Out of curiosity (and giving a shot at troubleshooting the item), I opened the bottom lid of a Gould 4072 DSO. Top view has been studied back in 2009.
The darker board consists of analog signal processing circuitry for the two input channels, while lighter green PCB carries flat connectors and interconnections for top-side boards. Fortunately this side is not subject to forced air cooling, so it has almost no dust accumulation and its side effects.
Edit 2014-02-11: I do not own anymore the Gould 4072.