Coustic Amp-560 distort on one channel and doesnt power up on the other part. - diyAudio
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Old 3rd September 2009, 03:45 PM   #1
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Default Coustic Amp-560 distort on one channel and doesnt power up on the other part.

I have a problem with a old Coustic Amp-560. One channel is working great, but the other channel on that card does distort the sound. I have replaced The tip142 and Tip147 because of the old ones was failing. I have mesaured the bias voltage on the both channels on the mpsa13s legs and that are different values with the working side and the distorted side.\r\n
The values i got on the distorted side i have
-1.86V on Emitter and -0.71V on the base and +0.18V on collector.
On the workingside i have -1.03 on Emitter and +0.09 on base ande +0.93 on Collector.

I have also checked with oscilloskope on The differentialsetup whit 2sc9014 and on the distorted side i have a high voltage gain on Collektor wich does clip. But on the workingside ther are a maximum peak of 5mV.

The power to the other channel does not work only fade the poweron led.

Regards Jesper Johansson
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Old 3rd September 2009, 05:21 PM   #2
jol50 is offline jol50  United States
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Did you look at the op-amps with a scope? I had a coustic that the op-amp or a cap on one was bad and had weak distorted output.
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Old 3rd September 2009, 05:40 PM   #3
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yes i have checked the one and only OP(NJM3530) and the signal looks similar on the both sides.

Therefore i think it have to do with the bias circuit to the mpsa13.
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Old 4th September 2009, 04:52 AM   #4
damage is offline damage  United States
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A wild guess, but it sounds like something pulling the voltage down on the collector side.
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Old 4th September 2009, 05:49 AM   #5
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Is there DC on the defective channel?

If so, is there DC offset across the two inputs of the differential amp?

If so, how much?
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Old 4th September 2009, 12:52 PM   #6
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I have 10mV across the two inputs on the differential amp, on the defect side but only 5 mV on the side working.

I have mesaured across the emitter resistors and all have 0V exept three on the tip147 , the values are about -1.86V to -1.91V
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Old 4th September 2009, 04:17 PM   #7
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If you have nearly 2 volts across an emitter resistor and the amp isn't drawing excessive current, the emitter resistor is likely well out of tolerance. Check the resistance (amp off) across the emitter resistors with significant voltage across them.
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Old 4th September 2009, 04:31 PM   #8
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There was a huge difference from the distorting side to the workingside.
12Kohm on the distorted side and only 1.9ohm on the working. I think i need to replace them.
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