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I am trying to repair a laptop where the back light has gone. I have tested the lamp by plugging it into another laptop inverter, the bulb lights up and is fine.

I have purchased a "tested" inverter board off of ebay but it did not fix the problem. I have tested the input pins for voltage. One of them reads 19 volts (pin 1 I think). Pin 2 in blank, pin 3 is 0.7 volts, pin 4 is 3.4v and pin 5 is 0v (presume this is negative).

Do these voltages seem right, in which case this would point to a faulty inverter wouldn't it?

The laptop is an Advent 7301.

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migrated from May 8 '11 at 22:23

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up vote 1 down vote accepted

From what I have read from researching this you typically have the following signals:

  • Vin - ~20V
  • Vcnt - 0V to 5V - low = disable, high = enable
  • A-dim - Analogue brightness control - anywhere between 0 and 3.3V - 0V is brightest, 3.3V is darkest
  • P-dim - Pulse Width Modulation brightness control - square wave between 0V and 1.6V
  • GND - 0V ground connection

A-dim and P-dim may be both there or only one there.

Looking at your voltages it could be that the Vcnt is in the low state disabling the inverter.

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Thanks for replying - on the 5 pin on the connector I only have 4 wires, pin 2 is blank. Of the 5 you mention, which one am I likely missing? – Fly_Trap May 9 '11 at 8:34
You probably have either A-dim or P-dim but not both. They both do the same thing but in different ways. I'd expect to see 20V(ish), 5V(ish) and somewhere between 0 and 3V on the 3 wires that give a reading. – Majenko May 9 '11 at 12:15
Thanks Matt, that makes sense. In which case I have the following: Vin=19V, Vcnt=0.7V, dim=3.4V. I'm assuming that due to the low voltage (0.7V) on my Vcnt that this is causing the inverter to fail, does this sound logical? I'm told that this model has a common problem with a voltage regulator chip. Could this be related? – Fly_Trap May 10 '11 at 11:54
Vcnt at 0.7V will be telling the inverter to switch off. As for your voltage regulator chip - I really can't say. – Majenko May 10 '11 at 18:13
Hypothetically speaking, if I were to bridge the dim 3.4V to the Vcnt 0.7V, would that be enough voltage to enable the inverter? Or am I likely to blow the mobo? – Fly_Trap May 12 '11 at 10:10

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