Help with NTP mic pre fix

[rafafredd]

For some reason I cannot understand, I´ve fried one of my NTP opamp boards. I was just experiment with defferent ground schems to hear the results...

Yeah, I´ve built some units but never fixed anything, so I need help.

Well, here is the condition:

The point indicated is at -15.74v
On the working board, it´s at 0.65v

So, I thought the 4148 diode might have gone. But, when testing the diode, it tests fine. So, I´ve replaced the BC117B transistor but it didn´t worked.

Do you think I´ve fried the output transistor? Or the diodes?

Thanks a lot for any help with my first fault-finding.

 

[tk@halmi]

Hi Rafa,

What voltage are you running these at?
I am thinking that one way you may have negative potential at that point is by shorting the BC177B or more likely the 2N2905 transistor on the lower half of the circuit. Trying to reason, but it is getting late here.
If a diode was open you would still have positive voltage at the red arrow.
If a diode was shorted than the output transistors still work, but they would not be properly biased.

Do you use heatsinks on the output transistors?

Tamas

 

[rafafredd]

no heatsinks on the output transistors. In fact they doesn´t get hot.

I´m runing this thing on +-18v...

 

[tk@halmi]

So going with 18 - 15.74 = 2.26 you have about three "diode" drops.
Other than testing the output transistors make sure that the 180 ohm resistor has not burnt and opened up.

 

[SSLtech]

If the BF115 is turned completely on, it looks like it will give you that result.

Keith

 

[rafafredd]

All resistors are fine. All diodes are fine. Will start substituting other transistors.

Thanks for all the help, guys!

 

[rafafredd]

replaced the 2n1711 output transistor and it works again. Thanks for the help, gentleman!

 

[Gus]

I would use heatsinks on the outputs. Years ago I was helping a friend with a stepper motor controler drive design. He was using MOSFETs, they kept shorting, they never got warm to the touch. We added heatsinks and the problem went away.

Are best guess was that the die heated up and did not have enought thermal mass with out the heatsink to slow down, equalize the heat up and cool down times of the die.

 

[rafafredd]

Interesting. Thaks for the tip, Gus. They are not even warm, thought... actually those transistors runs cold. But I´ll add heatsinks anyway, just to be on the safe side...

 

[tk@halmi]

Hi Rafa,

I am glad that your amp is working again. Even though they do not get warm during normal use when you short the output to the ground they will go through an intense heat cycle. With the heatsinks attached they have a better chance of staying alive. (No guarantee though)

Cheers,
Tamas