Repairing JVC 42" LCD TV

[dave.martin]

Ok TV plugged in and turned on. No screen image, blue power LED comes on and the TV must be doing a check of the system and then starts blinking, I was told by JVC to run through a button sequence to fix the problem. This did not work. Now I am not an electronic engineer but I am good with them but I am having trouble with checking the PCB. for some reson when I check a live voltage reading on the positive side on the large Capacitors and I am not even getting 1 volt out of them. none of the capacitors show bulging or leaking. I am wondering on a PCB what is the next thing I need to test and I need a little direction here. I would rather not have to order a new PCB for this TV if I can just find the bad componant and replace it. Thanks for the advice in advance. and if you need more info...
TV is a JVC model # LT-42X579
PCB # LCA90796

 

[(*steve*)]

OK, firstly, everything inside the thick white lines is at mains potential. It may bite you in a terminal manner.

Secondly, what do you mean by "large capacitors"? I hope you don't mean the big brown ones.

Check the voltages on the output rails (presumably near the top right corner of this photo.

My guess is that you may have a shorted diode. There will be 2 or more on the black heatsink. There are likely TO-220 package devices. If they have 3 leads, then they contain 2 diodes, and they will be connected up so they are in parallel.

It may be difficult to measure their resistance, but in circuit you should see some evidence of diode action. All the leads appearing to be shorted together is a bad sign. You may have to remove them to test them properly.

A failure here will cause the power supply to shut down, with the flashing LED the possible indication.

Also note that capacitors don't have to look bad to be bad. An ESR meter would be the right tool to check them.

 

[dave.martin]

(*steve*) yes with just this board plugged into the wall outlet and my meter (Fluke 115 true RMS Multimeter) set to DC voltage testing I get only 0.1volts with the positive test lead on the positive side of the large capacitors and the neg test lead on the common ground on the board... I also tried testing the capasitors onboard and found that they all have a reading of 5700 microfarid reading LoL so that was a wash guess ill have to unsolder them but I need to stop down to radeoshack and grab a solder vacuum so i can remove it easier.
and to check the output raild in the top right corner I would just plug it in the same way i did before with this board isolated from the rest of the tv correct?

 

[(*steve*)]

Does this supply only low voltage DC, or does it supply power to the backlight too?

Essentially yes, you connect it to power without connecting it to the TV. The problem being, unless it's a really simple PSU, it might start in standby mode with nothing really powered up.

And if it does power up, then shut down, you may not see anything either.

The DC output rails should be labelled and yeah, just connect the probes to them and power up.

Even in standby mode, the PSU should still supply one of the power rails, likely a 5V one. It may be labelled 5VSBY (or something similar)

 

[dave.martin]

Yes in the photo top right corner of the board there are two plugs there right one goes to one back light inverter board and the other goes to the other end of the TV to power that back light inverter board. So yes it powers the lights but not completely since this one has seperated boards with the light inverters/ballast. I will test it out more once I get home after work.

 

[(*steve*)]

OK, keep away from the stuff that goes to the backlight. You want the connections which have nothing to do with the backlight (so it may not be those in the top right corner).

A better photo taken so we can read what is on the board (taken from directly above) would be useful.

 

[dave.martin]

ok so i might have found something and it might be nothing... but here is what you asked for and i also overlayed text on one to possibly make it easier.

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I did notice upon instpecting the relay switches under the black covers that one will make contact and the other will not even if the coil is closing the circut it will not make contact as in this photo...

[/IMG]

 

[dave.martin]

ok so i bent the one contact back in a little so that it was making contact when the coil powers but I tried it again with everything hooked up and this switch is not activating at all just stays open all the time, so i followed the rout on the other side of the board and it runs to the plus side of the big capacitors (200v 1000uF) that might be partly why I was only getting a residual reading of .1volts on the positive side/faulse reading?
Radio Shack guy says, it's the IC on the board. not much faith in a blind diagnosis. but not ruling it out either.
I will trace this circut a bit more and see where it takes me possibly a diode that is open LoL I hope...

 

[(*steve*)]

I would probably withhold judgement on what the Radio Shack guy says. As you say, blind, and they're not known for technical expertise (which doesn't say he's wrong).

With the power in at the top right hand corner, you'll find that the circuit goes like this:

The mains fuse is near the 120V input, and then the path goes to the right, passing through a mains filter that keeps spikes out and noise in.

Next you go to the relays (and I'll talk about those more later)

Then to the smaller silver heatsink (which has a rectifier on it) and then to one or possibly both of the 200V 100uF capacitors. You would expect to find around 170V across them.

It is possible that the combined 340V (at the very least 170V) makes its way to the device on the larger silver heatsink neat the middle of the board. This drives high frequency AC to the transformer which straddles the white line (T9501).

One or more outputs of the transformer are rectified by the diodes attached to the small silver heatsink at the lower left corner and the larger black heatsink on the lower right.

I suspect the lower left produces 2 supply rails, and I suspect that the three small black cylinders (with black tops) are fuses. I would check all three of these for continuity (the board will probably label them Fxxx). If this is not the problem, then checking the diodes would be my next task.

In addition to all of this, I suspect that the components just above the middle of the board on the right are the standby power supply. See if you can trace where the output of this goes and check to see if you get an output voltage here.

The relays are almost certainly used to turn on the main power supply. I suspect that the power to (at least initially) turn them on comes from the standby power supply.

With this board disconnected, it is quite possible that the standby power supply has no way to enable the main power supply.

The relays may be a little sick. However you should see or hear something if they are trying to operate.

The photos are still not sharp enough for me to read, and you'll need to take them without a flash, it makes things even harder to read.

I suspect there's quite a few components on the underside of the board. It may be worth while looking carefully to make sure there's no burn marks on anything.

 

[dave.martin]

ok so this is what I have I hope that I am testing this correctly...
I put the black lead to the main fuse this is the black wire coming in from the 120V ac source. then turning my multi meter to V AC I test along the path of filters...
this is where it gets differant.
"Then to the smaller silver heatsink (which has a rectifier on it) and then to one or possibly both of the 200V 100uF capacitors. You would expect to find around 170V across them."
I only get 122.7V AC on the capacitors marked 200V 1000uF and I get the same reading off both sides + and the - pins. At the silver heatsink with the rectifier on it the same reading on all the pins off of it.
So if I am reading this correctly the there should be more voltage at the + side of the 200V 1000uF capacitor. and if that is the case then the rectifier should be making the higher voltage? Aslo just looked up data on the silver heat sink, its a...

testing the diodes in this on I tested it backwards and got an open circut for each diode and then across them the correct way and each diode takes 0.5V DC and when going from the - to + I get a 0.9 V DC reading as much as I can tell this is working correctly, next in line is the the first 200V 1000uF capacitor and that is were I also got a 122.7V AC reading and then from there it goes to the next 200V 1000uF capacitor in a chain style setup like so + -II- - then to + -II- - sorry for the bad symbols LoLand once through both caps I still have a reading of 122.7V AC. power going into the transformer is 122.7V AC and out is 108.5V AC once through the transformer it feeds the black heatsink witch then feeds the back lighting as well as the control board plugs all of those pins are getting fed 108.5 volts I also noticed that the relay switchs are activated by the control board, so I think its time to possibly hook this board to the rest and see if i can find out why only one relay is kicking in because both are activated buy the control board with the inputs for coax cable and HDMI and such. I am at a loss at this point on what else to test? can I jump the relays to a closed possition and see if the main power has out put or is that a very bad Idea?

 

[dave.martin]

ok last post and I am heading to bed because i just tested the...Sbd ,schottky Diodes
on the black heat sink and I have flow both ways on the first one I tested I dont think this is correct and I wonder if they are bad and letting the electricity flow backwards...
here is the data on them

no matter how i hook my multimeter to them I am getting a reading, and can not get it to give me an open lead reading... are these bad or am I testing them wrong.

 

[(*steve*)]

I only get 122.7V AC on the capacitors marked 200V 1000uF and I get the same reading off both sides + and the - pins.

You should be seeing DC across the capacitors and consequently across the + and - pins of the bridge rectifier.

If not, your bridge rectifier may be dead but this would almost certainly blow the mains fuse.

I suspect that if you place the meter on a DC range you'll see around 170V

(I also suspect your meter is a very cheap one) (120 * 1.414 = 170) Your meter may be trying (and failing) to give you the RMS value of a DC voltage, assuming it is AC

testing the diodes in this on I tested it backwards and got an open circut for each diode and then across them the correct way and each diode takes 0.5V DC and when going from the - to + I get a 0.9 V DC reading as much as I can tell this is working correctly

That sounds right.

can I jump the relays to a closed possition and see if the main power has out put or is that a very bad Idea?

I think it's a bad idea at this stage.

 

[(*steve*)]

ok last post and I am heading to bed because i just tested the...Sbd ,schottky Diodes
on the black heat sink and I have flow both ways on the first one I tested I dont think this is correct and I wonder if they are bad and letting the electricity flow backwards...

...

no matter how i hook my multimeter to them I am getting a reading, and can not get it to give me an open lead reading... are these bad or am I testing them wrong.

These diodes may need to be taken out of circuit to test them.

As I said earlier, these are one of the components I tend to suspect.

However Schottky diodes do have quite a bit of leakage, and there are capacitors around there which will mean any resistance measurements will start with a low value and then climb... Are you seeing that.

If you see slightly more conductance (less resistance or a lower voltage drop) in one direction than the other then the diodes are still acting like diodes.

 

[dave.martin]

So I took some photos while testing a few things just to make sure I am testing these correctly...

testing the capacitor

I would hope that my meter is good enough to do the testing I need to do here... I paid over $200 for it a few years ago... If not let me know what Fluke number i would need and I will order one at the end of the week... I have alot of LCDs to fix just sitting around and would love to get them running again.

 

[dave.martin]

those last 4 photos are of my testing the rectifier. I am a bit confused though as to why I am not getting a DC reading at the Capacitors does this mean that the Rectifier is not working correctly, since the full AC wave is still presant at the capacitors? or possibly I have it wrong in my head as to how it goes from AC over to DC voltage...

 

[(*steve*)]

Yep, the meter is fine. I expect the issue is what/how you're measuring things.

In the first image, you are measuring what appears to be the incoming mains (perhaps there is a capacitor there). I was assuming you were measuring the voltage on the large 200V (1000uF?) capacitors.

When you get to the bridge rectifier (if indeed it is configured as such) you need to measure across the AC input (~ and ~) and see 120VAC, then measure across the DC output (+ and -) and see what you get there. In theory it should be around 170V if the filter capacitors are connected across it.

Good to see you're working with one hand. (maybe it's because of the camera, but it's good practice). Even better is to get clip on probes so you don't have to juggle things.

 

[dave.martin]

Ok so I had. A few moment to check what you said on the rectifier. On both pins with the ~ I get a reading of 122v AC with red test lead on the rectifier and the black lead on the black wire at the AC input. I get the same reading on both pins with the ~. Now when I take the meter and switch it to DC voltage testing and put the red lead on the + pin and the black on the - pin I get 0.00V DC reading does this indicate that the rectifier is bad and should be replaced?

 

[dave.martin]

ok a few more tests I have done... first I put the board back in the TV and clipped my fluke to the + and - pins of the rectifier and turned the TV on I got 341.9V DC out of the rectifier, I then went to the end of the chain of 200V 1000uF capacitors and I have nothing... doing this I used the red lead to the end of the capacitors - pin and the black lead to the - pin off the rectifier since its a known ground testing DC I think?
since my rectifier is putting out 341.9VDC and the capacitors are rated for 200V this seems out or range to me?

 

[(*steve*)]

You are measuring things incorrectly.

What you need to do is measure the voltage from one pin of those capacitors to the other.

I can tell you there *is* a capacitor (or capacitors) there with voltage across them, or you would not have read 340V DC across the output of the rectifier.

I'm sure you'll find 170V (ish) across each capacitor.

Look, it's pretty clear that you don't know how to use a multimeter. I'm beginning to be afraid that you're going to kill yourself.

 

[dave.martin]

i dont mean to scare you LOL and before when I was testing the capacitors it was not hooked up to the rest of the tv so the rectifier was not powered up completly like I had thought, now the problem is getting a probe lead to the back side of the board while everything is hooked up to get a reading off the capacitors while its turned on. this is how I will see my 170VDC reading correct?

Also I do use my multimeter alot at work but its mostly for voltage drops, battery levels, sensor resistance, and I also have a amp probe for testing charging system outputs so this is just expanding my scope or use a bit. This is why I asked before hooking the meter to each pin off the capacitor because I dont want one to explode in my face...