Audio whistle in an FM4

Submitted by dada on Fri, 03/26/2021 - 15:24

I am indebted to my colleague John Matheson for some of this information.

Some FM4 tuners exhibit an audio whistle. This has been traced by John to a square wave appearing at pins 2 and 3 of the TL074 op-amp, which operates into a capacitative load provided by the 100uF bipolar smoothing capacitor across the L tuning reference voltage and ground.

The service manual shows a history of various other measures to combat this problem. It may be the reason for specifying a bipolar capacitor in this position, as there is no other reason apparent for it, the DC voltage across it being well-defined at about 2 volts: a bipolar capacitor would presumably have double the ESR of a single capacitor and thus present a more amenable load to the op-amp.

However John has devised another fix: replace the link near the TL074 that connects pins 2 and 3 to the L reference voltage line with a 47 ohm resistor. He reports that it cures the problem completely.

A very elegant solution.