Nakamichi AV-10 Digital Board

For the purposes of repair and troubleshooting, it is important to understand how and where the digital board comes into play in this Nakamichi AV-10. This amplifier relies heavily on LC78213, and LC7821, audio switch ICs to route signals to the correct subsystems depending upon stereo or surround modes chosen. The audio switch ICs operate in a coordinated fashion through a common communication channel of their serial bus. The stereo analogue RCA inputs CD, AUX, DVD, VIDEO, VCR1, and VCR2, pass through audio switch U530 and then to U506. If the amplifier is in stereo mode, then the front L/R signal passes straight through to U525/U526 and to the front PCB balance potentiometer, which forwards it to VR901 main volume control. Hence, as you can see, in stereo mode, the signal remains in its analogue form throughout. However if the surround sound mode is chosen, then U506 diverts the front L/R signal to the digital PCB where an analogue-to-digital converter (U804) samples it to convert it to digital form. Digital Signal Processors (DSPs) then decode this digital stream to resolve the rear, centre, and subwoofer channels, and return them to U525/526 audio switches, whilst the new front channel signals return to U506.

Here is a top tip for engineers. If you have an amplifier that exhibits a fault in the audio signal, then it is important to know whether the fault occurs in stereo mode or surround mode, because then you can eliminate the possibility of a faulty digital board. Alternatively, if the fault presists only in surround sound mode, then you can be confident that the fault may be in the digital board.

This Nakamichi AV-10 digital board D136 has a date stamp of 27 Jul 1999. It has the CS4926 DTS multi-channel decoder IC, which is responsible for the Dolby digital DTS function. There is also the QL2005 IC by the QuickLogic Corporation. The digital board is also responsible for the Dolby Pro Logic surround sound function, where the DSP decodes the stereo signal to produce the surround channels.

From memory, I seem to remember that in 2005 there were many amplifiers where the digital board failed resulting in the loss of surround channels. Usually the rear and centre channels would stop producing sound and it was due to the digital board failing. The problem was with the ADC and DAC ICs, which would begin overheating and drawing more current. Usually the 5 V regulator and bridge rectifier would heat up also resulting in loss of power. Replacement ICs are difficult to find, however there might be a modification where one can feed a signal to the preamplifiers to get sound from the rear speakers.

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