Dell 24" LCD monitor

[ppppenguin]

I have 2 of the excellent Dell 2408WFP 24" LCD monitors. One of them has taken to going into standby randomly for a few seconds. Usually it recovers by itself, occasionally I have to power cycle it. With some difficulty I opened it up. The power supply electrolytics all look OK, no bulges, but I suspect a mass replacement is in order. This approach certainly fixed my old LaCie monitor.

Anything else I should look at before doing the caps?

[ppppenguin]

One of the electrolytics, up at the mains end of the board, is 1uF 100V. I'd be inclined to fit a non-electrolytic unless there's some very good reason not to.

The tally on the PSU board is:

1x 150u/450V
5x 470u/35V
2x 47u/50V
1x 1u/100V

I may have some stock of HF rated 47/50.

Order to RS going in soon.

[Mark Hennessy]

Looks like it has active PFC, so two in series. I was initially surprised at that, but remembered how old it is - so before LED backlights - and apparently the maximum power draw is over 100 watts! I trust the tubes are still OK. We see large variations in light output with monitors of that sort of age, depending on usage as as you'd expect. It's worth looking at the inverter while it's all apart. Likewise, any local decoupling on the signal board.

In addition to caps, high value resistors exposed to high voltages are always suspect in these. Anything greater than a few 10s of kΩ can fail high. Usually these cause start-up problems - which you haven't reported - but even so, while the thing is on the bench...

[ppppenguin]

Looks like it has active PFC, so two in series.

2 what?

The backlights are fine. Very occasional starting problems when cold so I'll check the high value Rs.

No evidence of overheating that I've found anywhere so that's good.

[Mark Hennessy]

2 power supplies.

The first is a boost converter. This sits between the bridge and the big smoothing capacitor, and is a switch and diode (on heatsink) and inductor. It steps up essentially unsmoothed 100Hz from the bridge to a nominal 380V or so. But because this is active PFC (rather than a straightforward boost converter), the control circuit varies the duty cycle to draw a current from the bridge that is approximately proportional to the voltage from the bridge. In other words, it tries to look like a vaguely resistive load to the mains.

General safety note: This boost converter does not offer galvanic isolation.

Point of interest: The boost converter can be bypassed completely - it sits between the existing bridge and smoothing capacitor in a standard SMPSU. Useful to know for fault-finding occasionally, but more interesting to reflect that active PFC is very easy for a manufacturer to retro-fit to existing designs. On many PC power supplies a while back, if you had it, you'd have a sticker on the outside proclaiming as such, and a small daughter board added inside for just that purpose. Just a MOS-FET and diode on a small heat sink, an 8 pin controller chip (e.g. the MC34262), and usually a half-decent 1µF poly cap on the input.

The next stage is the conventional step-down (and isolating) SMPSU. Often a flyback type; might be half-bridge. It looks like there's only 1 device on the appropriate heat sink (second from left), so I'd suspect the former. The big transformer and feedback optocouplers (and EMC cap) crossing the isolation barrier is clearly visible.

It does look like a well engineered PSU. Some Dell stuff is very good. I speak very highly of my newer U2412 (LED backlight, IPS screen).

[ppppenguin]

Not only is there active PFC but there's another PCB full of stuff between this PSU and the mains input connector. That's all passive components that look to be doing mains filtering. Dell have clearly put a good deal of effort into doing things properly in this monitor. And it shows in the results which are excellent. A modern monitor may be thinner. lighter and use less power but I doubt the pictures will be any better. The 2408 has excellent viewing angle, not far short of a CRT, and passes this suite of tests with very few problems: http://www.lagom.nl/lcd-test/ The response time may be a bit slow for gamers but I've never seen any problems with video.

[ppppenguin]

Fitted a new set of caps yesterday. So far so good.....

This monitor is certainly good enough to justify a bit of work to keep it going. A nasty 15" LCD with poor viewing angle etc would have been binned.

One day I'll try to get the colorimetry of my pair of 2408WFPs to match. They sit next to each other and it's slightly irritating that things look different on each of them. I've tried in the past and got frustrated. I can line up the grey scale on a conventional colour TV or monitor but these have been tricky to adjust.