The bigass TV in the living room gave up the ghost the other day, but since it's such a bigass TV I'd like to try and fix it if possible. The sound still works but I don't get any picture, just a 1/4" line in the middle of the screen. Bumping the side of the case got it to go fullscreen for a split second, but then the screen just went black. I suspect it's a loose connection or something, but I'm not familiar with newer TV's enough to know where to start. Any ideas?

1st: don't turn on the set anymore unless you want to permently burn the line into the screen. The vertical sweep has failed.

2nd: Open it up, stay away from the picture tube and large Red lead, get underneath the board and start re-soldering joints. Giving it a smack to get the picture back reveals bad solder joints. YOu will need no more than a 30 watt pencil iron and some 60/40 solder.

If you're lucky, the sections will be printed on the top of the circuit board (Vertical, color, sound, etc). Find the vertical, then go underneath and resolder all the bad joints you find. Might as well do the whole section to be sure.

I agree. I have fixed lots of JVC (and Panasonic) with this problem. The pads to solder the vertical IC to are very small, and the tiny bit of solder on it often cracks. Resolder them and you'll probably be back in business.

I find it helpful to check my work with a magnafying glass to be sure there isn't a solder bridge between pins. Sometimes I use a Q-tip dipped in acetone to clean the flux off of the connections before checking with the glass. Dont want to make things worse :nono:

another thing: WHen resoldering around the ICs, be very careful not to keep the heat on the pins long. i would not keep the iron on the IC pins more than 3-4 seconds, and give about 10 seconds to cool between each pin.

If you heat the IC up too much, it will be damaged, and you will have more trouble on your hands.

I recall in 35" JVCs that there is a B+ line that gets cold solder joints causing the vertical to collapse. This is a bit tricky to explain to anyone who isn't into TV repair, but basically you need to go over the whole circuit board on those sets to resolder many bad connections. In particular, the vertical IC, the B+ resistors, the scan diodes, and any that you see with a bad connection. But don't try this if you aren't comfortable and experienced with fine PC solder work. Better left to a tech.

One more point on that set: There is a fire hazard with the degaussing thermistor also having bad connections. That is a 1/2" cube shaped part with 3 pins that is near where the AC line cord plugs into the board, and also where a white 2-wire plug is, which has 2 wires going up to the degaussing coils around the outer rim of the picture tube. The 3 pins that are soldered underneath the board are most likely black and cracked! That is a MUST to be resoldered!

Charles

Awesome, sounds like something I can do pretty easily. I figured it was a vertical problem, but I wasn't expecting something as simple as bad solder joints. Guess I'll fix it up after I get some sleep and hopefully be back in business...

One general tip on locating the area of the board where the bad joints are: Get a wooden Q-tip, or some kind of a nonconductive thing and gently tap around on the board while somebody is watching the TV. That way you can get an idea of where it is.

If you can't find the vertical output IC, which is likely an AN-5521 or LA7838, etc, follow the yellow wire out of the deflection yoke on the neck of the tube. It will go to a plug on the board, and lead you directly to the vertical output. The blue and red wires are the horizontal, and the yellow and (white or black) are the vertical. The vertical output IC will be a single inline IC, mounted on a fairly big heat sink.

Just a general caution: you mention you aren't familiar with NEWER sets, so I assume you're familar with the high voltage, etc etc in general same as old sets. Be careful :thmbsp:

Charles

Yeah, last TV I worked on was a '70s Zenith, so I'm all too familiar with the standard cautions. Getting zapped with the high voltage really sucks, don't wanna do that again.

I've already located the vertical output section and the IC (I love the labels on the board, makes things so much easier), but I'm just gonna pull the entire board out and work on any dodgy joints I find, since it sounds like the factory did a shitty job on a lot of stuff. Only thing I'm having a little trouble with is getting that large HV lead loose from the horizontal output. Do I take it loose from the tube, or should it come off of the big capacitor/transformer thing? And more importantly, how do I keep it from lighting my ass up in doing so?

You need to discharge the HV supply, then remove the HV lead from the CRT. Here's how:

You will need a LONG (at least 12") preferrably thin flat blade screwdriver and a lead with alligator clips at either end. Its always nice to have a rubber mat to stand on to isolate you from earth in the event you make a mistake.

1. Connect one lead to the ground strap on the aquadag coating of the CRT. its usually a thin metallic strap. DO NOT CONNECT IT DIRECTLY TO GROUND OR YOU WILL TOAST YOUR ICs. The ground coating on the CRT slowly bleads off the HV.

2. connect the other end to the screw driver about halfway up the shaft. Now slowly slip the screwdriver under the rubber cap at the top of the CRT while keeping it towards the middle. You should run into the Bare HV lead underneath and hear a healthy SNAP. Keep the screwdriver there for AT LEAST 30 seconds, I would do at least a minute. This will slowly discharge the HV.

3. carefully lift up the cap on top of the CRT and see how the HV lead connects. Usually you have to pinch the clip to get it out of the CRT. I usually gently push on one side of the clip (with the screw driver) until it its out of the hole, then gently pull it in the opposite direction to get the rest of it out.

While you are at it.........

There is a capacitor that WILL need to be replaced on your JVC.

It is in the pump-up ckt for the output of the chip. By now, it is surely leaky and of high ESR.

look for a 100uf at 35 volt capacitor off of the la7830,36,38, 7841 or whatever output it uses. It will have one lead connected to the cathode of a diode,(with B+ on it) and the other lead will go to the IC.

I could explain what the "pump-up" ckt is, but to put it in layman's terms, it gives the vertical output ckt. some "free" power obtained from the yoke during retrace period. IOW, it is like a "turbocharger on a small engine," that gives it way more power than it normally would produce. But when it goes bad, at first, it will cause a shrinking picture, probably with lines at the top or middle, and then, it WILL kill the chip.

So, while you are in there- REPLACE that cap!!

Ok, so first of all, I resoldered the cracked joints I found, and now I have no picture at all. The CRT still lights up for a split second when turning it on or off, so it doesn't seem to be a CRT problem, but it just keeps a black screen and tapping on the board makes no difference now.

I'll try that cap, see if it makes any difference, but is it more likely that the chip is just dead?

Honestly, if this gets too involved I think I may just take it in to a repair shop... I really don't know what I'm doing around small components like that, nor am I really properly equipped for working on things this small...

Looks like you bridged a couple of connections there.

Thought that was a possibility, but I checked with a magnifying glass and saw nothing wrong. The connections seem solid and are not shorted, all the little plugs were soldidly back in the right spots, and I saw nothing else that was an obvious fault. So I kinda doubt it's something I did to it... maybe there's a component somewhere that flaked out because of the shoddy solder connections?

Look around the video section. If your still seeing light on the screen even for a moment, and the HV is still there (hair stands up when you put your arm near the front of the set) then you most likely have lost video output.

Check the video section for bad joints.

Yeah, the HV is there, but I couldn't find anything obvious wrong with the video section. No cracked joints that I found, everything seems to be plugged in ok... So I dunno. Maybe I'll take another look at it later, but otherwise I'm just gonna take it into someone who knows better what he's doing...

Any display graphics (channel number, volume bar, etc)?

Did you bump any switches, such as possibly one on the CRT socket board (can't remember if JVC has those)? Some sets have a video kill switch for helping in set up the gray scale. I thought JVC has one, but I can't remember if it does, and if so, whether it's down on the main board near the vertical, or on the socket board.

Charles

Didn't see any switches or anything, though I'll check on that... I suppose it's possible one of the vertically-mounted video cards got jarred loose or something though. I'm not getting any display... no OSD, no static, nothing.