Improved circuit of Sanyo DC-SL mini stereo

Improved circuit of Sanyo DC-SL mini stereo

I have a Sanyo DC-Sl mini stereo in my bedroom. I have been using it for years. Although it does not have the popular VCD/DVD playback and HP3 decoding functions, I am still quite satisfied with it. The appearance of this stereo is quite avant-garde, with a large 10cm×6cm multi-functional LCD screen and a sliding CD tray panel. Even from today's perspective, it still looks generous and unique. Its functions and details are very convenient and considerate. The karaoke reverberation and song "voice elimination" functions allow users to sing without a special accompaniment tape. The "WAKE UP" alarm function can slowly increase the volume from zero when the machine is turned on, without causing the discomfort of waking up suddenly in a dream. In addition, the products produced in previous years pay more attention to quality and materials. They are small in size and have a large output power (using a thick film amplifier with dual 24V power supply). Their bass effect is obviously better than some similar small stereos currently sold on the market.

I like to disassemble electrical appliances and make some minor modifications in my spare time. This stereo is naturally not spared. After several disassembly and modification, the sound quality and performance of the machine have been improved to a certain extent. The modification process is introduced as follows for your reference when modifying a small combination stereo.

1. Modification of the speaker: The original SX-MS1 speaker of this stereo is a small two-unit plastic box structure with cross wooden strips for reinforcement. The speaker is a 6Ω25W 1-inch tweeter and a 6Ω25W 5-inch woofer, and a common non-polar capacitor is used for simple frequency division. Due to the limited volume of the box, it is inconvenient to make major changes to the frequency division structure. Only the frequency division capacitor is replaced with a 3.3μF RIFA capacitor, and then a proper amount of sound-absorbing cotton is filled in the box. After the modification, the high frequency of the speaker is more delicate and the low frequency is more stable and powerful.

2. Replace the capacitor of the audio circuit : The method of replacing the capacitor is simple and easy, suitable for modifying mini speakers and walkmans. The main method is to increase the capacity of filter capacitors and coupling capacitors at all levels to make the low-frequency response more solid, and adjust the capacity of individual high-frequency bypass capacitors to reduce the loss of high-frequency details. The power amplifier power filter capacitors C4902 and C4903 are increased from 2200μF/35V to 6800μF/35V. Because the printed board cannot accommodate larger capacitors, it is necessary to use thick wires to extend the capacitor pins and fix them in nearby positions. The filter capacitors C4918, C4919, and C1502 are increased from 47μF to 220μF, and the coupling capacitors C2705, C2805, C4701, C4801, C4517, and C4617 are increased from 1μF to 3.3μF. For the above, try to use audio-specific electrolytic capacitors from famous manufacturers such as ELNA. Reduce the high-frequency bypass capacitors C4700 and C4800 at the input of the power amplifier IC STK4132MK2 from 820pF to 100pF, and reduce the stereo decoding de-emphasis capacitors C2319 and C2320 at the ○14 and ○15 pins of the radio IC LA1830 from 0.015μF to 8200pF. If conditions permit, you can also replace the several SMD op amps 4558 in the machine with op amps with better performance. 3. Radio circuit: To improve the FH band reception effect, you can add a high-level amplifier, as shown in Figure 1. The coil L is made of 0.5mm enameled wire wound on the ballpoint pen core for 5 turns. The transistor 2SC3355 can be replaced by the common 9018, but the latter has slightly worse performance indicators. C1~C3 use high-frequency ceramic capacitors, and other components are selected with small specifications and directly welded on the copper foil surface of the printed board next to IC211 (TA8176SN). Note that the speed must be fast when welding high-frequency transistors. After adding high-frequency amplifier, listening to FM stereo broadcasts is no longer mixed with annoying noises, and the music sound is purer. The second is to increase the backup memory capacitor capacity of the audio microprocessor, and replace C4309 (1000μF) with an NEC5.5V 1.0F capacitor in series with a 51Ωn resistor, so that there is no need to worry about the loss of stored radio frequencies caused by occasional power outages. Since the newly replaced memory capacitor is relatively large, it can be pasted near the printed board with hot melt adhesive, and then connected to the circuit with thin wires. 4. Add standby auxiliary power supply This audio system uses a relatively high-power power transformer. As long as the power plug is plugged in, the transformer is always powered on regardless of whether the machine is turned on or not. The internal temperature of the machine rises significantly after long-term use, and the standby power consumption is also relatively large. Although the machine can be unplugged when not in use, this will lose the functions of timed alarm and remote control startup, and frequent plugging and unplugging is also a bit troublesome. After studying the original machine circuit, I added a low-power standby power supply to it (see Figure 2). The circuit consists of transformer T2 and peripheral rectifier and control circuits. On the one hand, it supplies power to IC491 (AN7809F) to supply infrared receiving circuit and microprocessor circuit, and is also responsible for the automatic on-off control of the main power transformer T1. In standby mode, T1 is no longer connected to the mains. Only when the microprocessor issues a "PO RY" power-on command, the relay J1 is closed and T1 is powered on. Since "PO RY" is enabled at a low level, Q1 is added as an inverter. D2 acts as an isolation to prevent the standby power supply from supplying power to irrelevant circuits. In the figure, Q4902, C4909, IC491, and R4957 are the component numbers of the original audio circuit, and the rest are newly added circuit components. T2 uses a small high-quality transformer below 10W, and is fixed to the cover of the rear of the audio host with screws (without affecting the appearance of the front). Relay J1 uses a model with a coil voltage of 12V and a contact current of more than 5A. The rectification and control circuits are made on a small circuit board and installed inside the audio. 5. Modify the LCD screen backlight circuit The original host screen uses 6 miniature incandescent bulbs plus a filter cover to produce green backlight. The circuit generates high heat and consumes a lot of power. Now it is replaced with the same number of super bright white LEDs, which not only reduces the backlight power consumption, but also makes the black body with white display look more "cool". Because the LF port is connected in series and parallel, the printed circuit board routing needs to be slightly changed. For the specific circuit, please refer to the relevant part of the upper part of Figure 2. Compared with incandescent bulbs, LEDs can emit light with only a slight current, so I use the standby auxiliary power supply to provide it with a continuous current of a few milliamperes, so that even after turning off the machine and the lights at night, I can still use the faint backlight to view the clock display on the LCD screen, which has the effect of a "luminous clock". It is beautiful and practical, and the additional power consumption is very small. The LED here needs to be a Φ5mm scattered light type, not a concentrated type, otherwise the brightness of the LCD screen will be uneven and ugly light spots will appear. If you can't find this "scattered light" type LED, you can use the chip LED of the mobile phone instead (first use fine enameled wire to add pins to the chip LED, and then solder it to the printed board), the effect is also good.