MTX RT2400X Rail voltages different

brandes.cm · 30th October 2011, 01:05 AM

The power supply fets were shorted in this amp, 75339's. After replacing them the plus rail reads 49.6V and the negative reads -42.3Vdc. The +- voltage on the op-amps is also odd, +18Vdc and -14Vdc, it looks to be derived from 1n4745 16V zeners.

The op-amp voltage could be messed up from the rail voltage being off, but why are the rails so different?
Should I adjust the bias pots full CCW?

Perry Babin · 30th October 2011, 09:23 AM

This is typically caused by defective rectifiers. One broken leg is most common. Confirm that you have the same voltage swing on the input legs of the rectifiers. If the amplitude there is the same for both rectifiers but the output voltage is not, replace the rectifier.

In some of the MTX amps, they use the B+ to produce part of the rail voltage. If that's the case in this amp, the B+ voltage could have an effect on the rail voltage.

For the regs, compare the voltage on the zeners and the voltage on the various points of the regulator circuit to see where the voltage isn't right. Many of the MTX amps use ±18v for the regulated voltage. I'm not sure what this amp has.

brandes.cm · 30th October 2011, 04:36 PM

The input swing is different between the two, the one with lower output has a higher swing???

With the outputs loaded in a bridged config the rails come to within 2.5Vdc of each other and the +- Reg voltages come closer as well. There must be some difference in the transformer windings?

Perry Babin · 30th October 2011, 05:20 PM

With no remote voltage applied, do you have ~12v on the positive rectifier?

Is there a lot of noise or a large spike on the input to one of the rectifiers? That could cause a high no-load voltage which would quickly come down when loaded.

brandes.cm · 30th October 2011, 11:56 PM

I don't notice much noise, a little ringing.

I did a search for this model here and found reference to a different fet for the power supply. I replaced what I found in the amp, but it was repaired by some other place. I know this by the sticker on the back of the amp.

Perry Babin · 31st October 2011, 07:15 AM

With no remote voltage applied, do you have ~12v on the positive rectifier?

30th October 2011, 01:05 AM	#1
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	MTX RT2400X Rail voltages different The power supply fets were shorted in this amp, 75339's. After replacing them the plus rail reads 49.6V and the negative reads -42.3Vdc. The +- voltage on the op-amps is also odd, +18Vdc and -14Vdc, it looks to be derived from 1n4745 16V zeners. The op-amp voltage could be messed up from the rail voltage being off, but why are the rails so different? Should I adjust the bias pots full CCW?

30th October 2011, 09:23 AM	#2
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	This is typically caused by defective rectifiers. One broken leg is most common. Confirm that you have the same voltage swing on the input legs of the rectifiers. If the amplitude there is the same for both rectifiers but the output voltage is not, replace the rectifier. In some of the MTX amps, they use the B+ to produce part of the rail voltage. If that's the case in this amp, the B+ voltage could have an effect on the rail voltage. For the regs, compare the voltage on the zeners and the voltage on the various points of the regulator circuit to see where the voltage isn't right. Many of the MTX amps use ±18v for the regulated voltage. I'm not sure what this amp has. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

30th October 2011, 05:20 PM	#4
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	With no remote voltage applied, do you have ~12v on the positive rectifier? Is there a lot of noise or a large spike on the input to one of the rectifiers? That could cause a high no-load voltage which would quickly come down when loaded. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

31st October 2011, 07:15 AM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	With no remote voltage applied, do you have ~12v on the positive rectifier? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

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30th October 2011, 04:36 PM	#3
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	The input swing is different between the two, the one with lower output has a higher swing??? With the outputs loaded in a bridged config the rails come to within 2.5Vdc of each other and the +- Reg voltages come closer as well. There must be some difference in the transformer windings?

30th October 2011, 11:56 PM	#5
brandes.cm diyAudio Member Join Date: Sep 2009 Location: Wisconsin	I don't notice much noise, a little ringing. I did a search for this model here and found reference to a different fet for the power supply. I replaced what I found in the amp, but it was repaired by some other place. I know this by the sticker on the back of the amp.

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