Steg K204 blinking protection

[Teo5584]

Good morning everyone.
In my hands I have a STEG K2.04 with green LED on and red flashing.

Let's get to the point: The amplifier had an entire channel shorted, so I removed all 14 FETs (FQP22P10 and FQP33N10).
I also removed some transistors of the preamps as they had values out of tolerance.

In theory, since nothing is shorted anymore it should turn on, but instead it doesn't.
I then tried removing everything from the second channel too, but the problem persists.
By measuring the microcontroller(ST7LITE1) that manages the protection, I noticed that in the PROTECTION pin, +4.9V is present even if I lift that pin from the PCB. (In the K2.02 diagram I should find +5V in input not output)
I assumed it wasn't exactly the pic that was generating the problem, but by separating the switching board and the amplification board (they can be divided using screw terminals and a flat) and keeping them connected only through the flat, as soon as I apply a voltage on the REM, the amplifier starts and is no longer in protection.

I have a feeling that some RAIL voltage checking is done and for some reason it fails.
Does anyone have any idea how to test the microcontroller?

Spoiler: K2.02 ST7LITE1 Pinout

 

[Perry Babin]

Is the output of U9 pulsing to ground?

Do you know for certain that the protect input of the MCU would be at 0v with no connection to the circuit?

 

[Teo5584]

If as U9 you are referring to the photocoupler defined as IC9, I get 4.9V between its two output terminals when the protection is enabled.

As I get back to the lab I will measure the MCU outputs when I remove the rails and the amplifier is not in protection and let you know if the PROTECTION pin has a voltage or not.

 

[Teo5584]

Here I am again, the only differences regarding the MCU voltages are that with protection inserted Pin 14 and Pin 15 are both at 5V, without protection instead I measure 0V on both.

Spoiler: With Protection

Pin 20 GND
Pin 19 GND
Pin 18 5V
Pin 17 5V
Pin 16 0V
Pin 15 5V
Pin 14 5V
Pin 11 5V
Pin 10 0V
Pin 9 0V
Pin 8 0V
Pin 7 3V
Pin 6 1,5V
Pin 5 1,5V
Pin 4 3V
Pin 3 5V
Pin 2 5V

Spoiler: Without Protection

Pin 20 GND
Pin 19 GND
Pin 18 5V
Pin 17 5V
Pin 16 0V
Pin 15 0V
Pin 14 0V
Pin 11 5V
Pin 10 0V
Pin 9 0V
Pin 8 0V
Pin 7 3V
Pin 6 1,5V
Pin 5 1,5V
Pin 4 3V
Pin 3 5V
Pin 2 5V
Pin 1 GND

 

[Perry Babin]

What did you do to get 0v? Did you directly ground 14 and 15?

 

[Teo5584]

I removed the rail voltages and secondary ground from the circuit. I split switching board from amplification and I kept them connected only with the flat.
I get those measurement differences on the MCU by removing the rail voltages.

 

[Perry Babin]

With rail voltage, do you see a pulse of DC on the 'bridging' output terminal of any channel? Alternatively, you can connect a rugged woofer to each channel to see if you hear a pop/click.

 

[Teo5584]

There is no CC on the channel outputs. Now there are no FETs. Furthermore, I notice that the voltage rises up to ±56V but drops instantly precisely because the amplifier goes into protection.

 

[Perry Babin]

If you bridge the LED on IC9 to prevent it from triggering protection, can you get the amp to power up with rail voltage?

 

[Lenin89]

Does the additional resistor between secondary and primary GND helps ? Something between 100-600ohms 1W?

 

[Teo5584]

Perry I'll try this evening and update you.

As regards the resistance, in the circuit there is a value of 6.8Kohm between primary and secondary ground. I carried out a test with a 680ohm resistor but the situation does not change.

 

[Teo5584]

With rail voltage, do you see a pulse of DC on the 'bridging' output terminal of any channel? Alternatively, you can connect a rugged woofer to each channel to see if you hear a pop/click.

Good evening, I am correcting what was said this morning: without Fets on all of the channel, the left channel (the one with the burnt Fets) I obtain a DC pulse of 1V.
On the right channel (the working one) I get 45mV.

 

[Perry Babin]

Can you reinstall the right channel FETs and confirm that that channel works normally?

 

[Teo5584]

No as it still goes into protection.

 

[Perry Babin]

In protect with the opto LED jumped?

 

[Teo5584]

Exactly

 

[Perry Babin]

When in protect with the LED jumped, is it producing rail and regulated voltage?

 

[Teo5584]

Yes, but the rail voltage drops instantly as the MCU forwards the shutdown command to the SG3525. The regulated voltage however remains as it is regulated by the two zeners via the +12v input.

 

[Perry Babin]

I don't understand a couple of things. Please explain.

When the amp goes into protect which pin from the MCU controls the 3525?

The regulated ±15v appear to be produced by the rails via regulator circuits. How are they produced differently (as in post 18 above)?

Why didn't you list all voltage on the MCU?

Pin 18 is the protect pin. Why are we discussing the voltage on pins 14 and 15?

 

[Teo5584]

Let me explain better why I made a mistake:
The ±15V is produced by the regulator circuit through the zener.
The MCU, on the other hand, is powered by the ±5V produced by the regulator powered by the 12V power supply.
(I explained myself badly in post 18, sorry)

As for the command that controls the sg3525 I imagine it is the VON at the output of the two digital transistors connected to pin 14 of the MCU.

As regards the two pins 14 and 15, they are the only two that change when the amplifier is out of protection because the rail voltages are not present. (But I don't think they can hit the mark.)