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Yamaha C2X DC at output

A

AndrewL

Member
Hi,

I have a C2X that I absolutely love, but I have a problem in that there is DC present on the right channel. While it is playing at a steady volume it's a negligible amount, but when I turn the volume up or down fast it puts out enough DC that it causes my amplifier to shut off. Also when I use the stereo mono switch or the subsonic switch it makes a huge and horrendous thump.
I know this preamp is CD coupled and I have looked at the schematics, but it is not clearly labeled and I don't know enough to isolate the DC servo circuit. I'm assuming the DC servo is not counteracting whatever DC may be present, but I don't know how to fix it. Could it be a bad capacitor, or would it be a bad op amp? I just don't know. Does anyone have any ideas?


Andrew
 
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First measure the output DC offset.. For both channels.
 
What voltage are you seeing at the emitter of TR432 ?
 
avionic said:
What voltage are you seeing at the emitter of TR432 ?
Click to expand...
I'll have to take it apart, but that case is not easy, and I don't want to scratch anything. I'll do it over the weekend when my family is busy doing something else, and I'll report back. Thanks for the help!!
 
O.K. so I finally got a chance to take it apart and measure. On TR 432 I'm measuring the following:
Emitter fluctuates from -2.5 to close to 0 to +2.5
Collector -1.5v fluctuates a bit too
Base -20v steady

Don't know what any of this means. Nothing looks burnt or out of order.
 
I don't know which way to count. But if pin one is the one closest to the back with the RCA jacks, then Pin 2 reads -1.5v if it's the other way around, then pin 2 reads +20V dC.
 
IC404 is a key part of a DC servo circuit. It is designed to constantly try to keep the DC offset of the 0 Db buffer amp. output ( emitter of TR432 ) as close to zero volts as possible.
 
When I turn on the subsonic or mono switch, it goes down to about 300mv and then slowly goes down more.
If I turn the volume all the way down, it goes down to about 300mV, then slowly goes down to zero and then stays down in the very low mV range like under 100mV.
 
AndrewL said:
When I turn on the subsonic or mono switch, it goes down to about 300mv and then slowly goes down more.
If I turn the volume all the way down, it goes down to about 300mV, then slowly goes down to zero and then stays down in the very low mV range like under 100mV.
Click to expand...
Well it looks like it is trying to correct.
 
If I leave the volume all the way down, the DC goes away from the output, but if the volume is higher than let's say 9:00 o'clock, then the DC at the output is about -1.6V, same thing at pin 2, it's like a steady -1.6V.
 
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