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Nakamichi OMS 5A mkII DAC board uses
Nakamichi OMS 5A mkII DAC board uses
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Old 14th July 2013, 06:47 PM   #1
jdamico is offline jdamico  Venezuela
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Default Nakamichi OMS 5A mkII DAC board uses

My old and beloved Nakamichi OMS 5A mkII CD player recently presented a fault with the Mute Relay (tickling and erratic on/off) and this made me involved for the first time in his electronics. Finally found there is a faulty (tired) optocoupler that drives the relay with difficult. This photocoupler is obsolete and no more offered by Mouser, Digikey or Farnell but I found one at ebay and I'm buying it now for replacement. The diagnose: the mute relay works at high (on) when reproducing music and the internal diode of photocoupler can't send enough ligth power now to drive the relay.

This trouble made me discover one of the the finest electronic circuits ever seen in my life: the DAC board of this equipment. No shortcuts and no cheap parts at all (even the relays are silver w/gold plating ones).
In the meanwhile I took time to measure the voltage accross the R601 resistor (this resistor on the other -main CPU and digital control- board senses the laser pick up aging) and made me feel sad: the laser pick up also is tired!

With this panorama in mind (future replacement of the laser pick up will cost more than $600 service and is no made in my country), I'm thinking to preserve the equipment but maybe with other functionality (just wondering).

First possibility: preserve DAC board as an independent 16 bit DAC that will last forever. But Nakamichi OMS series CD players don't have SPDIF's connectors at all, so I will need some adds. Looking at ebay saw a "multi input converter module" named 8805X-J based on Wolfson WM8805 digital receiver; looks nice: has spdif (optical, BNC and XLR), USB (CM102) and maybe could be used to provide external spdif and USB digital input interfacing.

But there is another thing to consider: Nakamichi DAC pcb (independent board) doesn't includes inside it the digital filter (the OMS 5A mkII uses an old SM5804 4x oversampling digital filter placed on the other pcb (main digital board with CPU, laser servo control and signal pick up processing).

At this point of my cavilations there are three possibilities: 1) future use the DAC as a non oversampling one (I dislike this option but is pretty simple). 2) use the SM5804 digital filter already on the other pcb. 3) use another 16 bit digital filter non obsolete.

THE DIGITAL FILTER: Nak's SM5804 digital filter works at 44.1 KHz fs (16 bit) and generates an internal bitclock of 96 fs (4.2336 MHz).
SM5804 Input data receives 2's complement serial (like I2S) data at a bitclock of 48 fs and also needs a 44.1 KHz clock as a wordclock.
Output data from SM5804 to DAC (Burr Brown PCM54 type K) is two channel offset binary serial with a bitclock of 32 fs (1.4112 MHz) and a control signal for latching of 4 fs (176.4 KHz).

I have read some threads and posts (mainly Salar and Anatech big contributions) but if someone here have clues and/or ideas related with the above will be gratificant for me.

I have some skills with electronics but I'm not a pro.

Will post a picture of the Nakamichi DAC board so you can undestand what I mean when say "old fine electronics".

Thanks in advance for any reply.
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File Type: jpg Nak OMS 5AII DAC.jpg (1.01 MB, 91 views)
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Old 14th July 2013, 11:24 PM   #2
jdamico is offline jdamico  Venezuela
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Just wanted to upload two more images related to the Digital filter SM5804B implemented in the Nakamichi OMS 5A mkII CD player.

This chip is the last stage before the jumper that goes to the DAC board.
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File Type: jpg SM5804 timing.jpg (503.9 KB, 79 views)
File Type: jpg SM5804 in NAK.jpg (106.8 KB, 74 views)
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