Dim display backlight (CCFL)

[ebastler]

I bought a rather old notebook computer -- a 1992-designed Siemens/Nixdorf PCD-3NCsl, which is actually a rebranded Panasonic CF-480C. That model was my first notebook long ago, I wrote my thesis on it and took it abroad for my postdoc year in Australia, so I have nostalgic feelings about it. A 386sl CPU with a TFT color display, which was a extravagant back then. (It made up for that by being quite small, an 8.4" VGA display which looks a bit lost in the notebook's lid. )

The "new" unit I got now has apparently seen very little active use: It has a 1993 production date, and the only files I found on its hard disk were from 1994. (It was used in a bank, apparently by someone who did not appreciate computers much. Its cosmetic shape is great as a result, including an as-new carry bag.) But it has of course been sitting in storage for nearly 30 years. The unit was known to be non-working, but with some care and feeding it is largely working again now: Basically everything electrolyte or rubber-related had worn out: capacitors, batteries, drive belts, hard disk bumper.

But the TFT display is not its former self yet: The backlight is rather dim, and also pulses slightly. It is a CCFL backlight with two thin tubes, each about 20 cm long. I have already replaced the leaky electrolytics in the high-voltage supply and the logic control board; this has fixed some shadow/stripe issues the display had before, but has not done anything for the brightness.

I have not had to fix one of these yet and am unsure about a few things:

• Do CCFL tubes age and get dimmer in storage, or should that only occur during operation?
• I tried to measure the voltage across the CCFL connections with a multimeter (1 kV range, 10 MOhm/50pF input), but the voltage collapsed immediately. Is that to be expected, or is it a sign of an issue with the supply?
• Is there anything else I could measure or otherwise test to narrow down the cause? (Tubes vs. supply?)
• If the CCFL tubes should be the culprits, is there any chance of getting replacements? Were these standardized in size; should there be a voltage rating on them?

Many thanks for your advice!

[Gyro]

CCFL tube ageing is an operating lifetime thing, they don't really degrade in storage (well if they've been really cold, a few minutes warm-up should bring them back). The classic signs or failing tubes are uneven brightness (blackening of the ends) and developing a pink hue.

Yes, you can still pick up replacement tubes on ebay (and doubtless, other places). Not as common as they once were but still available. Doing a search from here, I notice that there is a seller in Italy with several different sizes.

As long as the tubes aren't too difficult to access (two tube ones seem easier than single). The only things to remember are...

1. Make sure you get a good length and diameter match, there's almost no leeway.
2. Carefully recover and re-use the silicone rubber insulating boots - also check for any tiny transparent O-ring spacers down the length of the tubes.
3. Be very careful cutting the electrode leads to length (as with all glass to metal seals)
4. Make the solder connections to the ends of the tubes very quickly.

I've replaced several in old IBM Thinkpads. Not the most enjoyable activity but not too hard (on a friendly display) and satisfying afterwards.


Edit: If you've already got access to the tubes, blackened ends are a giveaway.

[ebastler]

Thank you, Gyro!

Yes, CCFL tubes only aging during operation was what I thought. That's what had me puzzled, with the presumed limited operating hours of the notebook. I don't notice pink discoloring or reduced brightness at the edges of the screen; but I guess I should have a look at the tubes directly.

I have not taken the actual TFT & backlight assembly apart yet, but with the back cover and circuit boards already out of the way, it does not look to difficult. The assembly seems be be held together by a metal frame which is mostly screwd, maybe with a few tabs which need to be bent open. I will look at the CCFL tubes tomorrow and see what they suggest.

[Gyro]

You're welcome.

I forgot to mention, the late, great, Jim Williams did a lot of good stuff with CCFL inverters. His AN65 is pretty much the authoritative reference on the subject (AN61 has a little too). Note that the series capacitor on the tube supply is typically around 27pF, explaining your voltage collapse with a 50pF+ meter load.

https://www.analog.com/media/en/technical-documentation/application-notes/an65fa.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/an61fa.pdf


P.S. The Appendices are excellent for measurement techniques in general!

[ebastler]

Thank you, Gyro, I will dig into these! Just what I needed; I felt uncomfortable poking around in the dark (no pun intented...) without knowing much at all about CCFL operation.

[ebastler]

It took me a long time to get this fixed -- because I was barking up the wrong tree, it turns out. Quick update for future reference:

The backlight of this display was indeed fine! What misled me to considering it dim was the messed-up, low contrast of the TFT array itself. And that was caused by damaged PCB traces and corroded potentiometers, due to leaky SMD electrolytic caps.

As mentioned in the original post, I had noticed the bad caps weeks ago. They were visible on the display PCB once I had removed the notebook's display back cover. Replaced the caps and cleaned things up; but I did not disassemble the display any further at the time. Only over the past weekend did I finally get desparate enough to do that -- fully removing the display from the notebook lid, and prying open its metal enclosure. And I found that the electrolyte had leaked through to the bottom of the PCB, through a few vias, and that this was exactly the area which held six pots to adjust various reference voltages for the TFT.

Trying to adjust the pots did not get me anywhere. Unsoldered them, cleaned everything up, repaired two traces below the pots which had actually corroded away -- and the display is back to it's former colorful, high contrast self!

So the assumption that CCFL backlights do not deteriorate during a few decades on the shelf (but only during operation) holds for this display. And I can finally put my nostalgia notebook back together again and play with Windows 3.11... 

[Gyro]

Well that's not what I was expecting! Good result!