Solid state audio amplifier slight saturation

[Rutherberg]

Hi,

I try to fix a bass guitar amplifier with a solid state power amplifier (SWR Workingman's 4004), it have a problem of slight "background" saturation.
I isolated the problem to the power amp, the preamp sounds fine.

It seems to be load related. When I connect it to a loudspeaker, the problem is present, even with low volume.
On the other hand, when I connect only phone output, which is connected at the output of the power amp via 100R resistor, no problem.

What could cause a saturation caused by load ? I didn't check the bias yet, is bad bias could be the cause ?

Here is the schematic, the power amp is page 5 : https://support.fender.com/hc/en-us/article_attachments/115007682386/Workingman_s_4004__Complete_.pdf

Thanks in advance

[frog]

Can you explain what you mean by saturation?

[Armadillo]

Follow the setting up of the bias procedure in page 6/7.

To check, short the input and measure the voltage at speaker output at L1 to +vcc, and compare it to voltage of speaker output at L1 to -vcc.

Edit: Also check the power supply +15vdc, -15vdc, +75vdc, -75vdc.

[David Hess]

I am not sure what "background saturation" is.

Besides confirming that the bias is adjusted correctly, I would check the output transistors for low gain, check that none of the output emitter resistors are open (and the collector resistors on the drive transistors), and that the supply voltage is not sagging.

Does it appear to just have low gain?  Maybe C4 dried out.  How old is the amplifier?

[Rutherberg]

Thanks for all the inputs !

I did the procedure to set the bias, it was completely out of range. Since, the saturation disappeared.