I have an old UT2020 (CE2000) that works, but when the "Signal Present" LED comes on it causes a little noise in the signal. It is mainly affecting channel 1, but is also present to a lesser degree on channel 2. The noise is greatest if the gain pot is in the middle and decreases as gain is increased, and disappears if the pot is all the way up.
The signal LED circuit is definitely causing the noise. As soon as the signal is large enough to turn on the LED, the noise appears on a sine wave and coincides with the switching waveform driving the LED.
Audibly, the noise is not terrible, but definitely noticeable as high-frequency static.
I have replaced the -15V regulator and its filter cap. Any ideas or suggestions?