Need some help with a CD player mod - diyAudio
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Old 23rd October 2009, 07:54 PM   #1
PJN is offline PJN  United States
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Default Need some help with a CD player mod

Hi all,

I have an old Sony CDP C8ESD that I've been using as a transport. The optical out crapped out on me the other day. Everything else works fine. I would like to just modify it to take the signal to the optical out to a coax out. It looks to be a simple modification, but there are three wires feeding the torx. Can anyone tell me which two I need to hook to the coax out, gnd and signal. I have attached pics of the board.

Thanks for your help,

PJN
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Old 23rd October 2009, 11:34 PM   #2
wwenze is offline wwenze  Singapore
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The DOTX (digital out transmit) and the unlabelled pin which looks like ground to me.
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Old 24th October 2009, 10:48 AM   #3
Joris is offline Joris  Netherlands
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Hi,

I did this mod to my XBOX, you have to connect the DOTX to the tip of your
RCA and the pin at the far right is the ground. (when you look at the underside of the PCB)

Best regards
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Old 24th October 2009, 10:50 AM   #4
Mooly is online now Mooly  United Kingdom
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One pin is ground, one is supply, the other data. The impedance may be wrong to feed a coax input directly, and is often in any case coupled through a balanced transformer to give a true isolated "floating" output.
Our you sure the transmitter is faulty ? They should be an easy item to source in any case.
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Old 24th October 2009, 06:11 PM   #5
PJN is offline PJN  United States
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Thanks for the input guys. I'll hook up the coax, and then see if I can source a new torx transmitter.

PJN
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Old 24th October 2009, 08:15 PM   #6
PJN is offline PJN  United States
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Done mod, works fine, I have my transport back for $0.00 in repair costs. It may be my imagination but it seems to have a tiny bit more detail coax out vs the optical. Many thanks guys.
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Old 25th October 2009, 06:52 AM   #7
Mooly is online now Mooly  United Kingdom
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That's good
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Old 26th October 2009, 09:12 PM   #8
Joris is offline Joris  Netherlands
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The bit of extra detail is likely to be true, the signal is less digitally converted,
some data can get lost due to the conversion process.

Best regards
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Old 27th October 2009, 02:02 PM   #9
ilimzn is offline ilimzn  Croatia
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Quote:
Originally Posted by Joris View Post
The bit of extra detail is likely to be true, the signal is less digitally converted,
some data can get lost due to the conversion process.
Data is not lost as such as the receiver has means to detect that. What happens is that optical outputs have worse jitter performance (often a lot worse). They do, however, offer electrical isolation which is a good thing, and especially important if your player is used as a transport and as a player at the same time (on non-transformer coupled cocax digital outputs, this will result in a ground loop if both the digital and analog outputs are conencted at the same time).

Regarding the mod, it is NOT a good idea to drive the coax output directly from a TTL level output pin because the coax input impedance presents a considerable overload for a TTL output. Also, TTL levels are about 7x higher than the standard for coax, and it's only because of the overload condition that the signal level falls within guidelines for the coax connection. All in all, you may end up with a working link, for some time - after which the pin driving the signal on the decoder chip in your CDP will die and you will have a much larger problem. At the very elast a resistive divider is necessary. This will typically be two resistors in series from the TTL output (input to the optical transmitter) to ground, with the tip of the coax taken from the point where the two resistors connect. 75 ohm impedance is standard, and the divider should be 200 ohms from TTL out, in series with 120 ohms to ground. If you can't get 200 ohms resistors, you can use 220 ohms, the resulting impedance is about 78 ohms instead of 75, but it should be close enough. A proper solution would be to add a pulse transformer at the output so it's electrically isolated from the rest of the system.
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