Power issues for old Viewsonic VP230mb

[jeff d]

Hi, I'm hoping to get some feedback from anyone who many have serviced one of these classic LCD monitors. I discovered that when connected to the display the supply appeared to be cycling the 12V output meaning you'd see a sawtooth wave from 13V down to about 12V then bounce back up to 13V and repeat. To me this is classic voltage dropout reset so I went after the favorite suspect the 8-10 electrolytics 4 on primary side, 3 on secondary. Cap C8 on the primary side was dead and was the only of the 3 smaller caps with a 25V rating. I'm familiar with spikes on the switching signal often exceeding ratings, so I decided to bump the replacement to a 50V part. All supply can caps replaced and ran a dummy load to check and things looked good. Reinstalled and monitor powers on view sonic logo display for a second then everything goes black. No edge light, no image. The display's audio board is working fine and the monitor does track connected (DVI or VGA) computer's power saving mode meaning if the computer shuts off the display the VP230 switches into standby mode (orange power LED), wake computer up and power LED returns to green, edge light illuminates for a couple seconds then another black out.

I had worked through the service manual DC/DC stage stuff and saw some strange behavior on pin 8 of U20 which should be roughly 5V but it was less stable than I expected. I figured that would be farily stable an not bouncing all over the place.  I swapped out all the radial electrolytics on the video processing board (damn tough with the amount of copper on ground!), I found three of the 470uF caps below -20% of their rating including closer to -27%. Two if the caps were in parallel along a single power bus (so that source may be something to look into).  

Since things weren't working as expected I created a little test load to see if I can identify the next suspect in the supply. I breadboarded a load of 3 12V car lamps and when connected in parallel all light up just fine, the 12V output from the supply is rock solid 12V! But, the supply is 84W (7A) supply, and the lamps are only drawing just over 1A combined. I expect the monitor can draws up to 5A so I wired in some 10K resistors between the source and load lines. The result was the sawtooth 12V signal and none of the lamps luminate. I dropped the inline-resistors down to 1K each and same result. I didn't drop resistors any lower, at this point I'm thinking the supply is only capable of supplying 1A. Or that's as far as I got... 

There's also a single 5V on the harness between the supply and video board (the one with video inputs) which appears to be feeding back from the video processing board to the supply but I don't understand that part of the circuit on both the supply side and the video processing board side. At least not yet.  

Hoping someone here has some thoughts on what to check next.
Service manual attached for reference.

 

EDIT: whoops, I forgot to mention when reworking the supply I noticed the U2 (LM358 dual opamp) has a strange look, almost like something leaked out, but ICs don't leak... also really interesting because aside from that the board is spottless not even a spec of flux. I know this could be flux that maybe was trapped under the part but that's my guess. Just not clear why it's there like that unless flowed when overheated? I got a replacement but stopped short of swapping it out and since it tested fine with my dummy load I didn't go back. But now... maybe that should be swapped?
 

viewsonic_vp230mb_vlcds22494-1b.pdf

Edited May 4 by jeff d
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[jeff d]

Thought I'd follow up with some new info and another question...

First, I believe the sawtooth type wave on 12V is as designed, basically a power savings feature (?) to only draw power recharge caps to maintain 12V and then cut power. It does look like the suspected SW-5V signal is an input to the supply board from the video board.

I verified all power signals are good 12V, 5V, 3.3V are all stable when switched on and flat when off. I confirmed i2c is working with reads from the serial EEPROM. With the scope connected to VGA input I can see H and V sync signals coming from U32 which feeds the LCD panel. Clocks look good.

So now I'm looking at the video inverter board as a possible problem, but the service manual only mentions checking it and doesn't provide and I didn't see any details on exactly what it is. Does anyone know details on the video power inverter board?

I've tried to aim a light at the display to see if there is an image, just without backlight but I couldn't see anything. I also can't bring up the menu so I'm not sure if there's a problem with the settings data on the EEPROM. I read the eprom, and data looks promising, I just don't know what should be there. Is there any way to reset the system back to factory settings?