Noise in Sony CDP-M95 left channel - diyAudio
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Old 31st August 2012, 05:11 PM   #1
wheijke is offline wheijke  Netherlands
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Default Noise in Sony CDP-M95 left channel

Found a cheap Sony CDP-M95. Specially the TDA1541A inside it intrigued me in buying it.

While playing a cd for the first time I noticed noise when music at lower volumes played, it sounds like wind combined with some distorted music. I switched to the optical out and the problem was gone!

Anyone who has seen this problem before? Any leads where to look?

I'm planning on modding this player but want to fix this problem first.

Wouter
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Old 1st September 2012, 12:26 AM   #2
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Manual here:
SONY CDP-M95 SM Service Manual free download, schematics, eeprom, repair info for electronics

I would start by checking the output of the first opamp after the DAC. If signal is OK there, then check after second opamp. Also check supply voltages to audio board and DAC, preferably with a scope. Also check grounds.

If you can unplug the headphone amp, that would help to eliminate it as the culprit.

Possibilities are endless without further info I'm afraid. If one channel is OK, then the problem is likely to be in the audio section. Muting transistor, perhaps, or opamp power supply cap.
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Old 1st September 2012, 02:53 AM   #3
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I have, a few times, seen a channel go noisy in the TDA1541, so the first place I would connect an oscilloscope is to the output pin of the dac. Being in one channel, everything before the dac is ruled out as cause, so it's either the dac chip or bad opamp chip after it.
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Old 1st September 2012, 03:59 AM   #4
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Quote:
Originally Posted by stephensank View Post
I have, a few times, seen a channel go noisy in the TDA1541, so the first place I would connect an oscilloscope is to the output pin of the dac. Being in one channel, everything before the dac is ruled out as cause, so it's either the dac chip or bad opamp chip after it.
Current output. There may not be a signal voltage at the output of the DAC even if it's working, depending on the circuit. Easier to start at the first opamp output. Only if there's no signal there and the supply voltages are OK do the more complicated tasks of checking the opamp and the DAC become necessary.
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Old 1st September 2012, 03:56 PM   #5
wheijke is offline wheijke  Netherlands
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Thanks for the replies! (Hey Stephen!)

I dont have a scope but since the player has a seperate audio board connected with wires to the main board I cut the wires for L+R and cross connected them... the problem moved to the right channel so I'm a bit confused but think the problem is not in the audio board but maybe like Stephen suggests in the DAC.

Wouter
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Old 1st September 2012, 06:49 PM   #6
wlowes is offline wlowes  Canada
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Default Interesting that I have just started having same problem

In fact I too am looking for the cause, and posted on an old thread in "digital source" earlier today.

Noise problem on TDA1541A output

I see that there have been people reporting this problem from time to time over the years. I have not found a case where someone really nailed it definitively.

In my case it is much easier to pin down as I am running a highly modified DAC that is striped to its bear essentials. Effectively I have a USB source (WaveIO) driving a TDA1541a DAC that drives a tube output. I can isolate everything. I built all the power supplies and know that the voltages are correct. I have had similar problems when a solder joint is bad to one or more of the 14 decoupling caps on the TDA1541a chip. This plus the DEM circuit at pins 16,17 are where I am focusing my attention.

I also think it is heat related. Could simply be that the decoupling caps got too hot for too long and have degraded. At first it would only appear on the right channel after the player was powered up for 5 or 10 minutes. Now it appears immediately while playing the first track.

Some posters have said that this is how a 1541a chip dies. Another poster said he changed the chip, but the problem returned quite quickly. This is why I think it could simply be the decoupling caps not doing their job on one channel. A bad solder joint or a bad cap could be the issue.
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Old 1st September 2012, 07:39 PM   #7
wheijke is offline wheijke  Netherlands
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I have desoldered TDA1541A and placed a socket. Reading the schematic I found that data comes (combined) in pin 3 so the stages before (filter/dsp) really can't be the problem. I checked all the voltages according to the service manual (easy with the socket in place) and all that is off is -5 v on pin 25 (AOL) where the service manual lists 0v...and AOL is the problematic Left channel!

How this is possible is still not clear to me.

Wouter
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Old 1st September 2012, 07:46 PM   #8
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Since you did the channel swap to "post" circuits, and read that dc voltage on left current output pin(AOL), it's pretty definitive that the chip is bad. Pretty near 100% sure that no decoupling cap or other external influence could cause this reading.
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Old 1st September 2012, 08:17 PM   #9
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Quote:
Originally Posted by wheijke View Post
Thanks for the replies! (Hey Stephen!)

I dont have a scope but since the player has a seperate audio board connected with wires to the main board I cut the wires for L+R and cross connected them... the problem moved to the right channel so I'm a bit confused but think the problem is not in the audio board but maybe like Stephen suggests in the DAC.

Wouter
Good you spotted the opportunity. Now you know both audio channels are working, too. Must be the DAC, assuming the tracks, solder and connector are OK. Plenty around. Is yours the A version?

You may as well swap the ceramic disks for film while you're at it.

A multimeter is often adequate for comparing AC signals. If you haven't got a test disk with assorted test tones, it's worth making one:

Wave Corrector Test and Calibration CD

Then you can know that both channels have identical signals.

Somewhere I have one of these machines, so I would be interested in your progress if you refurbish it.
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Old 1st September 2012, 09:06 PM   #10
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Quote:
Originally Posted by wheijke View Post
I have desoldered TDA1541A and placed a socket. Reading the schematic I found that data comes (combined) in pin 3 so the stages before (filter/dsp) really can't be the problem. I checked all the voltages according to the service manual (easy with the socket in place) and all that is off is -5 v on pin 25 (AOL) where the service manual lists 0v...and AOL is the problematic Left channel!

How this is possible is still not clear to me.

Wouter
Is that -5V with the chip in or out? If it's with the chip in, is it the same with it out? Is the audio board connections still reversed?
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