Author Topic: Help with Calibration and Analysis of 5050E Multimeter  (Read 141 times)

0 Members and 1 Guest are viewing this topic.

Offline miette_KruemelchenTopic starter

  • Contributor
  • Posts: 10
  • Country: de
Help with Calibration and Analysis of 5050E Multimeter
« on: January 18, 2025, 07:44:28 pm »
Hello everyone,

I have an older analog multimeter, the 5050E, 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 JFET 2N3958 is used. However, there are also provisions on the PCB for two separate K30A FETs, but I have the 2N3958 variant.
- Power Supply: The circuit is powered by a 9V battery, with the voltage applied to the drain of Q3-2 and Q3-1 (8.99 V).
- R34, R33: Two 1kΩ potentiometers 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 = 1kΩ and C10, C8 = 0.01µF.
- The circuit uses Q3-2 for resistance measurement and Q3-1 for voltage and current measurements.

Problem:

- In resistance measurement mode, the display works correctly when S1 is set to +: the needle moves cleanly from left () to right (0 Ω).
- However, when S1 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:
    - Across R36: 312 mV (at full-scale deflection)
    - Across R32: 355 mV

I suspect there might be a symmetry issue in the circuit, possibly with the gate control of Q3-2 or an incorrect setting of R33 and R34.

Questions:

- Does anyone have a calibration manual for the 5050E or a similar device?
- Could the issue be related to the component values (R33, R34, R32, R36), or might the 2N3958 be 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 this circuit or areas I should further investigate.

Thank you in advance!

Sven
« Last Edit: January 18, 2025, 07:58:51 pm by miette_Kruemelchen »
 

Offline Kleinstein

  • Super Contributor
  • ***
  • Posts: 15149
  • Country: de
Re: Help with Calibration and Analysis of 5050E Multimeter
« Reply #1 on: January 18, 2025, 08:47:35 pm »
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.
 
The following users thanked this post: miette_Kruemelchen

Offline miette_KruemelchenTopic starter

  • Contributor
  • Posts: 10
  • Country: de
Re: Help with Calibration and Analysis of 5050E Multimeter
« Reply #2 on: Yesterday at 08:59:18 pm »
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 V
  • Gate: 0.05 V

Interestingly, 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
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf