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Tek 7141 graticule backlight - no voltage on connector
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Nife Sima
Feb 20, 2020, 9:25:30 AM2/20/20
to
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
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
peterw...@gmail.com
Feb 20, 2020, 9:51:59 AM2/20/20
to
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:
https://qph.fs.quoracdn.net/main-qimg-4b55be07eaaa83bc46a640b083cc4e4f-c
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
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:
https://qph.fs.quoracdn.net/main-qimg-4b55be07eaaa83bc46a640b083cc4e4f-c
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
Michael Terrell
Feb 20, 2020, 11:06:02 AM2/20/20
to
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
Nife Sima
Feb 20, 2020, 11:21:52 AM2/20/20
to
On 20.02.2020 17:05, Michael Terrell wrote:
> 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 >
>
> 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
>
Yes, you are right.. It's 1741 my bad. Not 1741A as that one is
>
>
> 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
>
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.
Nife Sima
Feb 20, 2020, 11:49:15 AM2/20/20
to
On 20.02.2020 15:51, pf...@aol.com wrote:
> 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:
>
> https://qph.fs.quoracdn.net/main-qimg-4b55be07eaaa83bc46a640b083cc4e4f-c
>
Maybe there are? The part of backplane is under the CRT tube so I'll
> 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:
>
> https://qph.fs.quoracdn.net/main-qimg-4b55be07eaaa83bc46a640b083cc4e4f-c
>
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.
> 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.
>
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.
> 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.
>
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.
> 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
>
For the record as I already responded to a different person - my bad -
>
> Peter Wieck
> Melrose Park, PA
>
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
Michael Terrell
Feb 20, 2020, 12:21:43 PM2/20/20
to
Nife Sima
Feb 20, 2020, 12:50:25 PM2/20/20
to
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.
https://polprog.net/~ircjunk/vectorscope/6bulbs.png
More photos of the unit are in the same directory.
Michael Terrell
Feb 20, 2020, 2:07:30 PM2/20/20
to
https://groups.io/g/TekScopes
Nife Sima
Feb 22, 2020, 6:04:09 PM2/22/20
to
had to do some work here and there. I'll be sure to look at that
groups.io thing.
Michael Terrell
Feb 22, 2020, 8:27:11 PM2/22/20
to
The B prefix is Beaverton, so it was built in the United States. The 1720 is about the same vintage, it might help you.
groups.io is the replacement for Yahoo Groups. They were migrated to groups .io, along with all the old messages and files. The HP group was moved there, as well.
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