LaCie 20" LCD starts badly

[ppppenguin]

My lovely 20" LCD monitor, a LaCie photon20vision, has begun to start up badly. It's the second monitor on my main desktop PC. At least 8 years old it was very expensive and is still very high quality. I'd like to keep it in service.

For the last few days I've had to remove and reconnect the power a couple of times to get it to start. once it has started it runs without problems.

I presume this is something to do with the startup circuitry in the switchmode PSU. Believe it of not I've not (yet!) had to fix something like this so some guidance would be appreciated.

User manual: http://www.lacie.com/lacie-content/manua...ion_en.pdf

[pamphonica]

I had a similar problem with a nice 24" monitor. It was the main (large) caps in the switchmode psu. They were bulging! I replaced them and all was well again
Jeremy

[ppppenguin]

FIrst thing is to work out how to get inside it. Caps are the usual problem so I'd better take a look. ISTR some switchmodes have a startup circuit which can fail.

Time to shuffle some monitors around. Probably use my spare Dell 24" instead of the 20". Then I'll have even more desktop real estate, 2x 24" monitors Having robbed the 24" from the test position to the side of my desk I'm sure I've got a stray 17" or 19" LCD to use there for now.

The 24" Dells (2408P I think) are incredibly flexible devices with almost every conceivable input so very handy to use in a test position.

2 x DVI-D - 24 pin digital DVI
1 x VGA - 15 pin HD D-Sub (HD-15)
1 x component video input - RCA x 3
1 x composite video input - RCA
1 x S-video input - 4 pin mini-DIN
1 x HDMI - 19 pin HDMI Type A
4 x USB 2.0 - 4 pin USB Type A
1 x DisplayPort - 20 pin DisplayPort
Audio line-out - mini-phone stereo 3.5 mm

By contrast the LaCie only has DVI and VGA. It also passes this suite of tests with flying colours. The Dells aren't quite as good, others I've tested are not in the same league.
http://www.lagom.nl/lcd-test/

[Refugee]

I had one that also had intermittent start and it was also caps in the PSU.

[Mark Hennessy]

As well as capacitors, be suspicious of any high value film resistors on the primary side. It's common to use 100-300k to drop the 325V DC down to provide the start-up current to the controller. Or about half that if there are two capacitors in series for 120/240V switching... These resistors gradually fail high because of voltage stress. Metal glaze replacements are a lot more reliable than metal/carbon film types. But in extremes, several smaller ones in series reduces the voltage stress...

And when it's apart, be aware that if it didn't start last time it was tried, the main smoother capacitor(s) could be charged. Sometimes, bleed resistors might be fitted, but these usually fail in exactly the same way as the start-up resistor!

[ppppenguin]

I tell a lie. There's a label on the back of the monitor saying it was made in January 2003.

I've cracked it open without too much difficulty. The PSU module is a nice spacious PCB. Since this was a high end monitor this suggests that the makers weren't pushing things to the limits in the design. I hope that Mark H as a connoisseur of these things will offer an opinion.

The fault however is blatant. The main reservoir is very bulgy. 150uF, 400V. Its solder joints look a bit dry, suggesting it's been getting hot. Otherwise there's no evidence of anythng overheating apart from some very minor discolouration of the PCB near some semiconductors and resistors. I may have a suitable cap in stock, I'm sure it won't matter if the replacement is a 220uF if that's what I've got. Otherwise it's an RS order. All the other electrolytics look fine. Is it worth changing them on spec?

Any recommendations as to a make and type to use. Original height is 25mm but could take 30mm without problems. Diameter is 35mm. Mounting pitch is 10mm
http://uk.rs-online.com/web/c/passive-co...4294955811&applied-dimensions=4293501615,4294337815,4294457432,4294461338,4294463121,4294465759,4294466539&sort-by=Height&sort-order=asc

[ppppenguin]

I could have sworn I had a box or drawer with some HV electrolytics recovered from PSUs. Can I find it? No. I have found a nice little NOS PSU with a 180uF 400 V electrolytic. Not sure what make but according to a note with it it's a SWT40-522, looks like a +5V, +/-12V unit and was supplied to me as a sample in 1997. If I steal its electrolytic I think I'll reform it first.

If the assembled experts think that 100uF will do, I've got loads of them in the NOS Meanwell 5V PSUs I offered last year.

[Mark Hennessy]

Looks nice...

I wonder if those 68k resistors are in series for start-up? I'm guessing that the larger 75k jobs are part of the snubber, but could be wrong. The 1M jobs seem too high for start-up - wonder if they're for bleeding? Still, anything is possible...

I doubt there will be a problem with the 220uF replacement.

Given the age, I'd be suspicious about the rest of the capacitors. Before I had an ESR meter, I would have changed them on-spec. Now that I have one, I measure them, but frankly, I usually change them anyway. If they're OK now, how long will they last for?

There is a risk of leakage, of course. Not too difficult to clean up on a board like that. but still a PITA.

Guess it depends on how awkward it is to have it all apart again in the future... Personally, I'd just go for it.

If the caps in the PSU are getting tired, that might put some stress on the caps on the main board. Just a though

[ppppenguin]

Mark, thanks for your thoughts.

I extracted the bad cap and it's not as bad as I expected. 130uF on my Peak LCR meter which was measuring at DC. I don't have an ESR meter so used an audio generator at 10kHz with a 10R series resistor. Used a DVM to measure across the R and C. X at 10kHz is about 0.1R, total Z was about 0.4R which is nothing like as bad as I expected.

The 68K resistors are in series, one end to the mains input. They measure OK. The 75Ks are in parallel and measure OK. Most of the other caps are 2200u, either 10V or 16V. There's a 1000u near the output conenctors (I suppose I could use a 2200u) and a few smaller caps. Any indication as to which need to be HF rated low ESR parts? Or just assume they all need to be? Probably got the small caps around in low ESR versions. I'd need to order the 2200u to be sure of getting good small parts. Do these look about right: http://uk.rs-online.com/web/p/aluminium-...s/7395260/

[ppppenguin]

I've ordered a 180V 400V and 10x 2200u 16V from RS. Should be here on Monday.