Repairing old BK 3010 and 3011 function generator, odd noise

[JTM45]

New to electronic work, built/copied a few tube amps, now trying my hand at more in-depth stuff that will make me learn more.

I needed a function generator, so I have acquired a few.  I got one Heath 1274 that seem to work fine, so that’s for my guitar amp work, but I did get two old BK Precisions, a 3010 that is from the late 70s, and a 3011 from the 80s.  I know it’s worth nothing value wise, but it works enough it’s of potentially great value to me as something to learn on.

Both have a sine wave that has a noise at both peaks of the cycle.  (See image)

So know very little about the functions other than I understand the triangle wave is being used to make the sine wave.

I have the PDF manuals to calibrate both, and the schematics with part lists for both.

Any idea what this is? Or where to start testing for proper functions?

I should add, I have acquired an oscilloscope. Sig 1202x-e

[mawyatt]

Recall these old Function Generators utilized a sine wave approximation technique where a triangle wave was passed thru a network of diodes (or transistors) and resistors in an attenuator. This attenuator would vary the attenuation amount based upon the triangle wave input level, thus approximating a sinusoidal result at the output. Intersil and Xar made function generator chips that also used this technique.

The noise blimp on the sine wave is just the peak of the triangle wave coupling thru the diode R attenuator.

Best,