I'm looking to see if anyone knows where I can get a reasonably priced aquisition board for a TDS 640A, the mux chip on mine seems to have died, when the scope switches sampling modes going from 100us/div to 40us/div it gets psuedorandom noise on all 4 inputs, I've checked the attenuator, buffers, and the adc, all seem to check out, as well as all my voltage rails. Only thing left in the circuit is the mux chip. I'd be happy just to get a replacement one of those. As far as I can tell the substrate voltage on the mux chip is the issue, something internally is dragging it to +3V when according to the schematic it should be a negaitve rail. It comes out of an opamp and into a resistor divider to -5v with caps for filtering, with the caps showing ground above them, not below.
The interesting this is just how it seemingly doesn't care for the most part, it works fine aside from the noise. I can turn on averaging and get rid of most of it, but that is only useful when I'm looking at static waveforms, can't do averaging when looking for noise on a signal, and I won't be able to see it with the noise the scope is generating itself.