PT2399 Karaoke Circuit

KALAOKE circuit principle

1) Microphone The input is introduced by two microphone seven-pin sockets. When the microphone is not plugged in, the microphone output signal is shorted to GND. Only when the microphone is plugged in, the microphone output signal will be disconnected from GND and effectively output to the post-amplifier. The circuit diagram is not drawn. The 7-pin microphone has a set of switches. When the microphone is not plugged in, the output is short-circuited to the ground, which has a noise-canceling effect. When the microphone is plugged in, it will automatically short-circuit and the audio path will be unblocked. Find a 7-pin socket to study this.

2) After the microphone input signal passes through the positive-phase amplifier tube of the IC1A dual op amp, the 50K single-joint potentiometer (2W1) realizes the attenuation adjustment of the microphone MIC VOL. After the volume adjustment, the microphone signal passes through the first-stage common emitter amplifier circuit and is also amplified by the other side of the IC1B dual op amp. The microphone signal finally enters the RSM2399 for reverberation (ECHO) processing. The reverberation delay time can be changed by adjusting the resistance of the 6-pin ground resistor at the bottom of 2399. Generally, the standard is to hear 7 consecutive echoes that gradually fade away.

3) RSM2399 can process the delay effect of audio signals. For the connection of its peripheral components, please refer to the DATASHEET of PT2399.

KALOKE circuit maintenance

1) MIC circuit failure: The microphone is silent and has no output.

A) Check the power supply of the MIC circuit
The inspection steps are as follows: First, confirm that the power supply of RSM2399 is normal. Under normal power supply, the voltage of PIN1 of RSM2399 to ground is 4.7V~5V, and it is 5V when there is no signal input. Also, confirm that the power supply of the dual op amp of microphone IC1 is normal.

B) Check the MIC signal path
Input the MIC signal (insert the microphone to test the sound and test at the same time), and use an oscilloscope to observe whether C15 has coupled signal output.
If C15 has no signal output, it means that the fault is in the MIC dual op amp IC1 amplification circuit. It is necessary to focus on the IC1 amplification to check whether the power supply is abnormal or the coupling electrolytic circuit is open, the volume potentiometer is poorly soldered, the signal is short-circuited, etc., and the fault can be eliminated.
If C15 has a signal output, it means that the RSM2399 input signal is OK, but RSM2399 has no output. First, check whether the 2C11 coupled signal is shorted to the ground due to a bad microphone socket. Otherwise, check the RSM2399 peripheral circuit and RSM2399 quality.

Note: If the output signal is still short-circuited to the ground after the microphone is plugged in, the microphone socket is defective and needs to be replaced. In normal state, if neither microphone plug is plugged in, the signal is short-circuited to the ground. If either one is plugged in, the signal will no longer be short-circuited to the ground.

2) MIC circuit failure: There is noise when adjusting MIC VOL or ECHO.
The focus of the repair steps should be on the quality of the MIC VOL/ECHO potentiometer. Check whether there is a problem with the contact of the reed inside the potentiometer. You can use a special cleaning fluid or industrial alcohol to clean the inside of the potentiometer. If conditions permit, directly replace the defective potentiometer.

3) MIC circuit failure: MIC sound distortion
The focus of the repair steps should be on the two op amps (IC2 op amp mixes the MIC signal and other audio signals as an adder, so that the human voice and music are superimposed KALAOKE) and the quality of RSM2399 itself or the quality of peripheral components. Focus on checking whether their power supply is normal, replace components if necessary, and use the "replacement method" to narrow down the problem area until the problem is found and eliminated.