Unaohm R127D analog multimeter repair - Voltagemeasurement too high - Page 1
Forum section: electronics / testgear

author: kblue
author_rank: Regular Contributor
timestamp: June 13, 2025, 01:31:36 pm
content: Hello!Mayhaps someone here in this board has any idea what could be wrong with this unit. At first, there was some internal short problem which is already solved (lose mechanical parts). During this time IC02 (CA3086) and T6 (BC108) got quite hot but it seems they survived (i guess).Ohm and mA measurement is fine and quite "accurate" (for an analog meter) but volts is not (neither DC nor AC). Volts is always too high in all ranges. 200 mV shows like 240ish mV ... 300 mV is way off scale. For fun i checked the µA for the meter movement and sure enough e.g. 3 mA DC input and ohms (open leads) fullscale is 250,0 µA ... 300 mV input is like 264 µA in the 300 mV measurement.R16 in the divider was a bit off (schematic wants 7M ... in reality it was 7,15M). I changed it to 7MOhm - no noticeable difference in the results, still too high. I also checked the resistance between GND and Input in all ranges:- in the 300 mV range its about 1,4630 MOhm- in the 1 V range its about 9,41 MOhm- 3 V range and all the other ranges show 10,13 MOhmThe printing on the scale says V DC should be 11 MOhm and V AC 10 MOhm input resistance. The schematics are a bit ... special. Some numbering is wrong (like 2x R29 and so on) and i wasnt able to locate the 1 MOhm resistor below R14 (mA range). The only 1M resistors that physically exists in the unit is R24. With some web research it seems this unit exists in quite some variants with different internals ... whatever parts they got at the time.Best wishes!/edit: It might be the missing probe ... found the following topic:https://www.vintage-radio.net/forum/showthread.php?p=689519

author: marco58
author_rank: Newbie
timestamp: June 15, 2025, 08:44:28 am
content: Hi kblue,I too think the missing probe may be the solution.In an italian forum I've found pictures of "PT100"http://www.arifidenza.it/Forum/pop_printer_friendly.asp?ARCHIVE=true&TOPIC_ID=321018and a full italian manual, in another forum (https://elettronialtramonto.forumfree.it/?t=78344724&st=75), for model "R 127 F"https://drive.google.com/file/d/13GVYBeBvA0sjDwqpGfpUARShtl3TQp73/view?usp=share_linkManual states that the probe contains a 1 Mohm series resistor (that is inserted only for DC voltage) and:page 10:Measuring DC voltages up to 1000 VMove slide switch on probe P 100 in position "="page 13:Measuring AC voltagesMove slide switch on probe P 100 in position "~"page 14-15:Measuring ResistanceMove slide switch on probe P 100 in position "~"You have errors both on DC and AC voltages. Are they the same percentage?1 Mohm resistor in probe can correct only DC measurements...Nonetheless I'd give it a try and maybe a general calibration may solveciaomarco

author: marco58
author_rank: Newbie
timestamp: June 15, 2025, 09:17:31 am
content: If hope these notes for R 127 F are applicable to your model, so you can tryextract from manual page 22:Sensitivity adjustment (R 37)Check a DC voltage with another calibrated instrument. If is not correct adjust R37 trimmer.Zeroing adjustment (R 33)Set instruments to DC voltage and short input terminals. Adjust R 33 trimmer to zero the pointer.Polarity indication lamps must blink, indicating an unknown polarity.