Observation can be delayed for a few seconds after turning on the machine to hear the relay sucks and chorus. Matching speaker static noise you can hear a slight.bmTservice manual download site
bmTservice manual download site
Input music signal. In a severely distorted when turned up full volume output.bmTservice manual download site
bmTservice manual download site
Initially suspected the power amplifier circuit of "failure". Power to examine the intermediate voltage 0V, power tube bias 0.5V. This shows that the main amplifier circuit is normal, "failure" should be at the fore. Oscilloscope measurement input main amplifier signal waveform, it really was the occasional sharp pulse signal. Can be sure the lead circuit "failure".bmTservice manual download site
bmTservice manual download site
The machine front circuit shown in the picture. VCD audio signal input signal selection circuit and after volume potentiometer to RIN and LIN shown. Balancing potentiometers VR respectively left and right channel signal into the ICl, 4558 op amp IC2 blocks. 4558 (5), (6), (7) feet and the VRl and peripheral resistance and capacitance, VR2 pitch tone network. To compensate for this amplifier for signal amplification. Because the output signal distortion, two operational amplifiers should be checked. Regular testing is to check the positive and negative power supply, (4) foot -15V, (8) script +15V, then again (1), (2), (3), (5), (6), (7) pin voltage. Measured two operational amplifiers (7) pin voltage is close to Dianyuan supply voltagebmTservice manual download site
bmTservice manual download site
Judgments are both amplifiers have been damaged and replaced it with two new 4558. Transmission measurement, two operational amplifiers (7) pin voltage is still wrong. What causes (7) foot prints exception? Should be (5), (6) pin voltage is wrong, but measurement (5), (6) pin voltage is OV. Finally check the op amp peripheral components. This found two channel the pitch potentiometer (100k) open circuit damage, lose DC negative feedback amplifier, gain too strong causing these "failed". Potentiometers are generally the Middle sliding arm connection is bad, I didn't expect all the way. Resulting in this special "failure".

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