Phone noise analysis
The phone requires clear sound, moderate volume, and low distortion. If there is noise, it is quite annoying and affects normal calls in severe cases. How is the noise generated? From the breakdown phenomenon, the noise can be summarized as the following.
1? The sound of "click" (1) The fault is in the incoming line of the telephone. For example, the HA238 (111) P / TSD telephone often has an incoming line that is severely eroded, which is cut off. After replacing the new line, the noise can be eliminated.
(2) There is rust between the plug and the socket of the spiral coil of the handle. It should be eliminated and the elasticity of the steel pin in the socket should be adjusted to achieve close contact between the plug and the socket. Due to the frequent twists and turns at the plug of the coil, the internal strand of the coil is broken. Cut out the old plug and redo the new one. Finally, with the phone off-hook, hold the handle in one hand to listen to it, and shake the winding wire continuously in one hand to hear whether there is any "click" noise.
(3) The upper bias resistor of the starter tube or pulse control tube is a large-value carbon film resistor, and internal fracture often occurs, and the resulting "click" noise is quite large, affecting normal use. Take a resistor slightly smaller than its resistance in parallel with it to determine the point of failure.
(4) The printed circuit board is corroded, and the corrosion point appears on the main path of the current, and it is in a non-breaking state. The corrosion of printed circuit board is caused by the corrosion agent not completely removed during the production process. These suspicious points can be observed with the naked eye, scrape the surface corrosion material with a knife, find the right and wrong places, and weld them well.
2? Exchange sound (1) The dummy welding and de-soldering of the power supply decoupling capacitor. Due to the large volume of these electrolytic capacitors, the mobile phone is moved or even dropped during use. The capacitor bears a large impact force, and its root lead is easy to be welded off. During the inspection, the capacitor can be gently toggled. When a certain capacitor is toggled, the audible sound changes significantly, and then carefully check the capacitor for loose wires or virtual soldering or de-soldering. Capacitor failure or capacity reduction is relatively rare. To find such a fault point, use a capacitor with the same capacity as the original one in parallel with the original capacitor during the inspection. If the AC sound is significantly reduced, it means that the original capacitor has failure or capacity Failure to become smaller or poor welding.
(2) The phenomenon of hum caused by poor insulation between the drain of the handle or the hands-free electret microphone and the case, due to the moisture in the tone of the person when speaking, and to prevent voice feedback during production, Seal with paraffin or hot-melt glue to make moisture accumulate easily. Remove the sealing material around the drain to improve the degree of insulation. When installing the handle, be careful not to press the external lead of the drain output against the electret housing.
(3) The poor insulation of the printed circuit board will cause a slight hum. Although the fault is not serious, it is very difficult to find the fault point. The treatment method is to clean the printed circuit board with anhydrous alcohol, dry it with hot air, and sometimes repeat it several times. A 10 to 50μF capacitor can also be used to short-circuit the step-by-step input of the call loop, that is, to connect between the base and emitter of each triode, narrowing the search range.
3? "Rushing" running sound
(1) Try to replace the high quality noise caused by the deterioration of the electret microphone.
(2) The penetrating current of the transmitting and receiving amplifier circuit is too large, especially in the pre-amplifier circuit. Fault judgment can be done in two steps. The first step is to make the output of the sending loop short-circuited, and the noise disappears. The fault is in the sending amplifier circuit, otherwise it is in the receiving loop. The second step is to check whether there is any obvious change in the DC working point of each tube or to check each tube by an alternative method.
(3) Leakage of ceramic capacitors, especially ceramic capacitors in parallel with power decoupling electrolytic capacitors, are common, and their testing cannot be ignored.
(4) The reverse leakage of the zener diode increases and the quality deteriorates, which can also cause "rustle" noise.
4? Howling (1) Now the more popular is the circuit design of the telephone in which the handle is connected in series with the hands-free power supply. This type of telephone has poor contact when the group of contacts in the hook switch that control the hands-free working power supply. When using the handle, the hands-free still maintains the power supply, so that while the handle is talking, the speaker can hear sound, even call. Secondly, in hands-free operation, the microphone in the handle is not short-circuited by the AC, causing a sound feedback path and howling. Remove oxides from the group of contacts of the hook switch and adjust the contact pressure to ensure that the contact is absolutely good.
(2) The handle or hands-free microphone is displaced by vibration during use, or the paraffin wax or hot melt adhesive to prevent sound feedback is not properly sealed, for example, blocking the small hole into which the speech sound is introduced will cause howling Called malfunction.
(3) The variable value, failure, and desoldering of components in the side-tone suppression circuit are also one of the causes of howling.
5? Dual audio sound After the user picks up the phone, what they hear is not the dial tone or the other party's speech, but the continuous dual audio sound. Strictly speaking, this phenomenon is not among the noises. The dial circuit is out of order, but users often mistake it for noise.
(1) The dial pad has poor insulation between the plug-in points or the solder joints of the cable ends. Removal of dirt and debris at these critical points includes hot-melt adhesive, and the failure will naturally disappear.
(2) The dial-up integrated circuit is invalid. At this time, it can only be solved by replacing the dial-up integrated circuit or replacement of the same model.
The noise is roughly divided into the above five types, which is only based on the author's own repair practice and is not necessarily appropriate. However, common failure points are provided for peers to take shortcuts.
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