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LCD monitor - PSU ticking (no startup)

I trash-picked an NEC LCD1700M+ 17" LCD monitor (apparently a rebadged Acer) the other day, and it has what appears to be a PSU fault. When plugged in, an inductor in the PSU begins to "tick" at about 1.5Hz, and the power LED blinks at each "tick".

I'm not familiar except at the vaguest block-diagram level how such a PSU works, so I am not sure where to begin debugging this problem.

The PSU is a separate board that also integrates the audio amplifier. It takes 110-240VAC in and provides +12, +5, +3.3 rails according to the silkscreen. When I put a scope on any of those outputs, I see a ~1V sawtooth riding on a constant voltage.

There were a bunch of 1000uF/16V electros that were bulged and nasty (presumably the infamous bad electrolyte scandal from some years back). Since I didn't have the correct voltage rating 1000uFs to hand, I put two 470uF/16V in parallel at each of these locations.

My next thought is to remove the PSU board and put a dummy load corresponding to say 250mA on each of those outputs and attempt to power up the PSU with nothing connected, but I'm pretty sure I'll see exactly the same symptom - i.e. I believe the problem is in the PSU not the rest of the electronics.

Any hints, general things to try, etc?

Reply to
zwsdotcom
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? Sounds like it is cycling on fault.

Reply to
Homer J Simpson

Something is surely cycling. But I believe it is the PSU, not the microcontroller.

Reply to
zwsdotcom

The LCD monitors I've looked at had a pair of PC boards. A SMPS, and a video board. Check the electrolytics on the power supply board for high ESR. As far as "Acer, rebadged as NEC", NEC is an OEM, while Acer isn't.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
Reply to
Michael A. Terrell

OK, thanks.

The exterior of the monitor is labeled NEC... the PCB artwork has the Acer logo in it.

Reply to
zwsdotcom

If you cannot fix the psu but want some life out of the monitor, solder a 4 wire to a computer psu, i did that with an 18 inch eizo and it's still happy after a year.

Reply to
Bart Bervoets

I've temporarily hacked the monitor so it will run without the 3.3V rail (it only wants about 25mA on that rail, and there are THREE 3.3V LDOs on the board with spare capacity - I don't know why they used that rail at all). But this means I am without the on-board amplifier, since that is powered directly off the PSU board (the amp is actually on the PSU).

Reply to
zwsdotcom

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