[Duo2400] Re: ON Topic: Inverter Board

[Jani Tiainen]

> > Just curious:  what is the function of the inverter board in Duo
> > screen?  I know that inverters convert DC to AC and/or vice versa.
> > Why is there an inverter in the screen?

Actually AC -> DC converted is usually called (full/half) rectifier
(bridge). But that's another story.

> > Is the function of the inverter board in the
> > display to turn DC back into AC for the Cold Cathode Fluorescent
> > backlight bulb?  If that's the case, how does one test for a bad
> > inverter board as opposed to a bad CCF?
>
> The inverter board takes a DC input (12V?) and outputs a high voltage
> (90-150V AC) to power the backlight, if the bulb is failing then it
> will still glow, however it may be quite dim and often pink! However
> if the inverter has failed then the bulb will not light at all.

Let's add that AC output is usually that 90 - 150AC, and a very high rate
(something like 20 - 100 kHz)

> I have found that the DC fuse on the inverter board quite often fails
> due to old age, however the switching transistors, the transformer,
> and the output capacitor are all common failure points.

Failing transformer usually can be detected from high pitch sound.

> Please be careful testing the inverter board, although the current is
> quite low (a few ma) it can still be enough to give you quite a bolt,
> (powerbooks do not appreciate being convulsively flung across the
> room!!) also without a bulb to load it the output can rise to 400V or
> so.

Best way (IMO) to test is keep inverter connected to CCF, use DC big enough
voltage range for input and measure voltage level between connection points
where wires from inverter board goes to CCF bulb. If you should get half of
real voltagelevel, but still it's pretty informative. You probably don't get
nothing if inverter board is broken (or get very low readings).