How Can I Verify Existence of Video Signal with a Scope ?

[mmartell]

Can it only be done with the help of the service manual for that specific tv or are there generic guidelines that will work ?

[Kiriakos GR]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Imagine the Oscilloscope as to be a spoon, and the video signal as to be the sea.

What you can do with the spoon is to taste the water so to examine the saltines.
If you need to sea a picture of the sea waves, what you need to have is a digital monitor and not a spoon.

[tom66]

Re: How Can I Verify Existence of Video Signal with a Scope ?

^I'm not sure what that analogy is supposed to mean, but please remember that analog video signals have been analysed with analog oscilloscopes and vectorscopes for years prior. Budget digital scopes are worse than analog scopes for video analysis. You need to spend a lot of money to get a decent digital scope for video analysis.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

I just acquired an old analog scope (iwatsu ss-5711) and will be learning how to use it as I go along.

I have a tv that has no video/osd but good back light and had a suspect tcon which I replaced twice (thinking the first was faulty) but am doubtful now. Would like to simply see evidence of signal coming from Mainboard into T-Con before I make my next move.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

So where/how do you test to see these signals, at least enough to say they are present or not ?

[budm]

Re: How Can I Verify Existence of Video Signal with a Scope ?

What is the panel model number? look for it on the sticker, then look up the spec sheet to get the pin out of the 6 LVDS/or 5 depends on the LCD format (Channel A~E, plus Clock), the LVDS is conformed to EIA664, eash signal is terminated with 100 Ohms on the receiver side.
You can look for activity, dead main board will show no activity. signal is less than 2V.
To have all dead signals (no output) from all 6 or 5 outputs will have to be something that is common to ALL the signals that failed such as no B+ to the driver ICs.
You are using the scope to look for signal present, for analyzing the waveform to see eye pattern, you will need better scope. I use TEKTRONIX 2210 for checking signal present only.
Example:


https://cdn.badcaps-static.com/pdfs/...8254cd7cb6.pdf

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Found the schematic. Looks like there are 10 twisted pairs (plus clock) so 5 data lines ?

But there are plenty of test points so I will start there.

[tom66]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Probably 4 data plus differential clock.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Does it take one or two pairs to make a data line ?

[truep]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Do you have proper voltage at both ends of the tcon fuse? No voltage, no picture.
There should be either 12V or 5V at the fuse. Need a picture of the TCON, Main, power, and a general picture showing the back of the tv,(cover off please).

[budm]

Re: How Can I Verify Existence of Video Signal with a Scope ?

1 pair of data line (+/- diff signal, it is basically balanced signal lines), so you you have 4 pairs of data line and 1 pair of clock.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

There is A0-A4 (+/-) and B0-B4 (+/-) plus two differential clocks.

@truep, yes I tested for voltage on both sides of the fuse and it was there. I think it was 12v but that was some time ago.

I've found a few shorted capacitors on the mainboard and removed them but that circuit is still shorted. Perhaps this could be the root cause ?

I will attach a few pics and maybe I could get some input on what to check for.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

C816 and C831 were removed from board and tested ok. Many smaller smd caps in that area test for short consition as well.

Second pic is the area directly under the first pic.

[budm]

Re: How Can I Verify Existence of Video Signal with a Scope ?

As you can see, there will be small value (<1uF) SMD MLCC in parallel with the electrolytic cap. I.E. C817 is in parallel with C816. MLCC when it failed, it shorted out. You can also have shorted load.
What TV is this you are working on?

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

LG 42LB5DF.

I have no idea what this circuit is doing - the schematic I have for this tv doesn't even list a C831 and has a different value for C816.

What is MLCC ?

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Oh happy days I've found a schematic that fits what I'm seeing on board. Both are part of 3.3v regulator circuits - one for BCM and the other for TMDS_SW for whatever that's worth. When regulators fail do they often short out ? These are listed as AZ1117H.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Here are the relevant circuits.

[budm]

Re: How Can I Verify Existence of Video Signal with a Scope ?

MLCC: Multi Layer Ceramic Capacitor.
Regulator can have failed Output shorted to Ground, input shorted to ground, Input to Output shorted. You can remove it to find out.

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

Well I was busy patting myself on the back after removing the two regulators and the short still exists.

After removal one shows the center pad grounded and on the other the tab pad is grounded.

I have now officially managed to confuse myself further haha.

At this point I would *guess* the short lies further up where the 3.3v sources are attatched.

What do the trained eyes here see ?

[mmartell]

Re: How Can I Verify Existence of Video Signal with a Scope ?

How do you find a short ? Even with the schematic there are a thousand components in these three circuits that could be shorted so how do you find the short ?