DC detection circuit failure on Subwoofer

Hello,

First of all sorry for my English and for my poor knowledge in transistor behavior, I’ll do my best to be clear…

I have a problem with the DC detection circuit of a Panasonic 40W Subwoofer, model SW-P270.
The relay in the DC detection circuit never turns on thought there is no DC in the audio output signal.
Output signal is ok, controlled with an oscilloscope.

I found the service manual (main diagram attached to this post) and measured the voltage as shown below, pink is the expected value indicated in the manual, red is my reading:



First of all, I suspected a failure on Q13 (NPN Transistor), so I removed it and got those new readings:


Getting closer but still doesn’t trigger the 24V relay and created a problem with the R19 resistor which began to overheat.

I have multiple questions to ask you:

1. About the overheating resistor:

Voltage at B+ being +35V, if Q10 is on then, if I’m not mistaken, there should be a +35V voltage drop at R19.
R19 being a 330 Ohm / 2W resistor it cannot dissipate U²/R = 35²/330 =3.7W so I must be missing something…

Could you tell me what I got wrong? There is no voltage drop in the relay right?​

2. A curiosity question:

In the schematics I see that Q11 is supposed to be on (Vce = 0V and Ve=0.8V) with only Vbe = 0.5V when I understand it should be at least 0.6V to turn on a transistor… Maybe there’s a tolerance but then why creating a circuit so close to the limits.

Again, I must get something wrong, do you have any explanation for me?​

3. About my main problem:

The schematics says I should get Vbe = 0.7V on Q10, in my circuit I have reading of 0.6-0.1= 0.5V that, I believe is not enough to fully trigger the transitor and explains the 10.6 V voltage drop on Vce.
I also get lower voltage on many other rails as shown on the second schematics.
I thought of replacing Q11 and Q10 but I fear there are other problems in my circuit…
I double checked my measurements with to different multimeter one Fulke and one ChauvinArnoux.

Do you think changing Q11 and Q10 will do? Then I fear for the over heating R19 resistor…​

Thanks for your time and for your help, it was a long post!

Bye Bye

 

Hi Harald and thanks for your reply!

So you were right, D4 is short dead...
I'm currently looking for a diode to replace it as I don't want to power up the board without any flyback diode on the relay..
I'll probably put a regular 1N4001 or 1N4148 diode, I unfortunately don't have a 24V zener at hand...

The trace on the upper right is the audio input for the speaker, I controlled it and it seems ok (I looked for unexpected DC and checked the output signal when an audio input is connected to the board via the jack).

I put Q13 off the board because I thought it was leaking and consequently dropped the expected 1.3V at the base of Q11 to -0.5V.
Once I'll have my diode replace you recommend me to putting Q13 back in place if I got you well?

Again thanks for your help Harald !

Bye Bye