Sunday, January 16, 2011

【 Weak current College 】 common circuit repair basics (1) 】

First, the capacitor paper
1, capacitance in the circuit of general use "C" with a number (such as C25 said number 25 of capacitors). Capacitor consists of two pieces of metal film immediately, separated by the insulating material is separated from the composition of elements. Capacitance characteristics mainly at DC-AC.
The size of the capacitor capacity is expressed to the size of the storage of electrical energy, capacitance on AC signal impediment is known as the capacitance, and AC signal frequency and power capacity.
Capacitance XC = 1/2 to π f c (f represents the AC signal frequency, the c = capacity)
Telephone the capacitance of the commonly used in type electrolytic capacitors, ceramic capacitors, chip capacitors, monolithic capacitors, tantalum capacitor and polyester capacitors, etc.
2. identification: identification of capacitance and resistance of recognition method is basically the same, divided into direct standard method, color standard method and a few standard method 3. The basic unit of capacitance for farad (F) that the other units also: MW (mF), micro (uF), NA (nF), skin (pF). Of which: 1 Farah = 103-method = 106-method = 109 na method = 1012 skin method
Capacity of the capacitor capacity values marked directly on the capacitor, 10uF/16V
Capacity of small capacitor capacitance values in its capacity on the letter or number 6
Letter notation: 1m = 1000uF1P2 = 1.2PF1n = 1000PF
Digital presentation: generally uses a three-digit number represents the size, the first two digits represent a valid number, first 3 digits is multiplying.
Such as: 102 represents 10 × 102PF = 1000PF224 represents 22 × 104PF = 0.22uF
3. capacitance tolerance table
Allowable error of ± 1% ± 2% ± 5% ± 10% ± 15% ± 20%
Example: a porcelain capacitor-104J represents a capacity 0.1uF, a tolerance of ± 5%.
4. fault characteristics
In the actual repair, capacitor failure major findings are:
(1) pin corrosion caused by broken open failure.
(2) removal of welding and soldered open failure.
(3) leakage resulting capacity small or open circuit failure.
(4) leakage, serious leakage and malfunction.

Second, the diode
Crystal diode in circuit common "D" with a number, such as: D5 represents number 5 of the diode.
1, role: main character is a diode in one-way conductivity, which is in the forward voltage, on-resistance is very small; in reverse voltage of on-resistance of great or infinity. Because of these characteristics, a diode with a cordless phone in it is used in commutation, isolation, voltage and polarity protection, coding control, FM modulation and static noise and other circuits.
Telephones in use of Crystal diode by role can be divided into: rectifier diode (such as 1N4004), isolation diode (such as 1N4148), Schottky diodes (such as BAT85), light emitting diodes, Zener diodes, etc.
2. identification: identification of the diode is very simple, small power diodes n pole (anode), most of the diode appearance using a color circle marked out, some diodes with dedicated symbols to represent diode P pole (cathode) or n pole (anode), also have a symbolic sign for the "P" and "N" to determine the diode polarity. Light-emitting diode polarity can be identified from PIN length, long foot is positive, negative short legs.
3. test considerations: use digital multimeters to measure diode, diode red table pen, black table of cathode diode anode of the pen, the measured resistance is the diode is Wizard-resistance, this pointer table pen connected multimeter method is exactly the opposite.
Zener diode
Zener diode in circuit common "ZD" plus digital representation, such as: ZD5 represents number 5 voltage tube.
1 Zener diode voltage principle: Zener diode is characterized by the breakdown of both ends of the voltage has remained virtually unchanged. Then, when the regulator hose access circuit, if due to the power supply voltage fluctuations, orother causes circuit voltage changes in various points, load both sides voltage will remain unchanged.
2, fault characteristics: Zener diode breakdown mainly in open circuit, short circuit and voltage values are not stable. In these three failures, previous failures shows the power supply voltage to rise; after 2 failure performance for the power supply voltage to low to zero volts or output stability.
The model of the common Zener diode and voltage values are as follows:
Model 1N47281N47291N47301N47321N47331N47341N47351N47441N47501N47511N4761
Voltage value 3.3V3.6V3.9V4.7V5.1V5.6V6.2V15V27V30V75V varactor diodes
Varactor diodes are based on normal diode internal "pn" junction capacitance can vary plus reverse voltage changes this principle specially designed a special diode.
Varactor diodes in the cordless telephone is mainly used in mobile or fixed-frequency modulation circuit, the realization of low-frequency signal modulated to high-frequency signals, and fired away. In working order, normal voltage variable capacitance diode modulation to the anode, the internal junction capacitance diode capacitance with the modulation voltage changes.
2 maintenance basics
Variable capacitance diode failure, mainly for leakage or performance variation:
(1) occurs leakage phenomenon, high-frequency modulation circuit will not work or a performance variation.
(2) the evolution of performance variation, high-frequency modulation circuits of precarity, the modulation of high-frequency signal is sent to the other side is received after distortion.
One of the above situation occurs, you should replace the same model of varactor diodes.

Third, inductance
Inductor in circuit common "L" with figures, such as: L6 said number 6 of the inductance. Inductance coil is insulated wire skeleton in insulation around a certain number of turns. DC to DC resistance through the coil, it is the resistance of the wire itself, pressure drop is very small; when the AC signal through the coil, coil ends will produce from EMF EMF, since the direction of the applied voltage in the opposite direction, obstacles to communicate through, inductance characteristics is pass DC resistance, the higher the frequency, the greater the impedance of the coil. Inductance in the circuit can be composed of the oscillation circuit capacitance.
Inductance General direct standard method and color stop method, the stop method is similar to the resistance. Such as: Brown, black, gold, gold said 1uH (error 5%) of the inductance. The basic unit of inductance: Heng (H) conversion units are: 1H = 103mH = 106uH.

4, transistor
Transistor in circuit common "Q" with a number, such as: Q17 represents the number of transistors to 17.
1, features: triode (transistor) is an internal contains 2 pn junction, and has enlarged capacity of special devices. It is divided into NPN-and PNP-two types, both types of transistor characteristics from work on mutually compensate, so-called OTL circuits on the tube from PNP-NPN-type matching using and.
Telephone sets are commonly used in PNP-transistor: A92, 9015, model; NPN-transistor: A42, 9013, 9014, 9018, 9012, etc.
2, transistor is used primarily for amplifying circuit in the magnification effect, in a common circuit there are three types of connection. For ease of comparison, the transistor three connection circuit has characteristics are shown in the following table for your reference.
Name of the emitter circuit total collector circuit (emitter follower) total base circuit
Input impedance (hundreds of thousands of Central-Europe) and large (tens of thousand euro) and small (a few euro ~ dozens of Europe)
Output impedance (thousands of euro ~ tens of thousands of euro), small (a few dozens of Central-Europe) and large (tens of thousand euro ~ hundreds of thousands of euro)
Voltage magnification size (less than 1 and close to 1)
Current amplification large (tens of) large (TENs)and small (less than 1 and close to 1)
Power magnification (approximately 30-40 DB) small (about 10 decibels) (about 15 ~ 20 DB)
Frequency characteristics of high-frequency difference good

Application of multistage amplifier intermediate-level, low frequency amplifier input level, output level and impedance matching using high-frequency or broadband circuit and constant current circuit
3, online work measurement
In the actual repair, transistor has been installed on the circuit board, down to every measurement is a hassle, and very easy to damage the circuit board, according to the actual repair, I summarize a circuit on the work of the State charged measurement transistor to determine fault lies, for your reference:

Failure parts of test points

E-b drain Ved 1vVed = V + >
E-b short-circuit Veb = 0vVcd = 0vVbd rise
Re open Ved = 0v
Rb2 open Vbd = Ved = V +
Rb2 short-circuit Ved about 0.7V
Rb1-added many, Vec < 0.5vVcd rise
E-c interelectrode open Veb = 0.7vVec = 0vVcd rise
B-c interelectrode open Veb = 0.7vVed = 0v
B-c extremely short Vbc = 0vVcd very low
Rc-circuit Vbc = 0vVcd elevated Vbd unchanged
Rb2 resistance increases many Ved about V + Vcd approximately 0V
Ved voltage instability in and around components have triode Xu Han

Failure parts of test points
Rb1-circuit Vbe = 0Vcd = V + Ved = 0
Rb1 short-circuit Vbe about 1vVed = V-Vbe
Rb2 short-circuit Vbd = 0vVbe = 0vVcd = V +
Re open Vbd elevated Vce = 0vVbe = 0v
Re short-circuit Vbd = 0.7vVbe = 0.7v
Rc-circuit Vce = 0vVbe = 0.7vVed approximately 0v
C-e short-circuit Vce = 0vVbe = 0.7vVed rise
B-e drain Vbe > 1vVed = 0vVcd = V +
B-e short-circuit Vce about V + Vbe = 0vVcd approximately 0v
C-b drain Vce = V + Vbe = 0.7vVed = 0v
C-b short-circuit Vcb = 0vVbe = 0.7vVcd = 0v

IC testing methods:
Now the electronic products often as a part of integrated circuit damage, caused or sections not functioning, affect the normal use of the device. So how do you detect IC is good or bad? typically a device with many integrated, get a defective IC equipment, first under the symptom, determines that the failure of General position, and then by measuring the fault may be parts of narrow, finally find fault. Where to find the failure must be made through the test, usually repair personnel to use measuring pin voltage method to judge, but this can only determine the approximate location of the fault, but some pin reaction insensitive, even little reactions. In case of voltage deviations, and peripheral component damage factors must also be manifold internal faults and fault strictly distinguish the perimeter, and therefore rely on an approach to integrated circuit is very difficult to detect, and must rely on a comprehensive means of detection.

Now multimeter testing, for example, a general description of the specific method. We know that the manifold use, always has a PIN and printed circuit board "to" line is welding fluxes,In the circuit known as grounding pin. Because the IC's internal use directly coupled, thus, manifold other PIN and grounding there between the foot of DC resistance, this kind of DC resistance is called the foot internal equivalent DC resistance, hereinafter referred to as R. When we get a new chipset, the via multimeter measuring pins of the internal equivalent DC resistance it is good or bad, if the pins of the internal equivalent resistance R and standard values correspond to the description of this manifold is good, on the other hand if the difference between the standard value is too large, the manifold internal damage.

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