LCD's and Oscilloscopes

[UserName666]

For getting into more detailed lcd work what kind of oscilloscopes are needed? Analog/Digital, Single/Dual channel, How many MHz do you need?
Would any of these be good for *STARTER* units to learn oscilloscopes and LCD

http://cgi.ebay.ca/Pocket-Oscillosco...item230a0ac8bf

http://cgi.ebay.ca/ARM-DSO-Nano-Pock...item2eacfa936b

http://cgi.ebay.ca/USB-PC-OSCILLOSCO...item3cb0d7b5d1

or do you need one of those like big tectronics base stations for LCD work??

[Th3_uN1Qu3]

Re: LCD's and Oscilloscopes

You should be okay with any of the bottom two. I wouldn't use the PC based one though (risk of frying your computer = not good), so that leaves the DSO Nano. I remember i've seen a review on that one and it's quite decent. With just 1MHz analog bandwidth it kinda leaves to be desired, but hey, it's a digital storage scope and it's cheap.

With digital scopes, the bandwidth increases if the signal is periodic because the scope can then take more time to analyze it and produce an average. Ripple on a PSU is periodic, so you should be okay with the Nano. Another advantage is that it's battery powered, so you can probe just about anywhere with it without worrying about shorting the mains or needing an isolation transformer.

My scope has ~200MHz bandwidth, but it's an old Russian piece of gear, it's big, it weighs 15 kilos and has a rather noisy fan. And of course, it doesn't store anything because it's analog. I picked it up locally for $120.

Having a lot of bandwidth helps sometimes, as you may have extremely high frequency ringing on the switching transistors of a power supply (i've seen up to 64MHz ringing on some), which can generate unwanted heat in the power transistors or disturb nearby equipment. However it is not essential, for diagnosing a power supply with bad caps 1MHz will be enough. Most power supplies run at 25 to 500kHz.

[PlainBill]

Re: LCD's and Oscilloscopes

For getting into more detailed lcd work what kind of oscilloscopes are needed? Analog/Digital, Single/Dual channel, How many MHz do you need?
Would any of these be good for *STARTER* units to learn oscilloscopes and LCD

http://cgi.ebay.ca/Pocket-Oscillosco...item230a0ac8bf

I've spent enough time assembling Heathkit kits that I'm a sucker for anything I can assemble myself. But while this might be fun to assemble, I can't really see it as either an educational tool or as something to teach you much about oscilloscopes. One big concern - the lack of information. They don't even give the size of the display! And another - you are tethered to an outlet by the AC adapter.

http://cgi.ebay.ca/ARM-DSO-Nano-Pock...item2eacfa936b

This one really has some potential. I'd be concerned about battery life, but it has the capability of looking at the gate drive of the power FET in an SMPS. (I'd be a little leery of trying it on the source pin without a X100 probe.)

http://cgi.ebay.ca/USB-PC-OSCILLOSCO...item3cb0d7b5d1

This is a useful tool for many applications, but Th3_uN1Qu3 was exactly right - it is unsuitable for use in a test environment. WAY too much chance of blowing a motherboard by accidentally hooking the probe to the wrong point.

or do you need one of those like big tectronics base stations for LCD work??

All of the scopes above are adequate for demonstrating what the signal at a given point looks like. The last two actually are useful for some practical work. It depends on what you are working on. I would have no problems suggesting the DSO Nano for most LCD troubleshooting. If an X100 (or better yet an X1/X10/X100) probe were available it would be capable of handling any application in repairing LCD monitors or TVs. However, there are some adjustments on a plasma TV that require a 200 Mhz scope. And those are a whole different class of instrument.

One thing you should also consider is looking for used oscilloscopes. Barring doing something foolish (turning the intensity all the way up with no sweep and no input signal or feeding 5KV into the input), there is nothing you could do while learning to use a scope that would damage one.

PlainBill

[UserName666]

Re: LCD's and Oscilloscopes

Great thanks for all the info. As I say I am just looking for something to learn the scope for now. I have actually been finding used scopes on ebay for between 75-200 $ but they don't come with probes and need calibration. Is it difficult to calibrate and requires a generator?? I'm most likely gonna go the used ebay route if I decide to pursue this.
I'm considering getting into this LCD thing on the side because when you look at these things there isn't really all that much to a LCD monitor if you have the schematic (or so it seems to me though Im not dealing with a logic board problem...yet). Guaranteed that Daytek would have been running by now had I been able to find a schematic. The problem Im having with the Daytek is trying to translate everything form a general schematic to my specific board. I recently picked up a BenQ with a low backlight problem that I haven't even opened yet. Gonna look for a schematic first.

[Th3_uN1Qu3]

Re: LCD's and Oscilloscopes

Most (if not all) scopes come with their own internal calibration signal of known amplitude and frequency. x10 and x100 probes need calibration, this simply means that you connect the probe to the calibration output of the scope and adjust the trimpot on the probe until you get a perfect square wave. That is all.

You should not be concerned very much if the amplitude is a little off, my x10 probe is homemade and isn't too accurate on the voltage side of things due to the accuracy of the resistors... This hasn't been a problem so far, actually i've learned how far it is off, and i'm able to derive accurate measurements from that.

In the transformer side of a switcher you don't really care if you got 300 or 310 volts in the primary. What matters is that you get x% overshoot and ringing and what not on top of the switching pulse. If that x% isn't of much higher amplitude than the pulse itself, then the power supply is fine. Same applies to ripple measurements - who cares if it's 30 or 40 mV? If it's under 100, it's fine. One more thing - you'll want to do most measurements with the x10 probe. The x1 (aka straight probe) loads the circuit with the capacitance of the cable. This can distort the waveforms considerably.

All that you should be concerned when buying an used scope about is that it shows the waveforms correctly, and if it hasn't got bad caps (lol), then it's probably working to spec.

[PlainBill]

Re: LCD's and Oscilloscopes

Calibration of an oscilloscope is a job for a trained technician. It requires an accurate signal generator. However, that is necessary only if you are using it for very precise measurements or alignments. For general troubleshooting purposes it is not required.

PlainBill

[Krankshaft]

Re: LCD's and Oscilloscopes

Those PC and Wan Hung Lo digital scopes are complete trash. Trust me you're not saving any money and you get that you pay for.

If you want a decent scope buy an old analog Tektronix like the 465 or 475 there are plenty of them out there on e-bay for around 150. These analog scopes blow those OSO (oscilloscope shaped objects) out of the water.

I have read many reviews on those USB scopes and have been less than impressed with their specs.

Here's a decent video about USB scopes:

http://www.youtube.com/watch?v=Ev121xAt_k4

As for voltage calibration a scope is more of a visual aid to view the output waveform of a circuit to see if it's working properly. If you want to measure voltage use a good DMM.

[UserName666]

Re: LCD's and Oscilloscopes

A 465 you say?? Damn there is one for sale locally right now too that comes with a generator

http://winnipeg.kijiji.ca/c-buy-and-...AdIdZ226995863

Sadly I have to take care of rent and bills first lol. Ah well if there one there will be another.

I have to run out but I am gonna read this topic more in detail when I get home and thanks for the link to the video too

[PlainBill]

Re: LCD's and Oscilloscopes

Those PC and Wan Hung Lo digital scopes are complete trash. Trust me you're not saving any money and you get that you pay for.

Maybe not. I'll give you a very quick example. One recent thread, the poster was having a problem with a FPD2185W. The CCFLs weren't coming up to full brightness, then the inverter controller would shut down. Examining the various inputs to the inverter controller, the drive waveform from the inverter controller and the output of the drivers would have given a solid clue as to what was going wrong. This is well withing the capabilities of the $50 scope.

If you want a decent scope buy an old analog Tektronix like the 465 or 475 there are plenty of them out there on e-bay for around 150. These analog scopes blow those OSO (oscilloscope shaped objects) out of the water.

I have read many reviews on those USB scopes and have been less than impressed with their specs.

Here's a decent video about USB scopes:

http://www.youtube.com/watch?v=Ev121xAt_k4

Have you looked at the Tek 465s on eBay? With two exceptions, all of them under $200 are either 'Parts or Repair', or 'not tested'.

True, a 465B will blow a $50 scope away. My PT Cruiser will blow away the golf carts used by the residents of Sun City away, too. However, when all you are doing is running a half mile to the store, the golf cart does the job.

And that IS the point. You need a tool that does the job. We aren't talking bragging rights here, we are talking about doing a job.

As for voltage calibration a scope is more of a visual aid to view the output waveform of a circuit to see if it's working properly. If you want to measure voltage use a good DMM.

Obviously you have never tried to adjust a Y-Sustain card on a plasma TV. One I'm familiar with requires setting the voltage of a spike on a ramp waveform (not the peak) to 150V +- 1 V. Then you have to set the width of another to 100 µS +- 1 µs. No, the $50 scopes can't handle it. Actually, your 465 would be barely adequate.

PlainBill

[PCBONEZ]

Re: LCD's and Oscilloscopes

Most (if not all) scopes come with their own internal calibration signal of known amplitude and frequency. x10 and x100 probes need calibration, this simply means that you connect the probe to the calibration output of the scope and adjust the trimpot on the probe until you get a perfect square wave.

That's not 'Calibration' that is 'Compensation'.
It corrects for the error created by cable and probe.
.

[Th3_uN1Qu3]

Re: LCD's and Oscilloscopes

Thanks for the correction.

[Krankshaft]

Re: LCD's and Oscilloscopes

I've got a 475 with 200 MHz bandwidth good enough for general purpose use. Needed it to adjust an old TV signal generator originally. I don't consider a 200 MHz scope a bragging right just a good bang for the buck.

100 Mhz bandwidth should be a minimum for hobbyist use. Bandwidth is one of those things you'd rather have and not need than need and not have.

Why do you think you need a scope? Most LCD monitor repairs require nothing more than a DMM and some basic troubleshooting skills. As a matter of fact I'd recommend a good reliable, accurate, autoranging DMM before a scope.

In my opinion the best scope to learn on is an analog one all of the adjustments are right in front of you. You can experiment with the unit and learn much more quickly. A digital scope has the same adjustments as an analog but if you've never used an analog scope before it will seem more foreign.

Will the PC scopes display waveforms yes but their accuracy will be sketchy and there is a lot of inflation and inaccuracies in their specs especially the $100 and under models (made in China syndrome). USB scopes and portable handhelds are built to a price and many of the companies that design them don't have the design resources of Tektronix, Agilent, and other large scope companies. They just don't give enough bang for the buck IMO.

If you don't need one today then save up for a decent one. Good things come to those who wait.

</RANT>

[PlainBill]

Re: LCD's and Oscilloscopes

I've got a 475 with 200 MHz bandwidth good enough for general purpose use. Needed it to adjust an old TV signal generator originally. I don't consider a 200 MHz scope a bragging right just a good bang for the buck.

100 Mhz bandwidth should be a minimum for hobbyist use. Bandwidth is one of those things you'd rather have and not need than need and not have.

Why do you think you need a scope? Most LCD monitor repairs require nothing more than a DMM and some basic troubleshooting skills. As a matter of fact I'd recommend a good reliable autoranging DMM before a scope.

In my opinion the best scope to learn on is an analog one all of the adjustments are right in front of you. You can experiment with the unit and learn much quicker. A digital scope has the same adjustments as an analog but if you've never used an analog scope before it will seem more foreign.

If you don't need one today then save up for a decent one.

Will the PC scopes display waveforms yes but their accuracy will be sketchy and there is a lot of inflation on their capabilities especially the sub $100 models (made in China syndrome). USB scopes and portable handhelds are built to a price and many of the companies that design them don't have the design resources of Tektronix, Agilent, and other large scope companies. This is especially important in the triggering circuits. They just don't give enough bang for the buck IMO.

If they were $20-30 bucks sure pick one up and play with it but for the prices I'm seeing at your links it's just not worth it.

There you said it. Most It may have escaped your attention, but most is not ALL. I'm not going to look up the threads because you would only talk your way around the problem anyway, but there have been a few cases where even the cheap scopes would have given us the information to repair the monitor.

Your insistence that 100Mhz bandwidth is the minimum necessary is laughable. What board has the highest failure on an LCD monitor? Not the logic card, where a high end scope may be required, but the power supply / inverter. And what's the highest frequency on that? Both the SMPS, and the inverter run about 70 -100 Khz. How do you troubleshoot a SMPS that s trying to start, but not running? The 5 volt output pulses every time the SMPS 'ticks', but no sign of activity on the winding to power the SMPS controller. Diode tested good, cap had been replaced. Fat lot of good a DMM will do you there.

The 'two seconds to black' problems are a lot easier to solve if you can actually see the voltage and current sense pulses. Actually, the cheap digital scope are a lot better for those than any analog scope.

PlainBill

[Krankshaft]

Re: LCD's and Oscilloscopes

No I was very careful to put most in there .

Minimum bandwidth was strictly from a bang for the buck perspective.

[UserName666]

Re: LCD's and Oscilloscopes

OK if I decide to get a scope I think its gonna be a Tek 465 when I find one where the price is right. Probably waaaaay more scope than I'll ever need but better to have the features on hand and not use it than to need it one day and be caught without it. Thank for all the info on this.