DSP/Amp Problem (Reversed Polarity)

[dorke]

Rick,
Don't forget to check the 5v instead of 12v,
in post #12.

 

[Riley1]

When I check where the circle is I get 10.5V when I turn the power supply on and then it drops to 3.9 and starts falling. I get 0V where the square is.
For whatever reason I now have a steady 12V (VCAR) at pin 3 on U4 and 11.5V on pin 13.
Next move?

Thanks,
Rick

 

[dorke]

When I check where the circle is I get 10.5V when I turn the power supply on and then it drops to 3.9 and starts falling. I get 0V where the square is.
For whatever reason I now have a steady 12V (VCAR) at pin 3 on U4 and 11.5V on pin 13.
Next move?

Thanks,
Rick

View attachment 24940

Pin 13 of U4(TDA3681J) should be GND,
are you counting correctly?
What is the voltage at pin 15 of U4 is it stable?
What is the voltage at the base of Q10,is it stable?

 

[Riley1]

Pin 13 of U4(TDA3681J) should be GND,
are you counting correctly?
What is the voltage at pin 15 of U4 is it stable?
What is the voltage at the base of Q10,is it stable?

Caught me, I goofed. It should have been 11.5V on pin 14 (stable). I'll check Q10 tomorrow. I haven't been bringing the board home as I have it on a empty bench at work with the power supply, test equipment and a nice BIG lighted magnifying glass which without I wouldn't be able to do any of this.

I sure do appreciate your help....and patience.

Rick

 

[dorke]

Rick,
Lets hope pin 15 is only 5V...

 

[Riley1]

Rick,
Lets hope pin 15 is only 5V...

I'll double check tomorrow but I'm pretty sure on that one. My eyes are started to get crossed looking at this thing.

Rick

 

[elebish]

" Using a Fluke DMM set on the diode setting we get .618 and .468 and when we reverse the leads they both read 0."
That should not be!
When you reverse the leads to the DMM it should read INFINITE! .618 is ok for the zener D2 and .468 is ok for the protector diode D32. Those are the forward voltage drops for the diodes.

 

[dorke]

" Using a Fluke DMM set on the diode setting we get .618 and .468 and when we reverse the leads they both read 0."
That should not be!
When you reverse the leads to the DMM it should read INFINITE! .618 is ok for the zener D2 and .468 is ok for the protector diode D32. Those are the forward voltage drops for the diodes.

relax,
O as in O.L.(Over Limit),
it is not zero...

 

[Riley1]

relax,
O as in O.L.(Over Limit),
it is not zero...

Correct.

Rick

 

[Riley1]

Decided to go over a few of these again.

P1 – 0V

P2 – 0V

P3 – 12V stable

P4 – 0V

P12 – 5V and drops, power light goes out

P14 – 11.5 stable

P15 – 5V stable

P17 – 12V stable


Base of Q10 is 11.4V and stable.

 

[dorke]

Please measure voltages on IC4 pins 9,10,11

Base of Q10 is to high.
Please check Collector and emitter of Q10 as well

 

[Riley1]

Please measure voltages on IC4 pins 9,10,11

Base of Q10 is to high.
Please check Collector and emitter of Q10 as well

Will do, back at you tomorrow.

Thanks,
Rick

 

[Riley1]

Please measure voltages on IC4 pins 9,10,11

Base of Q10 is to high.
Please check Collector and emitter of Q10 as well

Quote from original post. " I know just enough about troubleshooting electronics to be dangerous"

With that reminder ......... I did a little checking and discovered I was actually measuring the collector.

Here's what I have at Q10
C - 11.4V
E - 2.62V
B - 3.28

Is this better?

Thanks,
Rick

 

[dorke]

Yes,
It is better(looks fine).
Now ,
please measure the same(B,C,E) for Q9.
and the voltages on IC4 pins 9,10,11

 

[Riley1]

Just in case it helps you should probably know how I’m doing these checks. I’ve got a 3A power supply with a toggle switch. I get my probe solidly on the pin and then switch on the PS.


Here’s the latest checks.

Q9

E – 10.3 and then drops to .4 and slowly falls from there.

C – 12V stable

B – 11V and then drops to 0


IC4

Pins 9-11 all come up at 3.28, power light goes out and they drop to 0.


Thanks,

Rick

 

[Riley1]

Something peculiar…at least to me. I left the power on to the board for a couple hours and decided to check the voltage at the terminals before I shut it off. It was around 9.5V and fluctuating a bit.


Rick

 

[Riley1]

I put power to the board this morning and after 4 hours it’s still holding at 12V, weird. I have the new U4 chip, I'm thinking it's time to swap them out.


Thanks,

Rick

 

[dorke]

Rick,
I would check a bit more before swapping.

The problem I can see is this:
U4 is "turned-off" by the voltage on pins 9-11
going from 3.28v to 0.
Why that happens is the question to solve.

What puzzles me the most is the voltage called 3P3V_ON.
I can't see how this is "generated".

Please measure the voltage at TP4.

 

[Riley1]

Nothing on P4.

 

[Riley1]

Nothing here either.