Help with Calibration and Analysis of 5050E Multimeter - Page 1
Forum section: electronics / repair

author: miette_Kruemelchen
author_rank: Contributor
timestamp: January 18, 2025, 07:44:28 pm
content: Hello everyone,I have an older analog multimeter, the5050E, and I would like to calibrate it properly again, but I am encountering a few problems.Description of the Circuit:-Display:A moving coil meter that requires 47.6 µA for full-scale deflection to the right.-Q3 Components:A dual JFET2N3958is used. However, there are also provisions on the PCB for two separateK30A FETs, but I have the2N3958variant.-Power Supply:The circuit is powered by a9V battery, with the voltage applied to the drain ofQ3-2andQ3-1(8.99 V).-R34, R33:Two1kΩpotentiometers are present in the circuit, presumably for calibration.-Switch S1:Allows switching of the polarity on the display (M+ and M-).-Other Components:R32, R36 = 1kΩandC10, C8 = 0.01µF.- The circuit usesQ3-2for resistance measurement andQ3-1for voltage and current measurements.Problem:- Inresistance measurement mode, the display works correctly whenS1 is set to +: the needle moves cleanly from left (∞) to right (0 Ω).- However, whenS1 is switched to -, the display tries to push the needle past the left stop, and I must immediately interrupt the operation.-Measurements show different voltage drops:- AcrossR36:312 mV(at full-scale deflection)- AcrossR32:355 mVI suspect there might be asymmetry issuein the circuit, possibly with the gate control ofQ3-2or an incorrect setting ofR33andR34.Questions:- Does anyone have acalibration manualfor the 5050E or a similar device?- Could the issue be related to thecomponent values(R33, R34, R32, R36), or might the2N3958be defective? (I've already replaced it once!)- Can anyone analyze the circuit more closely or provide tips for calibration?I appreciate any help, including general tips on the operation of thiscircuit or areas I should further investigate.Thank you in advance!Sven

author: Kleinstein
author_rank: Super Contributor
timestamp: January 18, 2025, 08:47:35 pm
content: It should be normal that the meter is not measung negative resistance. That combination likely has not use.The slight asymmetry between the voltages at R32 and R36 may not be an issue. The starting point the an offset in the dual JFET, but there is the trimmer R35 to compensate for this. The relevant point is that one can get a correct zero reading (same as the mechanical zero that one gets with the meter off). Very likely the zero trim would be one of the first steps in an adjustment. Symmetry in R32 and R36 at full scale is not needed - there is part of R35 in series. For the symmetry in the positive and negative readings there is R34 to adjust - possibly as one of the next steps.

author: miette_Kruemelchen
author_rank: Contributor
timestamp: January 19, 2025, 08:59:18 pm
content: Hello Kleinstein,Thank you very much for your response to my question about the analog multimeter! I really appreciate you taking the time to help me. 😊Today, I revisited the multimeter and measured the voltages on the dual MOSFET Q3. There were no external connections to the device during these measurements. Here are the results:Source terminal of Q3-2 and Q3-1:0.626 VGate:0.05 VInterestingly,the zero point on the left side ( ∞ ) of the instrument aligns correctly during these measurements.However, I noticed the following:When I perform a measurement—whether in resistance mode or DC voltage mode—and then stop the measurement, the needle sometimes does not fully return to the leftmost position (i.e., "∞"). Yet, if I turn the device off and back on, the zero point is restored correctly.Once again, thank you so much for your support!Best regards,Sven