If you have a problem with the triangle wave symmetry then I would begin with that before looking elsewhere. The operation is well described in the manual but I need the big screen to look at that, not the phone! If memory serves me right there are two constant current sources driving the integrating capacitor and limit comparators to define the end points. If the waveform is distorted at the integrating capacitor then you need to look at the current sources, any associated decoupling circuitry and the FET that buffers the integrator. If the triangle wave looks OK at its source then you can follow it through the switching and amplifying stages. The output stage is quite beefy to drive 50 ohm loads and I had to replace some components in mine to get it up to spec.
The push buttons on the 500 series are unreliable with age, there is a thread here about cleaning so try exercising the waveform selector buttons several times.