Tek 7141 graticule backlight - no voltage on connector

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

Reply to
Nife Sima
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First, the obvious, stupid stuff... (been there, done that):

a) Are there any internal fuses? Often, there will be board-mounted fuses and/or fusible links to protect sub-assemblies not immediately visible. If you have a schematic (you DO have a schematic??), look for one of these symbos:

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b) Corroded and/or broken trace in the voltage path. Follow the voltage for the lamps back to the source and look for anything that could cause a discontinuity.

c) Are you sure you are using/measuring to the correct 'ground' for those lamps? In some few cases, the lamps are part of the active circuit, such that voltage must be measured across the correct points, not to the common ground.

Again, I have no schematic, I am just enumerating the stupid-before-doing-surgery-diagnostics-as-general-practice. Please let us know.

Peter Wieck Melrose Park, PA

Reply to
peterwieck33

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:

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Reply to
Michael Terrell

Yes, you are right.. It's 1741 my bad. Not 1741A as that one is substantially different. The 1741 is mostly analog, no readouts on the CRT like in the later Tek devices. Sadly I can't find a schematic or even a manual for the early model.

Reply to
Nife Sima

Maybe there are? The part of backplane is under the CRT tube so I'll have to find a way to remove it accessing from the bottom side. The backplane's on the bottom, CRT on the right and a card cage on the left. I don't have a schematic. More on that later.

I will check that, however the traces are just under the power switch rod that goes all along the side to the mains switch. There seems to be no way of removing the backplane from the bottom side. Looks like I can only access it from the top, having to remove the CRT and the cards.

I was able to find where 2 of the 4 pins go on the backplane with continuity check mode.

Yes, I've measured DCV relative to chassis ground and voltages between each of the pins. None of these were higher than 0.3 ~ 0.4 V, which is odd.

For the record as I already responded to a different person - my bad - the model is actually 1741. And it's not 1741A as these are completely different, newer devices. There are service manuals for the -A version but i think these aren't the manuals we are looking for. I wasn't able to find schematics for the non-A version.

/Nife

Reply to
Nife Sima

All I have are a few photos from an Ebay listing. Even Tektronix claims the 1740A manual is correct for the 1741.

Reply to
Michael Terrell

That's weird. I've looked through the linked manual but I could not find a matching schematic. There are 6 soldered in bulbs on two PCBs located on the left hand side while the manual shows 4 replaceable bulbs in sockets behind the bezel, below the CRT, between the knobs.

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More photos of the unit are in the same directory.

Reply to
Nife Sima

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the 1740A manual is correct for the 1741.

The manual is for the A version, not your unit.I cant find a non 'A' manual for the 1741, 1751 or 1761. which are related series. What is the serial n umber? It can help show where it was built. You might join Tekscopes at Gro ups.io to see if anyone can help.

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Reply to
Michael Terrell

The serial number for my unit is B023287. Sorry for the late reply, i had to do some work here and there. I'll be sure to look at that groups.io thing.

Reply to
Nife Sima

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nual for the 1741, 1751 or 1761. which are related series. What is the seri al number? It can help show where it was built. You might join Tekscopes at Groups.io to see if anyone can help.

The B prefix is Beaverton, so it was built in the United States. The 1720 i s about the same vintage, it might help you.

groups.io is the replacement for Yahoo Groups. They were migrated to gro ups .io, along with all the old messages and files. The HP group was moved there, as well.

Reply to
Michael Terrell

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