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 Post subject: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 5:08 am 
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Joined: Dec Tue 10, 2013 4:36 pm
Posts: 162
Location: Kansas City, Missouri
Hi All,

I'm embarrassed to ask this, but could anyone recommend resources, or absent that write a few lines explaining how I would use an oscilloscope to check my television?

I'm needing to check the sync signal on my Predicta, and I have a scope (BK 1477 Dual Trace scope), but I'm not sure I'm using it right, as I'm not getting any kind of input when I use it. I've read the manual, but it doesn't really discuss practical applications. I want to make sure I'm using it right, and I could use some help. I regret I've never had a teacher when it comes to restoring electrical appliances, I've just been self taught. So any advice would be deeply appreciated.

BR


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 8:15 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
Do you know for a fact that your scope is fully functional? Is the TV basically working but just has a problem with the vertical or horizontal?

There are many circuits in a TV in which only an oscilloscope can be used for troubleshooting. Do you get any signal at on on the scope? If you have the Sams on the TV it will show scope traces. It should be noted that using the wrong sweep rate will give you (apparently) different results.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 3:39 pm 
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Joined: Dec Tue 10, 2013 4:36 pm
Posts: 162
Location: Kansas City, Missouri
I'm trying to trace my sync pulse, because I can't get horizontal sync. I have an oscilloscope, but I have no idea if it's the right one to use for this TV, or if I am operating it correctly, hence why I'm looking for any tutorials on using oscilloscopes for television repair.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 4:39 pm 
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Joined: Apr Sun 15, 2012 3:10 pm
Posts: 997
Location: Buffalo, NY
most every oscilliscope out there has a calibrate output, for like 1V p-p or sumthin.
You get anything on that? below is a service manual found online for the BK1477,
that should be helpful for it's usage / capabilities.
http://bee.mif.pg.gda.pl/ciasteczkowypo ... Manual.pdf


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 5:20 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 1578
Location: Dallas, TX
Good advice about trying to see the calibration signal coming from the scope itself. If you are using
a 10X probe you would use that signal (usually a square wave) to adjust the probe.
I have several scopes (all analog) and a B & K 1471 is one of them, yours is a bit more capable but it doesn't matter.
First off be careful you don't probe a point that has too much voltage on it. Check your probe and scope input specs, sometime they can't handle over 200 volts.
I found a good starter video, although he doesn't cover triggering enough.
https://www.youtube.com/watch?v=z2jp-VhXw_8
Be aware that digital scopes can be fool you about what is happening since that can store a trace.
There is a trigger mode called "AUTO" that would be the easiest place to start. It causes the scope sweep to run continuously, so the trace should always be there. The other mode "Triggered" will display a trace only when the conditions are right as set by the controls. Older scope such as in the 50's did not have the triggered mode. Another thing about old scopes ( like the ones in used before about 1965) is that the horizontal display rate was in terms of the frequency of the oscillator that produced the sweep. So on a SAMS of older vintage when it shows a trace it states the setting is something like 30 cycles (Hz now), this would display two vertical sweeps on a TV for instance.
You might not see the trace if the "Position" control is set too far.
If you have an audio generator or function generator you could look at the generator out put and play with the scope settings to see the effect.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 6:09 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 1578
Location: Dallas, TX
I found a good video on scope triggering. This scope has more complex possibilities than the B &K.
https://www.youtube.com/watch?v=OFGm-Pel4Hg

There are other scope details like, whenever possible connect the scope probe ground clip to a ground as close to the signal you are looking at. If you want to measure time on the horizontal axis make sure the variable sweep knob is set to "CAL", and if you want to measure voltage on the vertical axis make sure the variable volt/div knob is set to "CAL". Frequency is just 1/time.

Got more questions?

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Sun 09, 2019 7:13 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 26024
Location: Annapolis, MD
One point of view:
First, learn all about how scopes work.
Then, use the scope to learn how TVs and other apparati work.

Start with the manual for your scope.

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-Mark
"Measure voltage, but THINK current." --anon.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 7:24 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 3482
Location: Lincoln City, OR
Greetings to BR and the Forum:

All of the above advice is good, but I notice that most people here (because most people on this Forum are extremely knowledgeable) assume too much.

They therefore omit what an oscilloscope actually does. It is simply a gadget that will create for you a Cartesian coordinate graph of voltage versus time. If you hearken back to your high school algebra days, you will recall what a Cartesian coordinate graph is. Usually, it displays two variables, one as a function of the other. In this case, time is the independent variable or the domain, while voltage is the dependent variable or the range.

Most of the knobs on the scope are just to help you get the proper time range for the X or horizontal axis and the proper voltage range for the Y or vertical axis. The remainder are to get the picture to look right (intensity, focus, positioning, etc.) and to see to it that the timing signal starts consistently when you want it to (triggering).

The TV sync waveform is simply a voltage that varies with time in a specific way as shown in the waveform photos.... you want to compare what you actually have to what you should have. There should be some notes on the waveform photos that will aid you in setting up your scope.

Do check out the recommended scope tutorials, but always remember that all you are doing is generating a graph. If you were fast enough (and no one is) you could do the same thing with a voltmeter, pencil, paper and a stop watch.

Good Luck,

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Jim T.
KB6GM


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 8:53 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
We need a lot more information. Does the TV produce a picture? If so then you have high voltage. Is the vertical stable but the horizontal is not? If so then you are truly missing H sync.

Can you get any kind of sync pulses displayed on your scope? You may just have some leaky caps in the sync circuit, in which case you need to be careful when selecting replacements as you can "correct" the problem but have trouble later because of drift.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 3:54 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 1578
Location: Dallas, TX
There are two basic ways of troubleshooting. With a radio (and to some extent a TV) you can use a "signal tracer" that either allows you to hear the audio signal or detect the RF/IF signal. Because with a TV the shape of the deflection signals and video is important, an oscilloscope is better.
To trouble shoot you either follow the signals from the start of the signal path to the end of the path, or from the end of the path toward the beginning. You check the signal along the path and compare it to the way it should be. You usually have to supply the radio or TV with a known signal at the antenna end. Most of the time you follow the signal from front to back, if you were trying to find the source of hum from a speaker you would trace it from back to front.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 5:09 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11421
Location: Powell River BC Canada
One of the best ways to use your BK 1477 to get familiar with it, and TV sets, is to make
the scope display the TV picture on the screen. Connect the TV to an isolation transformer.

http://bee.mif.pg.gda.pl/ciasteczkowypo ... Manual.pdf

This comes in handy for seeing whats what on a TV with a dead CRT.

The scope needs three things to do this.

HORIZONTAL OR VERTICAL

X input HORIZONTAL OR VERTICAL

Y input HORIZONTAL OR VERTICAL

flip these to make picture sideways

invert the horizontal for mirror image (ie downward pulse if you can find one)

Z input. Video (positive or negative image)

Attachment:
BK 1477 snip  XYZ.JPG
BK 1477 snip XYZ.JPG [ 107.17 KiB | Viewed 420 times ]



Your scope has all three, the Z being on the back, the X and Y being channels 1 and 2
with the XY mode selected.



Have Fun. use your X-10 probes. Keep clear of high voltage points. If you aren't sure
buy a high voltage probe on Ebay, and read the voltage vs frequency derating chart.

You will learn a lot by looking at the wave forms, then seeing how they affect the
TV picture on the scope screen.

You may also learn the most secret of all....how to get the picture on the scope
with NO Z axis connection.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 5:13 pm 
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Joined: Aug Tue 28, 2018 9:22 pm
Posts: 774
Location: Sanford Fla 32771
I looked up the specs on this scope, and the input is limited to:
300V (DC+ACPEAK) or 600Vpp.
As mentioned befor, be careful when probing a live TV set. The horz output circuit can have some very nasty voltages, which can easily ruin your scope, and damage you, and really ruin your day (Sorry, don't mean to scare you, but just be careful.). With 300V times a 10X probe DOES NOT equal 3000V. That depends on the probe ratings.

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Paul of Florida


Last edited by pauls.ironhorse on Jun Mon 10, 2019 10:14 pm, edited 3 times in total.

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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 6:35 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 748
Location: Pewaukee, WI
One thing I'd like to point out is that most predictas and many other TVs are hot chassis designs, and most scopes are grounded...I you carelessly connect your scope to a hot chassis TV (or radio) one or both devices are likely to be damaged and you'll see a nice spark when you connect them destructively.

If you want to scope a hot chassis set the safest way is to power the TV through an AC line isolation transformer....if you have a wimpy transformer that can't power a TV then use the transformer to power the scope... Just don't plug both the scope and TV to the same isolation transformer at once ( that is as bad as no isolation transformer).


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 6:57 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11421
Location: Powell River BC Canada
quote: f you have a wimpy transformer that can't power a TV then use the transformer to power the scope..

If you do that, the whole bench is alive.

One of my techs once had a live chassis on his bench (a competent man). By a fluke
we brought him a rush, and he moved the chassis under test. A pin on the picture tube
touched the grounded bench power cable sheath and snuffed the CRT.

It was a crumby little imported set with capacitor filament dropping, unobtanium parts.

Took me forever to get customer to accept a new portable.. :D

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Using an Oscilloscope with TV
PostPosted: Jun Mon 10, 2019 8:02 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 748
Location: Pewaukee, WI
radiotechnician wrote:
quote: f you have a wimpy transformer that can't power a TV then use the transformer to power the scope..

If you do that, the whole bench is alive.

:D

Yup, but if all you have is a TV, scope and a weak isolation transformer and you NEED to see a waveform it is infinitely better than shorting the live line of your outlet to ground through a TV and scope. :mrgreen:

Even with isolation there are hundreds of ways to destructively short stuff and or get bit with electricity if you rush and or disregard safety methods and or common sense.

No matter what one works on there is always some calculated risk...The trick is not goofing the calculation, and working properly within the limits of what your situation dictates.


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