Fixing an intermittent ViewSonic LCD Computer Monitor VX924 VS10162

I recently decided to dust off an old gaming PC from around 2006 that was in the attic.

I'm interested in folding@home , a way of assisting medical science donating computer time from home , and  wanted to see if I could build a dedicated FAH (Fold at Home) rig using spare hardware we had.  I'm also interested in computational heating i.e better exploiting the waste heat from computer systems . This  will be the subject of another blog shortly, but when I got this PC down and booted it up, I immediately hit an issue with the LCD screen. It kept turning on and off approximately every second , but sometimes staying on for longer. I could see the boot screen, the BIOS and eventually the windows logo, but the screen wouldn't stay on. Very frustrating

I later realised that if I left the monitor on for at least an hour it would stabilize and become usable.

I looked around  on the internet and realised this was probably a problem with the capacitors in the power supply. If you have read my other blog about repairing some M-Audio BX5a speakers , they hit a problem , similar in cause, bulging power supply capacitors.

The monitor is from 2006 and not super high res, but I didn't want to throw it out for the price of a few caps so decided to have a look inside. 

First disconnect the monitor from the power and the PC. I laid it face down on a soft towel on the table and removed 4 screws holding the back panel on. This could then be unclipped gently around the edge.

the electronics on the back of the screen is protected by a metal box section, and again some screws hold this in place. There are also some adhesive foil screening material used to bond protect some of the cables from interference. I teased this back, planning to refix it in place after.

With the metal box screen removed I could see the two neat circuit boards, the left having the power supply functions (Top in the picture), the right the display and logic. Some gold pins and socket linked the two boards, and removing 4 additional screws enabled me to lift the power supply board away from the other board, for closer inspection. It remained tethered by a number of cable assemblies but I was able to invert it to access  the underside of the board.

I could see immediately that two identical power supply capacitors exhibited bulgin at the tops. No electrolyte had leaked , but the silver part at the top of the can was certainly not flat.

In addition a 3rd capacitor, wrapped in some kind of heat shrink, also exhibited the same problem

So I reckoned these might be why the monitor kept switching on and off. I desoldered the 3 caps using my trusty old soldering iron, and a mechanical solder sucker, and found it pretty easy to pull the offending caps down while inverted and  softening the soldering iron to loosen their connection

They came out quite easily. each had a blob of some kind of adhesive next to it, but this crumbled

The caps were two 470uF 25V Radial 105 degrees Centigrade (Max operating temp) and one 330uF 25v Radial 105 degree cap. I ordered replacements from Cricklewood Electronics which came in a few days and cost very little. I like Cricklewood , service is good and they are happy to sell you a single capacitor, no need to order more than I need.

When the replacement caps arrived, I fitted their leads through the holes. I'd taken note of the polarity (the white stripe down one side of the can denotes the negative terminal. However the circuit board also had indications of the polarity where they sat, with a cross hatched marking for a segment of the circle corresponding to the negative terminal. taking photos during the removal is always a good idea.

I resoldered the leads, and cut the excess wire from each. I fitted some heat shrink insulation around the cap that appeared to have additional insulation around it. It looked intentional and original, perhaps to protect it from the inductor next to it (Copper wire coil around a carbon core) 

I then refitted the Power Supply circuit board back connected to the logic board and refitted both the 4 screws that held it in place and then refitted the cover , making sure all the foil screen pieces pressed back.

I fitted the back to the screen, refitted the remaining screws and plugged both the signal and power leads back into the monitor. It immediately stayed on, no flickering and has worked perfectly since, so a correct diagnosis and easy fix. Very pleased to have saved this old monitor for a few pounds worth of caps and a bit of soldering.