I've picked up a Tek 7141 PAL vectorscope not a long time ago and fixed
various odds and ends in it. However I'm having trouble finding out the
cause of the broken backlight circuit. The SCALE knob on the front
doesn't do anything.

I've opened it up and the backlight is handled by 2 groups of 3
lightbulbs arranged in a way they light up the proper graticule
depending on the mode selected (waveform mon./vectorscope). There are 4
wires going to the backplane via a pin header. There's no voltage on any
of the 4 pins of the backplane (even with the light board unplugged).

Two of the lightbulbs are open, so that's one fault. However, what else
should I check that may be causing no voltage at all on the header?

Cheers
Nife