PL-600 is a landmark classic model of Desheng . It has been in production for more than ten years since its release. I have analyzed its circuit characteristics before. Interested friends can search and read it. The PL-600 has excellent performance, the sound is thick but muffled, and the treble is not transparent, as if the announcer has a cold, especially when listening to AM . I got this machine produced in 2010 and carefully analyzed the sound problems. The machine
was produced in 2010, about three years after the product was released. It is a relatively mature and stable stage of the product. There is no shrinkage. The chips are still imported. The speakers are also fixed with glue and screws. Later machines The screws are omitted for fixing, which causes the risk of machine vibration between the speaker and the chassis, because the glue cannot provide downward pressure. There is nothing wrong with this machine, no capacitor bulge or leakage was found, and there is no frequency deviation in FM or AM. The machine is in good condition, but the sound is still muffled. This is undoubtedly a design problem and component quality problem.
does not need to make any adjustments and is in good condition. There are two main reasons for the dull sound of
: the audio channel has too much treble attenuation; the second is that the intermediate frequency passband is too narrow, especially the second intermediate frequency. The passband will affect selectivity and frequency deviation. I have not touched it for the time being, focusing on analyzing and processing the audio channel problem. The starting point of the audio channel is pins 12 and 13 of the 2057 chip, which are the left and right channel outputs, and the end point is the speaker and headphones. It can be seen that the 2057 output de-emphasis RC is 4.7K and 0.022uF. The capacitor used here by Tecsun is slightly larger, which causes the treble to be attenuated too much. It can be changed to 0.01uF. The capacitor in the power amplifier part is very important. Although there is no leakage or bulging, it is completely necessary to replace the original low-end capacitor with a high-quality capacitor. As for what type of capacitor to use, it doesn't really matter. Aluminum electrolytic or tantalum will do. As long as As long as they are genuine audio capacitors, try to use tantalum for the two 4.7 negative feedback capacitors. The Sony original CXA1622M used in the power amplifier chip is not necessary to replace even if it is Huajing's. Its impact is much less than the capacitor. The power amplifier power supply filter capacitor Desheng uses is 1000uF. The large-capacity capacitor is inductive to high frequencies. A 104 ceramic chip can be connected in parallel to filter out high-frequency interference or decoupling. When the amplifier outputs BTL mode to drive the speaker, the speaker is directly connected to the chip. It does not have to pass through a capacitor like other models such as the 450 660, which is better. However, Tecsun did not handle the headphones OTL well, because when outputting OTL mode, the left and right channels do not use independent output DC blocking capacitors, but share a 220uF capacitor at the ground end for DC blocking. This is a compromise and cheap approach. Therefore, the key to the modification is to add two 47uF output capacitors. In order to avoid drilling and flying wires as much as possible, I adopted the method as shown in the picture. You can refer to it for reference. The old speaker has been fully boiled in and the paper cone has good elasticity, so I did not replace it. After
's transformation, the stereo sound of the headphones is no longer blurry, has clear layers, and has a clear sound field. In speaker mode, the treble part of the sound is significantly improved, making the sound more transparent and high-pitched.
capacitor positive electrode is connected to the chip output pin!
