T4D 40 - TV repair comments

[retiredcaps]

I suggest that the Fluke 114 is not suited for TV or any electronics troubleshooting because it lacks a diode test.  While you can test a diode using resistance, I prefer having a dedicated diode test function like the Fluke 115.

If the gentleman is going to repair TVs, I suggest an ESR meter is more useful because many capacitors be spot on in terms of capacitance, but have high ESR. 

After almost 3 years of participation at badcaps.net, the newer TVs that are failing (say 2008 or newer) are not failing due to the traditional bad bulged capacitors.  They are failing due to other components.

As for multimeter input safety, many TVs can generate nearly 400V DC on the power supply board if PFC is present.  Plasma TVs normally have 200V DC in many areas of the TV and may require turning the pot for adjustment.  Measuring these magnitudes of voltage is a requirement/mandatory when troubleshooting.  This is where having aligator clips to work hands free (or with just one hand) is really helpful.

LCD TVs can generate anywhere from 800 to 1600V AC in the inverter section for the ccfl tubes, but we never ask people to measure this area.  We can troubleshoot this area by measuring the resistance of the inverter transformers or by inspecting the ccfls for damage.

At badcaps.net we rarely ask anyone to measure current or ACV.  99% of time, it is DCV, resistance and diode readings.  The first two account for 95% of the measurements.
 
While participants at badcaps.net have used a variety of multimeters including the cheap $3 Harbor Freight ones, we have seen a handful of oops moments where the multimeter did not survive.  More than a few have been shocked by oops moments as well.

A multimeter, however, should be able to tolerate any oops including switching to ohms or hz when mains power is present.

We have also found that the cheaper multimeters also give erroneous readings when the battery is low, but without such indication on the multimeter lcd.

This youtube channel shows a competant TV repair person using a Fluke 115.

http://www.youtube.com/user/norcal715/videos?flow=grid&view=0

He should also browse/participate to get a feeling of the type of TV everyday problems.

http://www.badcaps.net/forum/forumdisplay.php?f=31

A couple of regulars there know their TV stuff.

[SeanB]

A little comment on the UNI-T is that as you wound up the frequency the amplitude shown got smaller, at 8MHz it was 70%of the amplitude that was shown at 1MHz, and it got smaller as the frequency got higher. This is as the input amplifiers start becoming bandwidth limited.

With the Agilent is is a termination problem, you need to terminate the cables in the characteristic impedance, those are most likely 50 ohm or 100 ohm cable ( depending if they are standard coax or thinnet cable) at the end by the scope, and the generator has a 50 ohm output impedance.

[jwrelectro]

Darn I lost my whole post.  Oh well try again.

I agree with SeanB on the termination of the generator.  However when measuring higher frequency waveform you should usually use a divider scope probe or active scope probe.  I believe you have the 150-MHz x10 probes with that scope.  I think you said you were going into the scope directly through coaxial test cables.  That means from what I could make out in the video you have an input of 1 Mohm shunted by 11 pf.  With one of the sine waves you showed you are only dealing with a fundamental frequency of 5-MHz.   Square waves are made-up of sine waves, fundamental and odd harmonics.  At 5-MHz the impedance of the scope's input is less than 3000-ohms.  The higher frequency harmonics of the square wave will be attenuated and result in a distort square wave.  You noted that at lower frequencies the square waves look pretty good.  For example a 1 kHz square wave has infinite harmonics but usually 7 to 10 give a nice display (21-kHz).    Your scope with a direct connection will have a limited BW.  When you used the 5-MHz square wave the 7th odd harmonic is 75 MHz so you should use the x10 probe.  As a general rule on basic oscope operation it is to use the x10 probe for almost all measurements and use a x1 or possibly a direct connection when you need additional sensitivity while understanding you are reducing useable bandwidth of the oscope.  Also another plus of the x10 probe is isolation and higher measurement voltages.  The x10 probe is basically a frequency compensated voltage divider.  Please don't forget to compensate the x10 probes.

I noted a few years ago that Agilent stated that one of the biggest errors technicians make is using the wrong scope probe for a particular measurement.  On their website they have several application notes on selecting and using scope probes.  Please note I am no expert but just trying to be helpful.

Looks like a very nice scope.

[SeanB]

W2AEW has a nice series on his YT channel about using a scope and about grounding as well. Very interesting, i learned a lot from that, little tips and such that are important.  Generally on as square wave you do not really have to be concerned much about ringing, as most of the time you will be going into a logic chip with it. Small amounts are not a worry, but if it is enough to cause false clocking it need attending to.

[retiredcaps]

Darn I lost my whose post.  Oh well try again.

I usually type long posts into notepad (so I can save locally once in a while) or something similar and then cut & paste when responding.

[jwrelectro]

Yea I usually type into Word and then paste but didn't do it this time... what's the odds? 

[MJLorton]

Thanks gents, I'm going to pop the gent in question a link to this thread.

Cheers,
Martin.