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Michael Terrell Michael Terrell is offline
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Default Tek 7141 graticule backlight - no voltage on connector

On Thursday, February 20, 2020 at 9:25:30 AM UTC-5, Nife Sima wrote:
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


Are you sure of the model number? Even Tektronix has no information on it. Ther are several 174x series TV scopes including a 1741A.


These are not simple lights with a Rheostat like older products, it uses an IC to drive them.


From the 1740A/1741A manual:

Graticule Lights

U1D is an oscillator with a 600 ms period. Its output drives U1A directly and
U1B through a comparator (U1C). The output of U1A and U1B is a 50% duty cycle, with each amplifier driving two of the four graticule light bulbs. Only two of the bulbs are lit at one time.

The schematic is on page 299 of the Tektronix 070-8469-03 manual:

http://w140.com/tekwiki/images/d/d2/070-8469-03.pdf