Mackie Th12a no bass sound

[paliddo86]

Hi sorry for my bad english but i'm italian.
I have replace the tda8950th in my th12a and the sound of the bass is nothing.
What is the problem please help me.

[Jazz P Bass]

I have replace the tda8950th in my th12a and the sound of the bass is nothing.
What is the problem please help me.

-Possibilities:
-the speaker is bad.
-the power supply voltages are not good.
-the new TDA8950 is not seated properly.
-the PMW signal is not present.

[paliddo86]

thank you for reply
I did some testing
i have a second mackie and from the tests I've seen
- the speaker is good
-the power supply is good because if i mount the amp board of the second mackie the sound is present
-the tda8950 is seated properly

my ask is:
how do i check if the pmw signal is present?

in the tda8950 i have 50v on the out...

[Jazz P Bass]

Can you post a schematic?
There ia a Mode pin that could hang up the output.
Standby the pin is at 0 volts, Mute: 2.5 v & Operate is 4.2 volts.(DC)
Also a Protect pin.
The PMW is internal to the IC.
I do not see how 50 volts can be on the output.
You do understand that this is a Class D device?
Here is the IC datsheet:

[paliddo86]

i have check the mode pin and i have 5,45 volts but on the stab pin (18) i have 5,5 volts and not 9,8-10v!!!
if I measure the output pin I have 50 volts and not as 150mV datasheet

[paliddo86]

Can you post a schematic?
I do not see how 50 volts can be on the output.
You do understand that this is a Class D device?

this is a schematic of the amp board:
TH-12A AMP SCHEMATIC 0021633-XX-E09-SCH.pdf

and yes i have understand that this is a Class D device but if i measure the pin out i have 50 volts DC

[Jazz P Bass]

Are you making the measurement After the low pass filter.
If not, then you are measuring the carrier wave.

[paliddo86]

Are you making the measurement After the low pass filter.
If not, then you are measuring the carrier wave.

my measure is on the connector pin, if i measure on the amp board good the value is 30mV-120mV change in according with the volume, on the amp board broken my value is 45-50V.
On the broken amp the value on the STAB pin is not regular, i see the data sheet and the value is 9,8-10V in my amp board the value is 5,45V.
i have measured the voltage on the pin STAB and VPPS.
I don't understand the problem...
P.S: if it can be useful a friend of mine before I replace the TDA8950 had mounted on the board without the pad, so I think he has short-circuited something

thank you for reply Jazz P Bass

[Jazz P Bass]

On the working amp, what is the voltage on STAB & VPPS?
The STAB pin is merely a decoupling pin, hense the capacitor, for the Internal power supply of the IC.
And check the MODE pin .
It should ramp up at turn on.
That 5.45 at STAB sounds as if the IC is in the Mute condition. (for whatever reason)

[paliddo86]

on the working amp the STAB pin voltage is 9,8-10 Volts.
I have check the mode pin and the voltage is 5,45 V (is regular and operate)

[Jazz P Bass]

Alright, how about the protect pin?
If that is o/k, then I can only assume that the ic, for whatever reason, is bad.

[paliddo86]

ok but the ic is new what is possible?

[Jazz P Bass]

I don't know.
This is a 'stand alone' IC.
Give it power, a signal, make sure the protect & mute pins are good & away it goes.

[paliddo86]

perhaps the problem is on input i don't check the input pin
on the datasheet the voltage on IN1M,IN2P,IN2P and IN2M referenced to SND have value -5 +5 Volts now check this voltage...