Causes of Supply voltage on output? Help

luckystrike · 14th October 2009, 10:06 AM

Hi all

I have a completed BrianGT LM1875 PCB and PSU however, I consistently measure the supply voltage on the output (25v). I suspect I may have a fried chip but I have assembled the board and psu using standard components and have checked many times. Can anyone point me in the right direction why I might be getting this short on the output?

Thank-you

luckystrike · 14th October 2009, 06:46 PM

Update: As read somewhere, I added a 47kohm resistor R6 between input and output ground. This seems to have stopped the supply voltage appearing on the output, but now there is NO output at all. What is the correct resistor to use or is a jumper all that is needed???

Thanks

pacificblue · 15th October 2009, 02:20 PM

A jumper is usually all that is needed. The resistor comes into play, if you have hum. Sometimes you can cure that with a small resistor, below 10 Ohms.
With a 47k resistor in that position you set the gain to 1, so you have no voltage amplification at all.

Usually full rail voltage at the output indicates a fried output transistor. Desolder the IC and measure with an Ohm-meter from pin 3 to 4, then from 4 to 3, then from 5 to 4, then from 4 to 5. If you have continuity in more than two of the cases, you need a new LM1875.

14th October 2009, 10:06 AM	#1
luckystrike diyAudio Member Join Date: Oct 2009	Causes of Supply voltage on output? Help Hi all I have a completed BrianGT LM1875 PCB and PSU however, I consistently measure the supply voltage on the output (25v). I suspect I may have a fried chip but I have assembled the board and psu using standard components and have checked many times. Can anyone point me in the right direction why I might be getting this short on the output? Thank-you

15th October 2009, 02:20 PM	#3
pacificblue diyAudio Member Join Date: May 2008	A jumper is usually all that is needed. The resistor comes into play, if you have hum. Sometimes you can cure that with a small resistor, below 10 Ohms. With a 47k resistor in that position you set the gain to 1, so you have no voltage amplification at all. Usually full rail voltage at the output indicates a fried output transistor. Desolder the IC and measure with an Ohm-meter from pin 3 to 4, then from 4 to 3, then from 5 to 4, then from 4 to 5. If you have continuity in more than two of the cases, you need a new LM1875. __________________ If you've always done it like that, then it's probably wrong. (Henry Ford)

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14th October 2009, 06:46 PM	#2
luckystrike diyAudio Member Join Date: Oct 2009	Update: As read somewhere, I added a 47kohm resistor R6 between input and output ground. This seems to have stopped the supply voltage appearing on the output, but now there is NO output at all. What is the correct resistor to use or is a jumper all that is needed??? Thanks

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