SWR LA15 power amp problem

[binito]

Dear,
I have a SWR LA15. The amp sound ugly and distorted . And the the cap Q8 starts to get very hot and inflate.

Observing the voltages (Photo attached). It seems that the Bias is not correct. But I suspect that some component is wrong.
I am also not clear on how to do the bias adjustment correctly.
Do I understand that I should have a voltage drop in resistors R3 and R4? Is correct?? How much?

[Jon Snell]

C8 will struggle and get hot if its voltage is exceeded.
Check for zero voltage between the 8A fuse and GND.

[binito]

C8 will struggle and get hot if its voltage is exceeded.
Check for zero voltage between the 8A fuse and GND.

But , the voltage in C8 is 41Vdc .
The voltage between the 8A Fuse and GND is 0,166Vdc.

[Helmholtz]

The voltage drop across R12 (5.4V) shows that C8 has a huge leakage current of 30mA. This means it's dissipating 1.2W, which makes it get hot.
Replace C8.

[binito]

I already replaced it. And the new C8 (47uF 63V) heated up and inflated again.

[Helmholtz]

I already replaced it. And the new C8 (47uF 63V) heated up and inflated again.

Correct polarity?

[binito]

OH MY GOD, The polarity of C8 is bad.
the amp came to me with that inflated capacitor because someone soldered it with the wrong polarity. And I installed a new one, in the same polarity as the previous one. And it was bad polarized.
Now I have changed it and put it in the correct polarity.
Now the C8 does not heat up and there is no voltage drop in R12.
But the amp continues to sound bad. (distorted)
The bias voltage is correct?
Attached image with the new voltages measured in blue.

[Helmholtz]

The bias voltage is correct?

No , especially Q1 seems to be not conducting. Base-emitter voltages need to be between 0.5V and 0.7V for conduction.

But first I would also replace C9 and C10.

Do you have a scope, signal generator and dummy load?

[binito]

No , especially Q1 seems to be not conducting. Base-emitter voltages need to be between 0.5V and 0.7V for conduction.

But first I would also replace C9 and C10.

Do you have a scope, signal generator and dummy load?

ok, i will replace the C9 and C10.
Yes , i have scope , signal generator and dummy load .

[glebert]

I don't know about the LA series, but many SWR combos have a problem with broken legs on the power transistors if they are mounted to a back heatsink.

[binito]

No , especially Q1 seems to be not conducting. Base-emitter voltages need to be between 0.5V and 0.7V for conduction.

But first I would also replace C9 and C10.

Do you have a scope, signal generator and dummy load?

I changed C8 before, and now changed C9 , C10 , Q4 , Q5, Q6 and Q10. The voltages are same.
And the amp continue distorted and ugly sound.

[DrGonz78]

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.

[binito]

I rotate de bias pot , and now the voltages are this: (Green colour in this schematic)



Are the voltages ok ??

[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.

This is the sound :

https://www.dropbox.com/s/fffii0g0n2...0LA15.mp3?dl=0

[DrGonz78]

How does the distortion translate on a scope?