Coustic Amp-560 distort on one channel and doesnt power up on the other part.

Jeja2000 · 3rd September 2009, 03:45 PM

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

jol50 · 3rd September 2009, 05:21 PM

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.

Jeja2000 · 3rd September 2009, 05:40 PM

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.

damage · 4th September 2009, 04:52 AM

A wild guess, but it sounds like something pulling the voltage down on the collector side.

Perry Babin · 4th September 2009, 05:49 AM

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?

Jeja2000 · 4th September 2009, 12:52 PM

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

Perry Babin · 4th September 2009, 04:17 PM

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.

Jeja2000 · 4th September 2009, 04:31 PM

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.

3rd September 2009, 03:45 PM	#1
Jeja2000 diyAudio Member Join Date: Sep 2009	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

4th September 2009, 05:49 AM	#5
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	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? __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

4th September 2009, 04:17 PM	#7
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	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. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

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3rd September 2009, 05:21 PM	#2
jol50 diyAudio Member Join Date: Nov 2007	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.

3rd September 2009, 05:40 PM	#3
Jeja2000 diyAudio Member Join Date: Sep 2009	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.

4th September 2009, 04:52 AM	#4
damage diyAudio Member Join Date: May 2006	A wild guess, but it sounds like something pulling the voltage down on the collector side.

4th September 2009, 12:52 PM	#6
Jeja2000 diyAudio Member Join Date: Sep 2009	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

4th September 2009, 04:31 PM	#8
Jeja2000 diyAudio Member Join Date: Sep 2009	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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