First power up testing procedure

[Mark245]

Hey everyone,

I have breadboarded a 20 watt amplifier. I want to make sure it is going to work right before I go to the trouble of making a pc board. The first time I plugged it up, the output stage blew. I replaced the output transistors and fixed the bad connection that I had. Now, when I adjust the pot on Qbias, I get a Vce of 0 volts and 1.3mV across R35 and R36 no matter how I adjust the pot. I read that you want 1.28v Vce on Qbias and 8mV across R35 and R36. Does the load need to be connected to set the bias current? I have checked all transistors and they are Ok. I have also verified all connections.

The circuit schematic is attached.

Someone mentioned using AM oscillator detector. What is this used for, to detect if the amp is oscillating? I have an O'scpoe and function generator. What is the correct procedure for testing if the amp works?

This is really my first time testing an amplifier with considerable complexity so I am not really sure as to what steps I need to follow. The schematic is attached. Any help will be greatly appreciated.

Mark

[AndrewT]

q7 is passing ~6mA.
The Vbe multiplier resistors are too low value for this value of VAS current.

The combination of C3 and R19 is too low, F-3dB ~11MHz

The ratio of C8+12 : C2 is wrong. This one is not the problem.

Use a mains light bulb tester to power up all your new or modified projects.

[nigelwright7557]

Could be Q7 died with the output transistors ?

I set bias without speaker connected in case DC offset gives false readings.