Audio System X 100.2

[DerSamoaner]

First time only Q6 was shorted between Base and Emitter.

4 Ohm bridge bench test passed.

During 2 Ohm bench test amp gone in protection near clipping point

This time Q6, Q5 and ZD6 failed.

Changed parts and during 4 Ohm bridge bench test amp gone again in protection.

This time only Q281 failed. N Channel Mosfet FQP33N10.

Both, Q281, Q181 changed. Set Bias and test it again.

Heat up amp with 8A Power consumption, clean sine wave, gain works properly.

After heating up, amp was a long time in idle current, maybe one hour. Remove remote, apply remote, Q6 again shorted between Base and Emitter.

Any idea?

 

[Perry Babin]

What do the points A and B go to?

 

[DerSamoaner]

Here the full schematic.

 

[Perry Babin]

Those are the mute lines. Are any of the resistors Rx60/61 out of tolerance or the transistors Rx50/51 shorted?

 

[DerSamoaner]

The Rx60/61 are in tolerance.

In schematic the Qx50/51 are BCP53 and BCP56. In board are BC327 and BC337. All not shorted, but 2 had a bit different values with MM diode test.

 

[DerSamoaner]

Desoldered all Qx50/51. All transistors fine.

 

[Perry Babin]

Are Q3/Q4 running hot?

 

[DerSamoaner]

Q4 getting hoter than Q3. But both not very Hot.

Q6 again shorted between Base and Source.

I order new BC850C.

 

[DerSamoaner]

How to set bias correctly?

Bias potis full counter clockwise.

If increase the bias to 1mV, on the N Fet's emitter resistor 1mV, on the P Fet's emitter resistor 1,8mV. Maybe normal?

 

[Perry Babin]

It's not normal for the voltage across two series resistors of the same value to be at a 8:1 ratio.

Did you have a load connected?

Are the 0.05 ohm resistors within tolerance?

 

[DerSamoaner]

The resistors are in tolerance.

Without load. Heatsink mounted and idle.

Maybe this is the issue because Q6 and other components of the mute line fails every time.

 

[Perry Babin]

Does the amp produce clean audio with a load, up to clipping?

 

[DerSamoaner]

No clean audio with bias fully counter clockwise and load(speaker). Plays audio but little noisy.

Without load I increase the gain up to clipping and measure the bias.

P Fet 2,2mV. N Fet 1,1mv. 2:1 ratio.

Picture is without load. Distortion are visible.

 

[Perry Babin]

Were you measuring the bias with signal applied?

 

[DerSamoaner]

On the source resistors. RR10/11 RR20/21