Thank you very much for the diagrams provided by BI4PMQ.
It has already been repaired.
Let's organize the repair process:
1. Replace a 0603 magnetic bead resistor with an impedance of 120Ω @ 100MHz at one of the audio ports.
2. Tested the original amplifier (model: LM4950TSX), and the problem persists.
3. Check the circuit again. Replace one of the 4-channel amplifiers in the LM2902PWR. This will fix it.
The likely cause, based on a post-incident analysis, is that there was an audio line shorting out and drawing 12V of current. This caused one resistor to burn out, which in turn also burned out the amplifier.
The 300D uses a dual-channel amplifier, and produces audio on both channels. It also separates digital and analog signals. The circuit board appears to be quite complex.
Lesson: If you are using external speakers (specifications: 4Ω 3W, the amplifier chip only supports this parameter), do not connect powered speakers. Even if you connect powered speakers, they must be supplied with separate power. Do not use the radio's 13.8V power supply.
Also, there are many external speakers available online, most of which use an 8Ω speaker. This needs to be taken into consideration.
Regarding whether the 100D is like that – I haven't used it, so I'm not sure. However, the 300D has two ports and uses 2-core 3.5mm connectors, not 3-core ones.
All accessories were purchased from Licheng Mall. The cost of the accessories was 8.09 yuan twice, and the shipping fee was 21 yuan.😂...I'm also exhausted. But it was worth it. Very efficient.
Thank you again, BI4PMQ teacher.
73!
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