Fixing intermittent faults? / Computer Science / Forums

Forums

4hv.org :: Forums :: Computer Science

« Previous topic | Next topic »

Fixing intermittent faults?

Move Thread

LAN_403

Conundrum

Sun Aug 26 2012, 09:40AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi all.
Anyone ever come across LCD >19" monitors which work fine with the cover off and as soon as it is replaced within a few minutes random lines appear and eventually the screen shuts off?

I can guarantee it isn't bad capacitors, none of them are the affected brands and the only parts which get hot are the main CPU (added heatsink, no effect) and the regulator ICs.

Seems a shame to waste $$ and throw it out when it is only a week out of warranty.

I did consider adding a small fan to the CPU but is this likely to help?
There was no heatsink to begin with so adding thermal mass might make things worse.

-A

testtest

Sun Aug 26 2012, 09:38PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

You might consider hooking up a few wires to externally monitor the output of the power supply to start with.

If still OK, your assesment about the CPU might be OK. This does not rule out a capacitor on the CPU board that is off value and heat sensitive. You mention there are no fans, thus adding a heatsink might just delay the onset of the problem.

I ran in a similar issues a few years back and it was the inverter board for the CFL that was the problem. Looked like the display went off but a strong flashlight showed that the LCD display still worked. Just made a hole near it to allow a better air circulation and it is still fine today.

Shrad

Mon Aug 27 2012, 09:04AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

have a look at the bonding wires connecting the LCD glass panel to the panel breaker board... they are usually bonded with a "U" shape at the border of the glass panel, and when the bond fails it is usually disconnecting itself when pressure is applied to the sides of the screen

it might also be a small bit of plastic or a small screw which got caught in the "U" or between the flat cable and the glass panel, which is bending the flat cable

if there is such a problem, be very careful and use a line of white silicon adhesive to secure the whole "U" flat cable, as the defect tends to spread a bit each time pressure is applied to the bond

I hope it is not this kind of failure though, as it is usually definitive

plazmatron

Mon Aug 27 2012, 08:12PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Shrad wrote ...

have a look at the bonding wires connecting the LCD glass panel to the panel breaker board... they are usually bonded with a "U" shape at the border of the glass panel, and when the bond fails it is usually disconnecting itself when pressure is applied to the sides of the screen

I hope it is not this kind of failure though, as it is usually definitive

I concur. I have had several people come to me with Laptops that had LCD's fail in this way, and I have even had it happen to an old second hand one of mine.

In that case it appears the flexible connectors along the edge of the LCD fail from repeated lid openings and closings, as well as heat.

As for monitors, I guess just heat and age. Remember that anything electronic generates heat, and repeated thermal cycling, will cause repeated, albeit imperceptible movement of components. A well known effect is 'chip creep' where an IC, given enough time, can creep up out of its socket.

I have tried all sorts to fix this including tightly taping around the edge of the LCD, and placing shims inside the lid to apply pressure at the affected points. These are poor fixes, and work for a short while, but failure is inevitable I'm afraid.

Conundrum

Sat Sept 08 2012, 09:40AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Yep, fixed for good by:-
1) adding a LARGE heatsink to the CPU with thermal paste then epoxying it down to ensure reliable long term contact
2) Shotgun replacing of all the capacitors which looked the slightest bit flaky and metering them when removed
3) Adding two ex-X360 HDDVD fans (finally a use other than the lasers) with rerouted temperature sensor placed on the flaky chip in question.
4) Sacrificing poultry in its general direction while uttering "Monitorus Resurrecto"

So far it seems fine, no signs of failure and its running a LOT cooler now it has airflow.

In the meantime, metering of the "suspect" capacitors showed at least two had lost value, so the ESR is likely to be high.

Thanks guys

Another related problem, a Geforce 8600GTS which seems to only work in DOS.
It loads up Windows, and the graphics goes away.
I have done the infamous "spread the dry thermal grease" trick and also have replaced the SMD fuse on the mainboard
in case it has gone high resistance or something..
So far it only works reliably if the case is off (facepalm), if you reboot when "hot" 100% fail.
The onboard graphics "glitches" but the board does that even with the card out.
I have replaced the power supply but this had negligible effect...

-A

Sulaiman

Sun Sept 09 2012, 08:29AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

You mentioned known brands of aluminium electrolytic capacitors failing,
as far as I am concerned, ALL small electrolytics are suspect, surface mount, radial or axial.
At work we replace all small electrolytics as a routine procedure and test the large ones.
This 'repairs' about 1/3 of all of the industrial electronics that we service !

We have noticed that many many manufacturers have replaced small electrolytics with tantalum or ceramic in newer equipment

If you have some faulty electronic equipment that uses the small surface mount electrolytics replace them all
the esr increases long before the value changes so a capacitance meter will not show most faults
I use an esr meter to check capacitors but we replace them all anyway as everything that we repair has a 1 year guarantee.
Our preferred brands are Panasonis, Nichicon and Rubycon.

P.S. the surface mount electrolytics can be removed easily by griping them with a pair of long nose pliers and twisting them left and right a few times, increasing the ammount of twist gradually to not pull off the pcb track .. works very well

Steve Conner

Sun Sept 09 2012, 08:39AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Interesting comment on the removal of surface mount lytics. On one board we designed, we put one too near the edge of the board, and it tends to get "removed" accidentally when the board is handled. The legs don't seem very sturdy. :(

We use tantalum or ceramic in our designs wherever possible too.

Conundrum

Sun Sept 09 2012, 10:40AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

I know a camcorder repair guy who has run into this repair technique.
He also says it works fine, IF the lytic hasnt already leaked.
If so then twisting it can destroy the tracks..

-A

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob