strange distortion on one channel of cd player

[Luke]

Hi,

I have a marrantz cd6000 which is modded.

I recently noticed that when soundtracks get loud, I get a strange distortion on one channel. If I turn the volume of the cd player down, the distortion goes away.

I put a scope across the output of the affected channel, and it appears to sort of clip, its a very faint line across the top of the signal, but its not hard clipping in that signal does appear to go higher. This faint line when clipping dissapears if I turn the volume down and the distortion stops.

I wondered if it was oscilating on one channel but I dont think it is. I changed the opamp with a spare (lm6172) and no difference.
I checked for dry joints but it really looks in good order.

The other strange thing is that the headphone amp doesnt have this distortion at all, regardless of volume. If you look at the cd6000 schematic, the headphone output comes after the opamps and hdam. This only leaves the muting circuit which is in tact (without output caps). Is it possible its the muting circuit and if so, whats the strange clipping on the opamp output.

any comments appreciated.





I have checke


Whats also strange

[Luke]

please ignore last two lines of my post. No one out there have any suggestions?

[poynton]

Try removing the muting by lifting the leg of the transistor .
Also restore the output caps as a temp. measure.

Sounds like one of the muting transistors could have gone 'leaky'.

Andy

[Luke]

thanks Andy,

Ill give it a whirl and see what happens.

[anatech]

Hi Luke,
These parts will test fine out of circuit, removing them is the only way to test them.

Now for the good news. These are special transistors made specifically for muting. You must use the same part number as these have a much higher E-B breakdown voltage. One example of an older number is 2SC2878.

-Chris

[Luke]

Hi Chris,

thanks for the tip. I will remove them first and see if the noise goes away. If it does I dont know if its worth replacing them. Freight alone will be rediculous for these. Perhaps I can source them locally.
Perhaps I get time in the weekend to check it out.

[poynton]

There is an alternative which does not rely on "special" transistors and that is to replace the muting with a relay (as used in the original Philips players before the beancounters got involved)


Andy

[anatech]

Hi Luke,
You really need to have some kind of muting in there.

Hi Andy,

There is an alternative which does not rely on "special" transistors and that is to replace the muting with a relay

That is the way I would go for sure! Not knowing how much Luke is comfortable handling had me hang back on that suggestion. You are talking about mounting the relay acceptably and modifying the drive circuit possibly. The high sensitivity relays would be the ticket there. The communications types sealed with a nitrogen gas fill would be my choice.

-Chris

[Luke]

Hi Andy and Chris,

I took the transistors out and one was leaky. I replaced the muting on my cd67 with relays. I know I kept them in one of my many parts bins (all new stuff organised replaced parts dumped in various bins) I finally found the parts and they were the same. I replaced the faulty transistor and now Im away

As for fitting relays, Im a bit over modding players for now, I tend to find they spend more sprawled over the workbench than playing music.

Again thanks for all your great help!

[anatech]

Hi Luke,
Good show!

Most defective muting transistors I have pulled over the years measure fine for leakage. That's why I test by replacement these days, although I do confirm the fault first.

I hear you on the mod thing. Same experience here.

-Chris