I fixed this one (replaced inverter board) a year or two ago. Now the
backlight is intermittent. On examination, D22, near the video
connector on the motherboard, appears to be toast. It's, of course, an
sm device with no visual clue of it's value.

The four terminals on the inverter a
Pin 1: Ground
Pin 2: +3.3 volts
Pin 3: The variable leg (1.8 volts backlight off, 2.8 volts on)
Pin 4: +19 volts

Metering reveals that one end of D22 is connected to ground, the other
to Pin 3 on the inverter (through the video flex cable). Touching D22
turns the backlight on, even though one end appears to be nearly burnt
away. Shorting Pin 2 to Pin 3 has the same effect, although it cannot
be dimmed in this state.

Would I be correct in assuming (actually 'suspecting') D22 is a zener
diode, somehow involved in dimming the backlight? Would the 2.8 volts
measured at Pin 3 be a clue as to it's value; or am I totally off base?

Any other ideas as to a suitable replacement; or should I just stick
something like a 1n4148 across there as an experiment? It's hard to
power the unit up disassembled, but it could be done.....

jak
jak

jakdedert wrote:
I fixed this one (replaced inverter board) a year or two ago. Now the
backlight is intermittent. On examination, D22, near the video
connector on the motherboard, appears to be toast. It's, of course, an
sm device with no visual clue of it's value.

The four terminals on the inverter a
Pin 1: Ground
Pin 2: +3.3 volts
Pin 3: The variable leg (1.8 volts backlight off, 2.8 volts on)
Pin 4: +19 volts

Metering reveals that one end of D22 is connected to ground, the other
to Pin 3 on the inverter (through the video flex cable). Touching D22
turns the backlight on, even though one end appears to be nearly burnt
away. Shorting Pin 2 to Pin 3 has the same effect, although it cannot
be dimmed in this state.

Would I be correct in assuming (actually 'suspecting') D22 is a zener
diode, somehow involved in dimming the backlight? Would the 2.8 volts
measured at Pin 3 be a clue as to it's value; or am I totally off base?

Any other ideas as to a suitable replacement; or should I just stick
something like a 1n4148 across there as an experiment? It's hard to
power the unit up disassembled, but it could be done.....

jak

Under extreme magnification, I can make out a single character "T" on
the uncharred end of the device. The only other diodes I can find on
the board are marked 'T4'. That is, unless the '4' is really a
triangle...can't really see the 'tail' on the digit; but the top part
looks exactly like the top of the 4 in the font I'm using to compose.

Does that ring a bell with anybody?

Thanks in advance,

jak

jakdedert wrote:

Any other ideas as to a suitable replacement; or should I just stick
something like a 1n4148 across there as an experiment? It's hard to
power the unit up disassembled, but it could be done.....

jak

Under extreme magnification, I can make out a single character "T" on
the uncharred end of the device. The only other diodes I can find on
the board are marked 'T4'. That is, unless the '4' is really a
triangle...can't really see the 'tail' on the digit; but the top part
looks exactly like the top of the 4 in the font I'm using to compose.

Does that ring a bell with anybody?

Thanks in advance,

jak

Okay, answering my own posts again...it's late. Google gives me this:

http://www.himfr.com/d-p113966738622199500-Diodes_SMD_1N4148/

which is indeed an sm 1n4148, around the correct size as nearly as I can
judge, and is marked--according to the listing--'T4'.

Unless somebody has a caution, tomorrow (today), I'll bridge this one
with a leaded 1n4148, and hopefully have this thing back together and
finally be done with....

jak