![img2](images/cr420/img2.jpg)
With the output loaded, I saw no problems on the scope until I switched the input to the phono channel. The left side disapeared. Notice on this circuit board how the potentiometers are mechanically coupled to the controls through universal joints.
![img4](images/cr420/img3.jpg)
I started to trace the phono preamp to see where the signal disapeared.
![img5](images/cr420/img8.jpg)
As I was probing the board, I noticed a very thin crack on one of the traces. The crack was right next to a solder joint for the PCB mounted potentiometer. Mounting pots like this is a bad idea for stereo controls. They transfer the torque from the knob directly to the brittle PCB causing stress and cracks.
![img6](images/cr420/img11.jpg)
To repair the crack, I removed the enamel coating from the traces along the length of the crack.
![img7](images/cr420/img14.jpg)
Then I added flux to the exposed copper and filled in the crack with solder.
![img8](images/cr420/img13.jpg)
I found another one next to another potentiometer and fixed it the same way.
![img9](images/cr420/img15.jpg)
Traces repaired
![img10](images/cr420/img18.jpg)
The left channel was back now and the amp was working as it should. A quick fix this time.