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