Museum of the Broadcast Television Camera

Marconi MkIII Television Camera CCU Valve side view

Marconi MkIII Television Camera CCU Valve side view

The CCU pulls out for access the component side is on top, the underside has the valves and is awkward to get at.

In restoring this CCU to working order, a lot of trouble was had with the black level drifting up and down. A lot of work was done on the clamp diodes, V3a & V3b, the surrounding components and the clamp pulse waveforms, nothing helped!

Suddenly there was some hum on the output! this was traced to the raw DC from the rectifier bank being low, which turned out to be because the mains voltage was down to 195 volts instead of the expected 240 volts and it was waving around all over the place.

An interesting conversation with the supply company ensued and eventually they admitted the local substation was out of action and our "volts" were traveling a long way! Eventually they fixed it and that fixed the drifting black level fault which must have been caused by the unregulated heater volts to V3, the clamp diodes, changing.