What frequency limit I should plan for when choosing oscilloscope for TV repair?

[lookimback]

I'm looking at older CRT digital storage scopes, but haven't a clue what to expect for frequencies in TVs.

[goontron]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

I'm looking at older CRT digital storage scopes, but haven't a clue what to expect for frequencies in TVs.

well, 4k TVs have frequencies in the GHz range, so....

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

OK, I had to Google 4k TV. I haven't seen any of them yet. I think the most I encounter is 1080p.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

OK, let me rephrase the question then. What frequency range would be most useful for TV repair? And, since I don't do repair services for the public (I only buy sets in need of repair, which are usually older technology), it's very unlikely that I will see anything better than 1080p anytime soon.

[goontron]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

100mhz. well, thats as high as i've seen anyway.

[Agent24]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Depends what signal you need to measure I suppose. I suspect only the video signal itself is going to be very high frequency, and if that's screwed up, you probably have some major problems anyway. Just about any scope is probably OK for measuring control signals etc. A digital may be nicer than a CRO, for the storage features. With so much digital stuff in modern devices, something with a logic analyser function may be a good idea too.

The input voltage rating\capability\probe attenuation factor might be more important, IIRC there was a thread here of someone troubleshooting a plasma, which he found difficult as the voltage was too high to fit the whole waveform on the display.

Without knowing exactly what you need to check, it's hard to say, except probably "the fastest\best you can afford" so it can deal with as much stuff as possible.

Learning about how to use scopes and what all the controls do is probably more important than the specifications anyway, at least to start with.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

So, if I found something that was 500mhz, it would work for any signal 0 to 500mhz right?

[goontron]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

yes. take into account what Agent24 says also.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Thank you.

[keeney123]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

What does everyone think about the Tektronix MDO3000 6 in 1 scope.

[Agent24]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

So, if I found something that was 500mhz, it would work for any signal 0 to 500mhz right?

Yes... but the signal won't look very good. The rule of thumb I see thrown around a lot is that a scope needs to have at least 5x the bandwidth of the signal you want to measure, if you want to 'see' it correctly. Of course if you just want to check if said 500MHz signal merely exists - then you don't need to have a scope that fast.

I am not an expert either, however. This should prove interesting: http://www.adafruit.com/blog/2012/01...width-matters/

I don't know if you've used a scope before or not, and how much you know if you have, but if you are starting out with one and don't want to drop a ton of money straight off, buy something 2nd hand and learn first. Try to measure things that are too fast etc and get an idea of what happens. The more you understand about signals and scopes the more you will know about what features you should look for when you buy something better.

Make sure you have a differential\isolating\whatever etc probe if you want to do high voltage measurements so you don't blow the inputs up with overvoltage.

If you want to measure anything on the hot side of an SMPS, you should have an isolation transformer for the DUT (device under test) also.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Thanks, I appreciate the help. I have actually never used a scope before. And definitely going for second hand. Regarding the isolation, I just watched an episode of EEVblog about that.

[saddle_au]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

I use an old 100Mhz Tektronix digital CRO (TDS220), the type that Dave at EEBlog laughs at (and straps to himself for a mud run), had it since new. There are times when I'd kill for another two channels, but then I'm mostly using the CRO as a logic analyzer at that point.

Most things that die that you have a chance of fixing, occur at less than 1Mhz. I have found, inverters/backlights run from 40khz to 100khz. Switchers on main boards and TCONs run around 40-60khz. The highest frequency that needed to be looked at was up to 65Mhz, which was the upper limit for the LDVS for TCons (according to the folks at TI), but a good 50Mhz CRO can spot things not right (differences in drive levels) at pretty high frequencies.

Modern processors have lower frequency oscillators on the outside (I see mainly 16 to 50Mhz Xtals) and then internally ramp up the operating frequencies.

Earthing is your friend
---------------------
You could look at this type of thing instead of isolating the cro/tv.

http://www.siliconchip.com.au/Issue/...+Oscilloscopes

Case in point
==========
I am working on a 32inch Samsung where the backlight turns off after an hour or two. The inverter is part of the Power Supply board BN44-00261A (if anyone knows what is happening from experience, speak up!). The backlight chip is a Samsung SEM2006 (joke of a data sheet, pin names, some trip level voltages, that's it).

The chip can stop due to 3 or so triggers and when they occur, it's output fail pin is wired to latch the circuit off until power down. Any of the tubes going open or shorted, any of the transformer voltages going high, it's internal reference is wired back to over voltage tripping as well. There's a bit of circuitry including two isolation transformers between the chip and the tubes.

Now the chip and the associated bits are under the board on the copper side and the board only runs when it's in location as the tube connectors are edge connections. How do you measure whats going on? What ever is happening is either temperature related, dielectric related, mechanical related.....and underneath a high voltage board.

If you wire some wires to four or so points and bring them out to observe, you need to set some trigger on the CRO to capture which one is doing the dirty on you because you don't want to sit there watching and waiting. You could buy another P/S, but then this is not a business for me, wheres the fun in that and where I live, it's a 3 week transit from almost all directions.

(Well, I don't want Fop, goddamn it! I'm a Dapper Dan man! Well, ain't this place a geographical oddity. Two weeks from everywhere!).

In this case, more than two channels means a quicker result. Maybe buy two cheap CROs, or a good one and another.

Whatever you buy, the real ongoing cost is probes. Cheap ones don't usually combine high frequency and long life.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

I recall one of those edge connecting Samsung boards having a fault with one of the big diodes. Not sure if it's the one you have. Sounds familiar though. Thanks for the info too. I'm going to watch a few on eBay and probably grab something mid-February. No need to rush. This gives me more time to learn. Also glad you pointed out the part about the purpose of multiple channels. I'm considering getting one that needs to be fixed. I've seen some of the older HP-Agilent ones which don't light the full screen and I know that it's caused by dried out caps. If I'm lucky, I can snatch up a 4ch 100mhz or better in that condition for under $100. I tried a few months ago to get a couple like that, but got sniped at the last second. I think they went for $76 plus shipping. Beat me by a penny, then I said F it.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Regarding a HV differential probe, what voltage rating would I need to probe the secondary on a large inverter transformer? I have rewrapped a few large inv transformers and would have liked to see how they compared to stock windings.

[tom66]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

What do you want to test? 1080p LVDS and HDMI signals are into the 300MHz range. You need special probes - costing more than the scope itself - to properly monitor them. I use a 100MHz scope myself.

[lookimback]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Depends on the fault. I think the biggest issue I have is with intermittent problems. Kind of like what Saddle_au was talking about. I also have a pile of main boards which I was unable to fix, and I think a scope would have helped. Not having ever used one, I can't really answer your question Tom. What I can test with one is still a question in my mind. I'm mainly trying to ensure that I choose something which will be versatile. It would suck to have something that big taking up space on my bench and then have it be not suitable for common tasks.

[tom66]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

In my mind bandwidth is just one of a few things that would decide the scope for me. I can't recall the last time i needed the full 100MHz. Intermittent faults are hard to track down with an analog scope due to the lack of storage. (Though some storage tube based analog scopes exist they are rare and limited in use.)

[CapLeaker]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

I use a 100Mhz scope. All I need.

[saddle_au]

Re: What frequency limit I should plan for when choosing oscilloscope for TV repair?

Grab a CRO for the $100, it's nothing compared to the time saving you will get back. If you want experience with using a cro for repair work, use it on the next working tv to get a feel for whats expected in various spots around the boards. Wish I could plonk $100 down and pickup a 4 channel 100Mhz around here, alas not to be.

Probe round a couple of the chips on a running TCON, you'll find its really just a mini-computer with voltage generators to excite the LCD. An on-board eeprom tells it the comms method for the panel and the resistor dividers around the gamma chip corrects colour for the particular panel as well. The gamma chips like the notorious AS15s are only there to reduce processing power, its cheaper to divide up incoming voltages in a non-linear way with resisters and some buffers than to increase the processor power there to compute more.

Look at the signals coming from the IR receivers and how the main board clocks the key/buttons in. They're all different in some way, even on the same brand.

Chips on tabs, incredibly small chips and single large mainboard "I do everything" chips are the real killers here. Its coming to a point where it all works perfectly or not at all. One dry joint under a 400 pin bga and it's a throwaway.

I have a Sony 40inch that reports over temp and shuts down when there's really nothing that hot. It turns out the ambient daylight sensor on the Infra-red and button board plays up and muddles the i2c bus which the over temp monitoring chip is attached to. How someone worked that one out is beyond me. I went to inspect the ambient chip and to my horror, I couldn't even see it on the board, it is a bga chip on (under) glass (doh, the top needs to be clear of course). I know there's four connections under that spec of glass, I can see them through it with my microscope, but it makes an 0402 part look gigantic. Two choices here, get a busted TV and swap boards or cut the traces and hope the TV ignores the fact it's not there (turn off that setting).

Ooops,rambling again.