api 550a problem

[ciaudio]

I have a 1990 api 550a with a gain problem.
When I adjust high frequency control all frequencies get boosted  not just the high end and in cut all frequencies get attenuated. All other frequency adjustments work normally.
I have tried replacing high frequency control with a good unit with no difference. I replaced the data mix transistor buffer board with good unit but still have same gain problem.
Electrolytic caps have also been replaced previously.
Anybody have any ideas.
Thanks
Chris

 

[jsteiger]

Is the HF set to peak or shelf? Or does it happen in both?

 

[ciaudio]

It happens in both peak and shelving positions

 

[pucho812]

check for a shorted resistor