Entertainment Center: TVs, Stereos, VCRs and DVDs - JVC 20" screen shrinks to half-height

I have a JVC 20" color tv made in '92 and after about 5 minutes of warmup, the screen shrinks to about half-height in the center of the screen. It stays full width and does not roll or do anything else abnormal, just squishes the picture.

A good swift hit on the lower right side of the cabinet restores the full picture for about another 5 minutes maybe. I began to feel a bit like Ralph Kramden by getting up to bang the side of my tv to restore the picture.

I pulled the back off, blew it clean with compressed air. Full picture for another 5 minutes. I found that wiggling the PC board would also restore the picture for about 5. It seemed to be more sensitive on the power supply side than on the flyback xfmer side.

I cleaned the pins on the crt with wd-40, cleaned every connector the same way...no success.

I pulled the board out to where I could see the solder joints. Fired up the soldering iron and reflowed all joints on the power supply side of the board, and near the flyback...as well as any that might have look cold. There are no visible cracks, and the board is clean and corrosion-free.

Is there a quick fix for this before I dumpster it (or at least a Rube Goldberg mallet-swinging contraption already invented that will hit the side of my tv every 5 minutes)?

Thanks in advance

Mark in NH

markjmay, Check the solder joints on the pins of the vertical output I.C. to see if there cracked. This I.C. is a mulit-pin and is mounted to a metal heatsink. If they are cracked resolder them, if they are not resolder them.



Fred:)

Old Smokey concurs. When you can rap on the side of the set and correct the problem, the problem is usually a bad solder connection.

Take off the back of the set and power it up.
Use the bedroom mirror so you can see the picture from the
back of the set.
Use a non-metallic wand (not a pencil) and poke around the
vertical circuit.
When you hit the sensitive spot that causes the picture to change, mark it. Then power down the set, unplug it, etc.

Use a low heat soldering iron (not a soldering gun) and a little
60-40 electronic solder (all available from Radio Shack) and
resolder the connections on the board in the "sensitive area".

Power up the set and check it again. You may have to do this
three or four times until you hit the right solder joint.

When everything is solid, put the back on the set, turn it on to the local football game, have a beer, and enjoy life as it should
be.

Smokey ;)

FreddyG and Smokey,
Thanks for your quick response. There is an IC attached to a rather significant heat sink. Without a schematic, I'm guessing that it is the vertical chip you referred to. It's a 4 pin chip in a TO-220 like package, maybe a little bigger. Those were among the pins that I reflowed. Perhaps I'll try them again.

I did the whole mirror thing with a plastic poker. The difficulty is that once the picture comes back, it's another 5 minutes to wait for it to wig out again. That make the poking thing a painfully long process.

Maybe it's time for a can of freeze spray?

Mark in NH

Mark, This I.C. has at least 7 pins and there will be two letters, either LA or AN then some numbers on the face of the I.C.. The vertical circuit is usually in front of the high voltage/horizontal circuit area of the board.



Fred:)

Fred,
I found the vertical IC. There are a couple of them with LA prefix. Reflowed all the joints as well as nearby discrete components. The shrinking-screen problems looks like it is fixed. However, it seems that I traded one problem for another. The on-screen info for channel #, volume, and menu no longer works. The info was working when I had the shrunken-screen problem.

Does that same chip generate on-screen info?

Once I realized my new problem, I reflowed and reflowed and reflowed again all components in that area. Picture is still full height, but still no info display.

I've done alot of soldering in the last 30 years, so I'm pretty confident I didn't destroy anything from heat.

Any ideas?

Mark in NH

Recheck all the areas you resoldered and very closely look for any solder bridges. Especially if you soldered any pins on the I.C. chips.


Fred:)

Well I went away on business for 3 days. When I returned on the weekend, I turned on the tv and the onscreen info worked properly! How bizarre. I put the back cover back on, hooked it up to cable, ran it for about 3 hours...still works fine.

Perhaps flying out of Boston for business on 9/11 and returning on Friday the 13th had some effect on my TV!

Thanks for all the help on this forum.

Mark in NH