How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

[Robb]

I have a Hitachi P50S602 plasma TV that is going into Safe Mode (auto shutdown) after about three seconds when I power it on and then the front LED blinks 5 times. The voltages off the PSU are testing good for the few seconds before it powers itself off. All the capacitors, resistors, fuses, etc. appear to be in good shape. Unplugging the various main boards - one at a time - does not make the unit stay powered on.

I was encouraged to get an oscilloscope to help me find the failed board/component since all other methods of diagnosis were failing. I got an oscilloscope with two channels, a TV trigger & a Line trigger. Now I just need to learn how to use the scope to test component/board in order to find the fault in the TV. Any advise will be greatly appreciated. Thank you in advance for any assistance in the seemingly daunting endeavor. Cheers

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

I figured I should post some pics of the boards for reference.

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

You need to find the sustain board outputs. These are near the plasma panel ribbons, can we see a picture looking at the sustain boards from that angle?

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Gladly. First 3 photos are YSUS & last 2 are XSUS.

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

I'm trying to see an obvious test point but I'm not seeing one, do you see any point marked TPnnn where nnn is some number?

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Next to the outer screws on the buffer side of the YSUS I am seeing TPnnn points.

Right side of YSUS has TP80E, TP81E, TP82E, TP85E, TP86E & TP87E. Left side of YSUS has TP80, TP81, TP82, TP85, TP86 & TP87.

The only TP points that I see on the XSUS are on the input side/half of the board.

[kingsapper]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Test points are handy, but if you can get a service/training manual for the boards you don't neccisarily need them if you can back-pin or solder on a takeoff point of your own.

When it comes to using a scope there are a few basic things to consider..

1. SAFETY: you're going to be poking around in a running plasma TV. I think we all know what that means. Don't work alone, and think about what you're doing before you do it.

2. Think about what you're going to measure before you measure it. If you don't have an idea of what it should be, how do you know if what you're looking at is correct?

3. Triggering takes some fiddling. Unless you have a fancy scope that does auto-trigger and waveform capture you may have to fool around a bit.

That being said, here are some ACTUAL tips for using a scope..

• Don't use them for measuring DC.. use a voltmeter. It's both more precise, and more accurate
• frequency = 1 / time so therefore if you're setting the scope on 1uS per division and trying to measure a low-frequency switching wave you're not going to see much. Once again, think about what you're going to measure. That being said, I usually start with a "guesstimate" and fiddle a little if I'm not using the scope's auto-capture
• You're not going to see much if your voltage settings are too high or too low.. or the probe magnification is set incorrectly. I suggest always using the probe on 10x if you can, as it loads the circut-under-test less. Learn how to set this on your scope.
• Start at the high end of the voltage range.. at 50V per div you're looking at 1000Vpp range on the screen.. so if you're probing something that may have a hundred volts on it or so, make sure that you're above 10V per division to start with
• Once you've found the waveform you're looking for, try and always adjust the volts per division to the level that maximizes the waveform on the screen. This will give you the most accurate reading.. if your scope also has a zoom capability it will make it easier to look at the waveform transitions for ringing etc

Scopes are great tools. But they can also confuse you more than they can help if you're not sure what you're doing. If you have trouble, post it up and I'm happy to guide you along.

Cheers
KS

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Thank you very much for the tips, kingsapper. I haven't been on a learning curve this steep in a while.

I tested a few of the YSUS Test Points just to get familiar with the scope before I saw your post here.

Unfortunately I don't know what the wave should look like. I was guessing I was going to see a square wave.

I couldn't get the Trigger to make a wave on the screen.

I don't know the frequency of the signal either & the largest division on my scope is 5V/div.

I found one of tom66's old posts that said "With your scope, check Zsus output. 10X probe, 5V/div, 20ms/div. TV trigger works sometimes, if you have it." and I adjusted the scope to those settings and finally got something like a wave on the screen.

I had to compress the video to upload it and took a screenshot just to give an idea of what the wave looked like. I'll post em' below along with a pic of my scope's control panel.

Thanks, again, for the input. I can use all the feedback I can get.

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

I've gotten some minor shocks from accidently touching heatsinks while also touching the metal chassis. Maybe now would be a good time to remind people - and myself - that you should never touch any part of the plasma boards with your hands **especially when it is powered** Even when it is not powered the capacitors can hold deadly current.

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

On a hitachi plasma, the sustain board outputs go both positive and negative, so you would expect a swing from about +85V to -85V (or whatever Vs is.) The TERES design kind of automatically generates a negative rail from the positive input. So I'd play around with 5V/div with a 10X probe, equivalent to 50V/div. The X-SUS (=Zsus) output is found where the plasma panel ribbons connect, on the X-SUS board.

[kingsapper]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

I've gotten some minor shocks from accidently touching heatsinks while also touching the metal chassis. Maybe now would be a good time to remind people - and myself - that you should never touch any part of the plasma boards with your hands **especially when it is powered** Even when it is not powered the capacitors can hold deadly current.

Yeah, important safety tip: When this system is operating there may be FLOATING parts of the system. Not directly connected to safety earth, the heatsinks or other parts of the system may float through capacitive coupling to a high frequency (ie a switching waveform) signal up to who knows what. This is not detrimental to the system, but if you happen to form a path to ground for it, that energy would like to flow from high potential to low potential.. so don't be that link!

tom66 is a stud. His information is invaluable. I'm new to TV systems but I'm a qualified professional tech and have done a lot of work on other high power systems so this is a bit of learning curve for me as well so don't worry about it. Big thing is safety. Keep that in mind at all times.

If you're not sure what you're looking for, it's time to search and ask for info.. once you get an idea of what's supposed to be around the place it's easier to find it on the scope, and see how far off the system is from where it should be.

What brand/model scope are you using? I have both an analog 300MHz scope I use for communications equipment and a DSO (digital storage oscilloscope) that's a 100MHz that does all the fancy stuff. I may be able to help you a bit more if you advise the model of your equipment.

If you can't get the scope to trigger correctly, or you're in doubt at any time, the scope should have a test point right on the front of it.. most of them give you a 1Vpp square wave at 1KHz.. that will give you a chance to find and trig a waveform and play with the controls.

Not sure what your original fault was, but if your scope is relatively modern you should be able to do just about everything you need to inside a modern TV.

As tom66 said, keep in mind the probe/scope settings. Some scopes have a probe setting switch or setting, so it will calibrate the display to the probe setting.. automatically multiplying for a probe set on 10x etc. If not, you're going to have to do the math yourself. I find a bit of masking tape and a sharpie marker come in handy if I have any tricky input values etc that I'm looking at.. I just mark what is going on and tape it to the top of the scope where I can see it as a reminder. Good if you're probing voltage dividers on SMPS feedback lines etc. Reminds you of what you're really looking at.

As always, here to help.

Cheers,
KS

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

I'm still getting familiar with the controls but from the looks of things on the YSUS the signal is at 0V for x period then oscillating +yV & -yV for the same x period. Once I learn more about the controls I'll try to fill in the x & y values.

I can't get the Trigger to do produce anything not even when it's connected to the Probe Adj point. I think it's dead/not functioning.

My scope is an old Tenma 72-5025 and there doesn't seem to be any manuals around on the webz. It's an analog dual channel 60Mhz 250W max. The only hits I get with a search engine is on ones for sale.

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

The trigger control will do nothing to the wave until it goes off the screen.
Try this:
- connect probe to cal/test point.
- adjust controls to get clear view on screen of waveform.
- try adjusting the trigger level; when you go past a certain point, the wave will either disappear completely, or turn into a mess of interleaving lines.

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

I have tried seemingly every variation and combination of settings with the probe connected to the Trigger input. The only change I observe is a slight dimming of the horizontal line if I touch the probe to Probe Adj point when the Source Mode is set to Ext Trig In and in the 1X position. I get the same dimming if I touch the probe to my finger. The Line Source Mode gets no response in any situation. There is no vertical movement at all with the probe connected to the Trigger.

I've probed around on the XSUS and found similar waves at various points. They all (XSUS) have a similar characteristic of a falling slope after the oscillation whereas the YSUS appears to have a spike above and before the oscillation point. Below are some pics. 1st pic is XSUS, 2nd pic is YSUS.

I apologize for the poor picture quality. The YSUS picture falsely appears to bend.

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

You need to adjust the FOCUS control on the scope to improve the sharpness of a trace. What V/DIV are those at?

You only use EXT TRIG IN with the external trigger input; use CH1 trigger and CH1 for main input for now.

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

The XSUS was set at 10mS/Div & 2V/Div

The YSUS was set at 10mS/Div & 5V/Div

The measurements were taken on Ch1

With a DMM the XSUS measured 35V and the YSUS started at +66V then went to -140V.

The poor focus may have been from the camera.

Not sure if this is relevant but there is a Test Point near the power input to the YSUS that I can't get either a DMM voltage or an oscope reading on.

I must say I'm having alot of fun probing around. Thanks for all the feedback. It wouldn't be as fun without it.

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Not sure what your original fault was

The TV powers on and displays maldischarge (deepspace-like static) for 3 seconds then turns off leaving a 5 blinking LED code.

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Was that with a 10X probe?

[Robb]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Was that with a 10X probe?

Yes

[tom66]

Re: How to Troubleshoot a Plasma TV (p50s602) with an Oscilloscope

Are you sure you got those the right way around? 1st wave looks like a bad Y-sus wave, 2nd looks like OK X-sus.