SWR LA15 power amp problem

[Helmholtz]

and now changed Q4 , Q5, Q6 and Q10.

Why this? Did they measure bad?

Most of SS amp repair can be done without any load. DCV at the output without load should be close to zero. The voltage drops across R3 and R4 should be identical and preferably lie between 2mv to 5mv. Any transistor with a base-emitter voltage below 0.45V isn't properly conducting. Doesn't mean it's bad, though.
Also check the remaining electrolytics (like C13, C16) for leakage.

Is there a service manual that specifies the biasing procedure?

Actually I'm hoping someone more experienced in SS amp repair than me will take over. I'm sure there are more methodic and efficient strategies than mine.

[Enzo]

If we have a scope, please post either a picture of the scope output waveform. Or at least a good description of hte distortion. Listening to it doesn't help much. I want to know if the problem is clipping on one side or both or excess crossover distortion or some other thing.

[Jazz P Bass]

From post #8: "No , especially Q1 seems to be not conducting. Base-emitter voltages need to be between 0.5V and 0.7V for conduction."

Have you replaced Q1?

[DrGonz78]

Even a cracked leg or trace connection could be suspect. I would check ohms readings right at the base of Q1 and make sure of continuity. Also test Q1 too as has been suggested.

[binito]

Helmholtz asked "Do you have a scope, signal generator and dummy load?" So we have to ask how you are monitoring the output? If you are only using a speaker to monitor output then make sure the problem is not the speaker. Also, describe the distortion so we can have a better understanding of it. Does it distort lightly or heavily? Less distortion when signal injection is stronger?

Edit: I do realize that you answered yes to the Helmholtz question. I just noticed that you mention the sound of the distortion when describing the symptom rather than a wave form description.

Input 1khz tone and measure with oscilloscope in output and there isnīt distorsion . The wave is perfect in output.
Ok, i change the speaker . And the amp sound perfect.

Summary:
The speaker is broken.
The C8 is broken and someone before me, welded it bad polarized.

Now, i need adjust de bias in the output transistors. but I don't know where to measure. The voltage drop in R3 and R4??

Thanks you .

[Helmholtz]

Now, i need adjust de bias in the output transistors. but I don't know where to measure. The voltage drop in R3 and R4??

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