yilin fu

it seems the BNC gnd is unconnected, because it works well after i fix the BNC connection

i got a old TDS210 oscilloscopes, when i want to use channel 1 to measure signals, it will display a 100v peak to peak signal as soon as i connect the gnd. When not connect probe to BNC, channel 1 also display a similar sinewave.
does anyone know what the problem is ? or any suggestion for a quick debug
i will apart it and find the bug.