How to repair the crosstalk of doorbell?

Building intercom doorbell system uses many discrete components and complicated circuits, but if there is a schematic diagram, the maintenance operation will be easy. As can be seen from the circuit diagram, the low voltage generated by the transformer is introduced from the socket V 1 2 through bridge rectification, capacitor filtering and+12V generated by the three-terminal voltage stabilizing block 78 12, and directly supplied to the relay K, and the other route is isolated by VD3, which generates VCC for other circuits. When someone presses S 1 outdoors, the signal at pin 1 of socket V6 passes through S 1, pins 2 and 3 of telephone switch SA and speaker LB, forming a current path, which is equivalent to grounding the lower end of R 17, so the base of VT5 is extremely low, VT5 is saturated, and the two pins of music ic have a trigger high level. After being amplified by IC 1(LM386), it is output by pin 5 of IC 1, and sent to speaker LB through pin 1 of V6 and pin 2 and pin 3 of telephone exchange SA to make sound. Due to the special structure of telephone exchange SA, audio signals can also be coupled to four pins of telephone exchange SA, and then coupled to seven pins of IC2(TDA2822M) through two pins of socket V3. After amplification, it is output by 1 pin, so that the doorbell ringer can hear the sound from LB 1.

Press any key in S 1~S 10. If the doorbell rings, it means that VT5, music ic and IC 1 basically work normally. Both parties can talk, indicating that IC2 and the indoor telephone basically work normally. When the 1 pin of socket V3 is normal, it is high potential. When the 1 pin is short-circuited to the ground, you should be able to hear the pull-in sound of relay K, indicating that the unlocking circuit is normal. When this point is closed, the electromagnetic lock is electrically actuated and the door of the building is opened.

Due to frequent vibration, the main circuit is prone to open welding failure.

When something goes wrong, check the power supply first. Because all the interfaces are clearly marked, the motherboard, keypad and telephone can be removed for maintenance, so it is generally easy to find the cause of the failure.