Sylvania TV dark

[Gregory T. Buchar]

I'm working on a 1993 Sylvania (25S104 chassis) TV. It's picture is too
dark. The problem happened suddenly.

With either an RF or a composite signal, the picture appears very dark,
with red and blue highlights. With signal removed, picture is made up
of black and white dots, but there is no grey background. sound is OK,
horiz and vert size is OK. HV is around 27KV. Filament has 6 V.
Screen has about 385V.

In examining the schematic, I see a transistor labeled ABL. It connects
to the flyback and the (chroma, luma, sweep) chip. Is this some sort
of brightness limiter? On the schematic, it shows 17v on the base and
2.7v on the emitter. I only measure 4 v on the base. The emitter
voltage is OK. An Ohm check does not show the transistor to be shorted.
What could I check next?

 

[James Sweet]

"Gregory T. Buchar" <Gregory.T.Buchar@grc.nasa.gov> wrote in message
news:402A9C92.5040703@grc.nasa.gov...

I'm working on a 1993 Sylvania (25S104 chassis) TV. It's picture is too
dark. The problem happened suddenly.

With either an RF or a composite signal, the picture appears very dark,
with red and blue highlights. With signal removed, picture is made up
of black and white dots, but there is no grey background. sound is OK,
horiz and vert size is OK. HV is around 27KV. Filament has 6 V.
Screen has about 385V.

In examining the schematic, I see a transistor labeled ABL. It connects
to the flyback and the (chroma, luma, sweep) chip. Is this some sort
of brightness limiter? On the schematic, it shows 17v on the base and
2.7v on the emitter. I only measure 4 v on the base. The emitter
voltage is OK. An Ohm check does not show the transistor to be shorted.
What could I check next?

Is there a resistor in that circuit? I just recently fixed a nice Toshiba
with the same problem, someone pointed me to the ABL circuit, something I'd
never heard of before, but it got me looking in the right area, turns out a
resistor was open.

 

[Ampdoc]

ABL = Automatic Brightness Limiter
Check whatever feeds the base of that transistor, and work back until you
find the fault. Try lifting the ABL pin on the Jungle IC (chroma, luma,
sweep), if the IC has shorted internally your voltage should come up.


--
Jammy Harbin
J & J Electronics, Inc.
227 S. 4th St.
Selmer, TN 38375
731-645-3311

"Gregory T. Buchar" <Gregory.T.Buchar@grc.nasa.gov> wrote in message
news:402A9C92.5040703@grc.nasa.gov...

I'm working on a 1993 Sylvania (25S104 chassis) TV. It's picture is too
dark. The problem happened suddenly.

With either an RF or a composite signal, the picture appears very dark,
with red and blue highlights. With signal removed, picture is made up
of black and white dots, but there is no grey background. sound is OK,
horiz and vert size is OK. HV is around 27KV. Filament has 6 V.
Screen has about 385V.

In examining the schematic, I see a transistor labeled ABL. It connects
to the flyback and the (chroma, luma, sweep) chip. Is this some sort
of brightness limiter? On the schematic, it shows 17v on the base and
2.7v on the emitter. I only measure 4 v on the base. The emitter
voltage is OK. An Ohm check does not show the transistor to be shorted.
What could I check next?

 

[Gregory T. Buchar]

Sounds like I need to further check in this area. Is it common for the
jungle IC's to go bad? Or is it usually a bad component? Lastly, are
there any websites where I could read about the ABL circuit?

Ampdoc wrote:

ABL = Automatic Brightness Limiter
Check whatever feeds the base of that transistor, and work back until you
find the fault. Try lifting the ABL pin on the Jungle IC (chroma, luma,
sweep), if the IC has shorted internally your voltage should come up.