Repair of Common Faults in Assembly of a Pointer Multimeter
Overhaul of Common Faults in Assembly
The electronic components assembled by the assembled multimeter have been screened and meet the quality requirements, so it does not involve the maintenance of faults caused by the quality problems of electronic components. The schematic circuit diagram of the pointer multimeter is shown in Figure 1:
All tests are unresponsive
Rotate the function conversion knob to each measurement position for testing, and the meter pointer does not deflect (hereinafter, "the meter pointer does not deflect" is referred to as "not deflected" and "the meter pointer deflects" is referred to as "deflection"), and the buzzer The instrument does not sound (hereinafter, "the buzzer does not sound" is referred to as "no sound" and "the buzzer sounds" is referred to as "sound"), the light-emitting diode does not light (hereinafter, "the light-emitting diode does not light" is referred to as "no sound" "bright" refers to "light-emitting diode" as "bright"). If these conditions exist, the most likely reason is that the tube is not installed or blown out, or it may be caused by the horizontal installation of the brush. If the contact between the brush and the brush track and the brush contact is poor, pressing the function conversion knob will respond. If there is an error in the horizontal installation of the brushes, you will feel that the rotation is not smooth and stiff when you turn the function conversion knob, so the brushes must be installed according to the assembly process. If there is a problem with the fuse, it is easy to find and can be replaced.
Reverse installation of the gear indicating knob
To debug the assembled multimeter, it is usually to detect the resistance gear first, then short-circuit the test leads to observe the ranges of the buzzer and the resistance gear, and then test the DCV, DCmA and ACV function gears in turn. Short-circuit the test leads to detect that the buzzer should be "loud, bright, and deflected", and the other ranges of the resistance file should also be "deflected", but there is no response, and when the function conversion knob is turned to DCV1K by short-circuiting the test leads, " Loud, bright, deflected" (this phenomenon should not happen in this file). From the printed circuit diagram, it can be found that the indicator knob (disc) of the function conversion knob is installed backwards. On the printed circuit board, the DCV1K gear is exactly different from the buzzer gear in the electric block, so the gear indicator knob in this fault indicates the DCV1K gear, and the brush is actually connected to the buzzer gear of the resistor. The way to eliminate the fault is to unplug the gear indicator knob on the function conversion knob and turn it over, so that the arrow position of the gear indicator knob is in line with the position of the brush groove on the brush knob below, and then align the gear indicator knob with the Just tighten the brush knob.
The function conversion knob is composed of upper and lower knobs (discs). The upper piece is the gear indicating knob with an indicator arrow, and the lower piece is the brush knob with brush slots and side holes for placing springs and steel balls. The brushes are Installed in the brush well. The upper and lower knobs are connected by a rectangular sleeve, but one side of the shaft is a small arc to facilitate the alignment of the upper and lower knobs. Since the arc is not obvious, it is easy to reverse the shaft.
Once it is reversed, it is easy to burn out the meter head if it is tested blindly.
Troubleshooting of Common Faults of Resistance Gear
There are many faults in the resistance gear, and the causes of the faults are mainly wrong installation, false welding, false welding, and poor contact. The analysis of the resistance file failure does not involve the assembly error of the resistance element and the short circuit phenomenon. Because there are too many failure phenomena of the resistance gear, the components that are prone to assembly errors in the resistance measurement circuit are now classified to find out the cause of the failure and propose a maintenance plan.
Buzzer file failure
The debugging of this file is the most concerned by the students, because the successful realization of the function of this file will have the phenomenon of "loud, bright, and deflected", which is a "visible and audible" affirmation of their work. The normal situation is to install the battery, turn the function conversion knob to the buzzer position, the buzzer should sound, the light-emitting diode (LED) will light up, and the hands will deflect. Symptoms are:
(1) Loud but not bright; (2) Bright but not loud; (3) No sound but not bright; (4) The light emitting diode is on when it is rotated to the ×10Ω position (in this position the light emitting diode should not be on). The above faults are the fault combination of reverse polarity of LED and buzzer. The "normal installation" mentioned below means that the components are correctly assembled according to the polarity of the circuit diagram. 2) The case of the buzzer is broken or reversed. In the case of (3), besides the buzzer and the LED are open or reversed, there is also a phenomenon of poor contact of the brush. In the case of (4), if If the LED is on but the buzzer has a weak sound, it means that the LED is installed reversely and the buzzer is installed properly. If the LED is bright but the buzzer does not sound, it means that the LED is installed reversely and the buzzer is installed incorrectly.
