ViewSonic VX924 No power - Not the usual suspects

[Player2]

I am having a great time with ViewSonics this week

I'm working on a VX924 which does not power on, nor does the green LED blink. I replaced all of the CrapXons (including the lil 100 uF and the large filter cap) on the power board, replaced the 4 transistors under the inverters, even replaced the ceramic caps by the inverters.

For a while it was ticking when I applied power to it, sounded like it was going into protection. As I tested parts and started swapping components the ticking would come faster or slower. Now it does not tick at all and it still does not power up. The fuse is still good. I've got voltage at the main filter cap, but no 12v DC output.

I'll get pics up today, but I think we all know what this board looks like Any suggestions?

[PlainBill]

Re: ViewSonic VX924 No power - Not the usual suspects

I am having a great time with ViewSonics this week

I'm working on a VX924 which does not power on, nor does the green LED blink. I replaced all of the CrapXons (including the lil 100 uF and the large filter cap) on the power board, replaced the 4 transistors under the inverters, even replaced the ceramic caps by the inverters.

For a while it was ticking when I applied power to it, sounded like it was going into protection. As I tested parts and started swapping components the ticking would come faster or slower. Now it does not tick at all and it still does not power up. The fuse is still good. I've got voltage at the main filter cap, but no 12v DC output.

I'll get pics up today, but I think we all know what this board looks like Any suggestions?

The ticking sound indicates the SMPS is trying to start, but either the load is too great, or the running supply for the SMPS controller is not functioning.

I'm not certain why you felt it necessary to replace the caps in the inverters. Most (all) I have examined have either a fuse or a jumper that can be lifted to disconnect the inverter.

I am certain you are sure you soldered in the replacement caps with the proper orientation. Double check that. A cap in backwards was the cause of problems with one recap job I did.

PlainBill

[Elab]

Re: ViewSonic VX924 No power - Not the usual suspects

Check D660, D661 = MBR10100CT

[Player2]

Re: ViewSonic VX924 No power - Not the usual suspects

D660 is definitely bad, didn't see a D661 though. Something else is still keeping it from coming on though. I'm going to work my way out from D660 and see what else could be affected.

All of the caps are oriented correctly, double checked against a working board. I replaced the ceramics near the transformers based on a tip in another thread. It was a shot in the dark.

[Maugli]

Re: ViewSonic VX924 No power - Not the usual suspects

Player, I am working on the same problem. 12 volts is not stable. any progress on your end?

[EGuevarae]

Re: ViewSonic VX924 No power - Not the usual suspects

Player, I am working on the same problem. 12 volts is not stable. any progress on your end?

Post a pic of the board, so suggestions can be made.

[wsqlim]

Re: ViewSonic VX924 No power - Not the usual suspects

I have "the blinking" power LED problem as well. After replacing all the capacitors, it still fail to power up, still blinking LED. I'm getting unstable 8-9DC on the power output side. What can be causing this and how to fix it?

[Dgtech]

Re: ViewSonic VX924 No power - Not the usual suspects

The last guy never posted pics of any of these boards. I'm hoping you will so that there may be a solution. Take pictures (front and back) of both PC boards. Post them as attatchments - no inline pictures. People will jump onboard quicker when there are pictures.

[wsqlim]

Re: ViewSonic VX924 No power - Not the usual suspects

I left the monitor alone for some time. Then I remembered 1 CAP 330uF 35V I replaced with 330uf 50V. So I got another 330uF 35V and put it in place. The monitor works, but only for 1 day. The next day I try to power up, I get the blinking light again.

[retiredcaps]

Re: ViewSonic VX924 No power - Not the usual suspects

So I got another 330uF 35V and put it in place. The monitor works, but only for 1 day.

What brand and series did you use for replacement? It matters because the SMPS requires low ESR caps like Panasonic FR capacitors.

[blutch]

Re: ViewSonic VX924 No power - Not the usual suspects

i also have a vx922 with fluctuating power from the 12v rail between boards. i have replaced all caps but the large 400v. still the power goes from 12v to 4v and repeats over and over. and the screen and green power light blink in time with it. what else could be the problem?

[retiredcaps]

Re: ViewSonic VX924 No power - Not the usual suspects

i also have a vx922 with fluctuating power from the 12v rail between boards. i have replaced all caps but the large 400v. still the power goes from 12v to 4v and repeats over and over. and the screen and green power light blink in time with it. what else could be the problem?

Cross posting is frowned upon here and confuses people and makes for more work. Keep your discussion contained to your thread at

https://www.badcaps.net/forum/showthread.php?t=13873

[wsqlim]

Re: ViewSonic VX924 No power - Not the usual suspects

I took out the monitor and replace all caps again. It seems to be working, but something bothers me. The red circled heat sink feels very hot within minutes, while the blue circled one is stone cold. And I am hearing this faint rattling sound while power is on. And with power off I can hear faint beep sounds, even if I off the power supply but leave the power cord plug in, the sound still remains. Any1 can advise what seems to be the problem?

[retiredcaps]

Re: ViewSonic VX924 No power - Not the usual suspects

The red circled heat sink feels very hot within minutes, while the blue circled one is stone cold.

The picture is pretty tiny, but if the power mosfet is connected to that heatsink, that could be normal operation.

Look at the power mosfet part number, then find its datasheet. The datasheet might tell you on the operating temperature.

Same suggestion for the diodes on the blue heatsink. The diodes are responsible for your secondary voltages.

And I am hearing this faint rattling sound while power is on. And with power off I can hear faint beep sounds, even if I off the power supply but leave the power cord plug in, the sound still remains. Any1 can advise what seems to be the problem?

When the power cord is plugged in, 5V standby is always on.

[wsqlim]

Re: ViewSonic VX924 No power - Not the usual suspects

Thanks for your advise retired, but I can't make out the number.

Well I have another working VX924, maybe I can open it and compare the two. But it does not have such noise while in operation. The repaired one makes this faint rattling sound while powered on, kind of like what a damaged fan will sound like. And is it normal for the beeping sound to be there?

Here's a bigger pic, if it helps. Thanks

[wsqlim]

Re: ViewSonic VX924 No power - Not the usual suspects

I think the heat problem is quite serious, I actually smell something burning after leaving the monitor on for 5 mins.

[jetadm123]

Re: ViewSonic VX924 No power - Not the usual suspects

If the one circled in red is silkscreened on the board as Q320, then it may be a NDF04N60ZG mosfet rated at 600V, 4.4A, 28W.

[pedro]

Re: ViewSonic VX924 No power - Not the usual suspects

I took out the monitor and replace all caps again.

Can you remind us which brand/series caps you used?