Re: gEDA-user: Any TV repair gurus lurking?

[Richard Rasker <rasker@xxxxxxxxxx>]

Op vrijdag 21-12-2007 om 20:52 uur [tijdzone +0000], schreef Peter
Clifton:
> On Fri, 2007-12-21 at 15:17 -0500, Darryl Gibson wrote:
> 
> > This is a very weird set of problems.
> > 
> > RGB lines indicate a failure of the vertical amp, which the shrinking
> > picture also indicates.
> > 
> > Check the coupling capacitor between the vertical amp, and the yoke?
> 
> Thanks, will do when I've got the set open again. It doesn't seem to be
> AC coupled. Do you mean the supply cap to the vert. amplifier?

Hello Peter,

I'm an experienced TV repair technician, and I've encountered similar
problems with this chassis.

Apparently, there's something wrong with the vertical drive pulldown or
its negative supply. Especially when the picture gets compressed and
shifted to exactly the upper half of the screen, a supply problem seems
the most likely candidate. A very common cause is a bad solder joint on
the V-drive IC(*).

These are the parts:
V-drive IC: IC500 (STV9379)

Positive 15V supply: flyback pin 6 - R509 (0.47 ohm) - L503 - D503 -
C520, C515 (470uF/25V) - IC500 pin 2

Negative 15V supply: flyback pin 8 - R510 (0.47 ohm) - L502 - D502 -
C517, C506 (470uF/25V) - IC500 pin 4

Also check C510 (220uF/50V flyback pulse cap)

I'd start by checking the PCB for bad solder joints -- use a small
flashlight and if necessary a good quality magnifying glass, as not all
fractured joints are as obvious as the ones below. If nothing is found,
switch on the TV and see if it's sensitive to vibrations -- lightly tap
the board and the components between the flyback transformer and IC500
with a bit of PVC tube. If this doesn't provoke the fault, there's
probably a non-mechanical failure. Most likely candidates are IC500
itself, R510, and D502; after this, the actual V sync signal (TP19) is
suspect.
You can measure the DC voltages behind D502, or (with an oscilloscope)
the flyback AC voltages before D502. Also, you can swap D502 and D503 if
you want to see whether D502 saometimes fails.

*: This happens a lot with components which are mounted on a vertical
heat sink. As the component heats up and cools down, its pins expand and
contract, causing the solder joint to fail. I routinely check all such
components on all boards. You'll find a clear example here:
http://www.linetec.nl/electronics/badsolder2.jpg

Other candidates for bad solder joints are larger pins in general,
because these conduct the heat away from the solder joint in the
manufacturer's wave soldering process, causing 'cold', brittle joints
from the onset. And don't forget to check the SCART connector too.

Hope this helps,

Best regards,

Richard Rasker



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