steps 9.1 to 9.6, because these include L and C ranges.Without further calibration steps, try to measure in different R, L, C ranges with known devices. Do you see a pattern in the errors? That could be a hint for more troubleshooting.

author: bborisov567
author_rank: Regular Contributor
timestamp: May 24, 2024, 12:24:29 pm
content: Thank you for the reply! After a lot of hours i find a short on the pcb right at the input of the ADC. It was there from the previous owner - i bought the unit as non functional. Now the i managed to calibrate the resistance measuremnt and it is working as it is supposed. But now i have a problem when measuring capacitance. It measure about 3 times less the actual value and that is on all ranges. I tried calibrating it using R186 but i can make only fine adjustent. The generator (M101) working as supposed to - it has 7v rms at 1kHz. I checked C123 and C124 but they are also good. I also did some measurements - U4 is a 1kHz sine and its amplitudes varies depending on the measured capacitance. The peak detector (M107) also seems to be working. Also the same offset of roughly 3 times less is present when measuring inductance. Do you have any clues where to look at next?

author: Sensorcat
author_rank: Regular Contributor
timestamp: May 24, 2024, 09:29:26 pm
content: Usually, I try to pick all the low-hanging fruit first. That means, testing everything that can be tested without a deep-dive into the schematic and that is known to be a frequent source of problems.In your instrument, that would be:The switches. Do they have all good contact resistance and good insulation?More general: Do you have low resistance on everything that is supposed to be connected in the schematic?Does the respective power supply voltage appear on all stages where it is needed?Also, a thorough visual inspection is useful: Suspicious dirt anywhere? Corrosion?An anecdote from my latest repair effort: The outdoor thermometer from my weather station failed, unless it gets completely fresh batteries. It worked and measured, until it had to transmit the data. Turned out that corrosion creeped into the cable from the battery compartment, invisible because of the insulation layer. Cable had several ohms. Almost no voltage drop during measurement, too much with the transmitter turned on. Lesson to learn:Very often, the problem is in the connections, not the components. Like the one you just found in your meter.

author: bborisov567
author_rank: Regular Contributor
timestamp: May 25, 2024, 04:26:12 pm
content: I was hoping to see something obvious but the unit is looking really good so i had no choice but to dig into the schematic. I figured out that if the problem is related to both inductance and capacitance measurement it has to be something related to both and that is of course the refference voltageVo. In Resistance measirementVois generated from a simple resistor divider(R106,R107) but in the other modes it comes from the op amp M105. They basically rectify the AC sinewave coming from the generator (op amp M101). I am not sure what is the idea behind that, maybe for better stability. But it turned out that the refference voltage was around -9V, instead of -3V so after quick check of D108,D107 and C119 i had to replace the op amp M105. The unit started showing correct values right away! But another problem came out - the op amp M105 starter heating up a lot - maybe that is the reson why the original one was bad. I looked at the schematic and figured out that D108, the way it is connected to ground, is almost always conducting since i have -3V at M105 output. Also there is a note on the schematic saying"Dioda zostala usunieta w modelu z lat '80"meaning the diode was removed in the late 80' models. My unit is made in 1989 but still had the diode placed. Now my question is what is the exact purpouse of D108? Should i leaved it removed or maybe put a resistor in series to limit the current? Or maybe there is a mistake in the schematic and the diode has to be connected the other way?

author: zepto
author_rank: Regular Contributor
timestamp: May 26, 2024, 02:14:07 am
content: Quote from: bborisov567 on May 17, 2024, 11:17:21 am1. If the device was not functional to begin with I am not sure why you are performing a calibration/adjustment procedure. You might make things worse and more difficult to diagnose with incorrect adjustments.2. 4k7 is not between 5k and 10k. When performing calibration/adjustment procedures you should pay close attention to numbers.