Museum of the Broadcast Television Camera

Marconi MkVIII Television Camera, camera PSU units

Marconi MkVIII Television Camera, camera PSU units

The camera power supply module. This is an inverter that converts the 180 volts DC from the main channel PSU to the voltages required by the camera circuits. There is a feed back signal to the channel PSU to compensate for load variations and the length of camera cable.

It can be a troublesome unit, all has to be correct or it fails!

The start up sequence is interesting. The inverter relies on +5V to run the drive to the switching transistors, but on switch on there is no +5V untill the inverter runs, but it won't run till there is +5V! Marconi overcame this by having a resistor (R3 330ohms) straight off the +180v to a 5V zener, this then allowed the inverter to run and provide +5V for the logic drive circuits and a relay disconnected the resistor from the +180v and all was well.

In correct operation this resistor was in use for a very short time, too short for it to get hot. However if the inverter did not start for any reason this resistor got very hot very quickly. Most PSU's were modified by the service dept. with a much larger resistor, as can be seen in the picture above, the gold object near the bottom of the heat sink.