The volume is messed up. I can turn my TV up all the way and put my ear next to the speakers and barely hear a faint voice. The TV itself is fine though and its not the DvD I'm using, I've switched it a...
You will probably need instruments (voltmeter, oscilloscope) to track down this problem.
At first I was going to suggest there is a fault in the audio buffer amplifier, but the only way that would be the case is if it is the bias circuitry inside a chip shared by both the left and right sound signals, or a power supply problem. The other possibility is the voltage reference for the Digital-to-Analog Converter (DAC).
How it works: the DAC gets the digital audio information from the DVD data processor. It generates an output voltage that is a fraction of the voltage reference value. For example, suppose the voltage reference is 2.5 Volts, and the audio data for the right channel is a 1 followed by fifteen 0's (half of full scale, which is sixteen 1's). The audio DAC should produce 1.25 Volts on the right channel. The audio DAC is almost certainly a dual channel device, and uses one voltage reference for both. If the voltage reference is putting out a very low voltage because it is bad or overloaded by another part, you would get the symptom you are having.
What to look for: identify the audio buffer amplifier IC (trace the circuit back from the sound output jacks). While a DVD is playing, use an oscilloscope to compare the input waveform with the output. You will probably have to search for and download the datasheet for the amplifier IC so you can see where the left and right input waveforms go in. If they are the same amplitude, or the output is larger than the input, the amplifier is probably OK. If not, check the voltages on the other pins on the chip to make sure they are correct. In particular, look at the voltage on the pin labeled "Vcc" or "Vdd" on the datasheet. It should be the same as one of the voltages printed by the connectors coming from the power supply board (give or take several tenths of a volt). If it isn't you'll need to check the power supply voltage with an oscilloscope to see if you have a bad capacitor. It is common for the first capacitor in a power supply filter to fail while the second one is good. This can produce a low voltage output that may still look like a clean straight line on the scope. However, for an audio amplifier, the result would be a low-amplitude, distorted sound.
If all is well with the audio amp, you will have to find the audio DAC chip. Start by downloading its datasheet and identifying the voltage reference input. Measure the voltage there. If the DAC has an internal voltage reference, look for a pin that provides this voltage externally.
Cautions: the laser in the DVD player can cause permanent eye damage if it is accidentally viewed directly. If the player mechanism does not have a cover built into it, put a sheet of opaque material (e. g., cardboard) over the DVD drive while you work on it. Keep your fingers away from the power supply, especially the high voltage section where the power cord comes in (often outlined on the power circuit board with white or black dashed lines, bridged only by transformers and several small parts). Get an anti-static wristband, put it on and connect the lead to the metal case of the DVD player. This is so any static electricity you build up in your clothing won't destroy parts in the DVD player. The transistors inside the integrated circuits, especially the data processor and DAC, are very delicate, and can be ruined by a discharge too small for a human to feel as a shock.
This is probably a "hobbyist" repair job. It's my understanding that this model is worth under $35, and most professional technicians want a minimum of $50 just to look at it. If you get lucky, you may find another unit of the same model that is no longer reading discs (try your local electronics recycler). You can swap the mechanism or main board with that unit to get one working (observe anti-static precautions with both parts - the laser circuit is also vulnerable to electrostatic discharge).
Dec 05, 2010 |
Philips Magnavox DVD Player MWD200F