Help: Bad colors with 55"-Sunstar (have schematics online)

A

Adrian Glaubitz

Guest
Hi there,

I have an 5 year old 55" Sunstar-TV PAL (schematics available online, see
below, you'll probably recognize this set as a relabled Philips,Samsung
etc) which has bad colors (from Composite, Tuner and Teletext): Red appears
too weak, blue and green too strong.

But:
The set has a multi-color on-screen-display (red,green,yellow,blue letters)
and all colors of the OSD-letters are normal proving that the CRT-neck-board
and CRT are okay (thus I didn't scan the respective schematic-part).
This brought me to the conclusion that there must be something wrong
with the chroma-decoding circuit. According to the schematics, the
OSD-chip (CTV352S) is directly connected to the RGB-lines of the
CRT, together with the RGB-lines from the jungle-IC (TDA8362 PAL/SECAM).

The set came with schematics and I scanned the part for the main PCB.
I split it into eight parts due to it's oversize. Please see the
following links for the schematics:

http://home.arcor.de/adi007/part1.gif
http://home.arcor.de/adi007/part2.gif
http://home.arcor.de/adi007/part3.gif
http://home.arcor.de/adi007/part4.gif
http://home.arcor.de/adi007/part5.gif
http://home.arcor.de/adi007/part6.gif
http://home.arcor.de/adi007/part7.gif
http://home.arcor.de/adi007/part8.gif

I also already checked suspect components:

- R,G,B at CN-103 (which connects to CRT-neck-board) all measure approx.
4Volts AC
- all resistors on CRT-neck-board are okay
- R121,122,123 at IC101 (R,G,B out) are okay
- Q304,Q305,A306 are okay
- R327,328,329 at IC301 (R,G,B out) are okay
- R124,125,126 and D104,105,106 (part6.gif) do not exist, so not tested
- all on-screen-display is okay: red,blue,yellow,green letters of OSD
appear normally

I didn't check the scope-shapes with my scope yet however.

1. Do you also think it's the chroma-decoding which has failed ?
2. What can be the reason for the chroma-decoding circuit to fail ?
3. Which components do you think are suspect in these circumstances ?

I own a digital multimeter (for Ohms, Volts AC/DC, Amps AC/DC, diode- and
transistortesting) and a 10MHz, 1 channel scope.

greetings,
Adrian
 
You need to first PROVE that the problem is either before the jungle
ic or after the jungle ic to the red crt. The fact that the osd seems
to look ok does not prove that the problem is not in the final video
drive stage.

You need to swap the red and green drive signal just past the jungle
ic to verify that the green is now weak and red too strong, this will
prove it is in the signal path before the point where you switched it.
If the red stays weak, you have now proven the problem is past the
point in the signal path where you switched it.


Adrian Glaubitz <adi007@germanynet.de> wrote in message news:<brkqfe$4ahnu$1@ID-206674.news.uni-berlin.de>...
Hi there,

I have an 5 year old 55" Sunstar-TV PAL (schematics available online, see
below, you'll probably recognize this set as a relabled Philips,Samsung
etc) which has bad colors (from Composite, Tuner and Teletext): Red appears
too weak, blue and green too strong.

But:
The set has a multi-color on-screen-display (red,green,yellow,blue letters)
and all colors of the OSD-letters are normal proving that the CRT-neck-board
and CRT are okay (thus I didn't scan the respective schematic-part).
This brought me to the conclusion that there must be something wrong
with the chroma-decoding circuit. According to the schematics, the
OSD-chip (CTV352S) is directly connected to the RGB-lines of the
CRT, together with the RGB-lines from the jungle-IC (TDA8362 PAL/SECAM).

The set came with schematics and I scanned the part for the main PCB.
I split it into eight parts due to it's oversize. Please see the
following links for the schematics:

http://home.arcor.de/adi007/part1.gif
http://home.arcor.de/adi007/part2.gif
http://home.arcor.de/adi007/part3.gif
http://home.arcor.de/adi007/part4.gif
http://home.arcor.de/adi007/part5.gif
http://home.arcor.de/adi007/part6.gif
http://home.arcor.de/adi007/part7.gif
http://home.arcor.de/adi007/part8.gif

I also already checked suspect components:

- R,G,B at CN-103 (which connects to CRT-neck-board) all measure approx.
4Volts AC
- all resistors on CRT-neck-board are okay
- R121,122,123 at IC101 (R,G,B out) are okay
- Q304,Q305,A306 are okay
- R327,328,329 at IC301 (R,G,B out) are okay
- R124,125,126 and D104,105,106 (part6.gif) do not exist, so not tested
- all on-screen-display is okay: red,blue,yellow,green letters of OSD
appear normally

I didn't check the scope-shapes with my scope yet however.

1. Do you also think it's the chroma-decoding which has failed ?
2. What can be the reason for the chroma-decoding circuit to fail ?
3. Which components do you think are suspect in these circumstances ?

I own a digital multimeter (for Ohms, Volts AC/DC, Amps AC/DC, diode- and
transistortesting) and a 10MHz, 1 channel scope.

greetings,
Adrian
Click to expand...
 
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