LG f700b raster problem
If the lines are slanted, you are seeing 'retrace' of the horizontal sweep.
The distortion at the top is probably caused by a failing electrolytic capacitor in the vertical deflection circuit or in the power supply filtering for that circuit.
If you want to try to find this problem on your own, use great caution! There are exposed parts that can carry very nasty if not fatal voltages.
Go to any electronics parts source and buy two cans of 'freeze spray.'
Expect to pay $4-5 dollars each for them.
Using this on a 'live' monitor makes it still a little more hazardous since the cooled parts will form frost that may be conductive and I recommend keeping one hand behind your back so even if you touch something 'hot' you won't have currents passing through your heart.
With the rear case removed, you might drape a clean towel over the guts to let it warm up as it would if cased.
When you notice the distortion going away, remove the towel and start spraying short bursts at the cylindrical parts, mostly those of 'medium' size ranging from ~ 3/4 to 1 1/2 inches in height and 1/2 to 3/4" in diameter.
These are the parts with values most likely to be failing.
If you can identify the power transformer, a largish block, mostly black, check those capacitors first.
You will need a mirror to watch the screen for the worsening of the distortion again and be patient; the parts you are cooling have some thermal mass so the effect is not immediate.
If you find one that seems to affect the picture, then, if you have one, direct a hair dryer (blower type) at that area to see if you can recreate the problem several times.
If you can isolate one specific part, note the value of it; it will have several printed items:
- Capacitance - this will be such as this: '100 uF'
- Voltage - '63V'
- Polarity - this will be marked with a '+' and maybe a minus on the other leg. This is important! Any new part must be installed with the correct polarity if any is shown and it will be present on this type of part.
These parts are very inexpensive so if you cannot decide which one is at fault, replace all of them in that area; they cost from 20 cents to a buck.
If you have no local electronic parts store (Radio Shack doesn't have much any more) you can order them from Mouser Electronics.
Mouser has NO minimum charge and adds only real postage/shipping charges.
You'll find them at mouser.com
If you decide to go this route, you will have to acquire and learn to solder with a soldering iron.
Or if you have a friend who is an amateur radio operator, they will probably do this for you.
Removing the board from the bottom of the case is pretty easy, just make sure the monitor has been OFF for a while otherwise removing the thick cable from the CRT will be a really memorable experience!
Looking at the bottom of the board, note the look of the other solder joints there and do your best to duplicate them when installing the new part(s), bad solder joints can make your repair futile.
Or, you can stuff the wole thing in an envelope and send it to me ;-)
If you have been happy with this unit, it is worth the repair, since LCD monitors are often dead soon after the warranty has passed on.
Jul 26, 2009 |
LG Flatron F700B 17" CRT Monitor