Viewsonic VA 903m shutting off/on

[Panther]

Hi everyone. Been reading a bit of the valuable info on this site to get a better understanding of how these things work, but I'm stumped with this problem so this is my first post asking for help.

This monitor was given to me. The symptom is that it turns on ok but after about a minute it will shut down for a few seconds, then turn on and shut off almost immediately, then keeps repeating this cycle.
You can also hear a prominent click from the inbuilt speakers during the cycle.

When I opened it up it was obvious someone's already had a go at this at some stage, but even so there were 5 bulging CapXon's that I've replaced with some spares I had on hand. I've also tried 2 spare ccfl's but the problem remains.

I can't see any other obviously damaged or distressed components except for U101 (mounted on the small heatsink next to the big cap) that gets extremely hot, well over 100C and you can smell it.
I suspect this component is overheating and shutting down due to it's built in thermal protection.
There's a datasheet here, but you need to scroll down a bit.
http://www.datasheetpro.com/181607_d...datasheet.html

I only understand the very basics of SMPS's so I'm at a loss at what to try next, so any help would be greatly appreciated.

I hope the pictures I've supplied are suitable.

[Dgtech]

Re: Viewsonic VA 903m shutting off/on

On page #4 of the PDF, it shows this chip's maximum operating voltages. If this thing is getting hot, I'd check to see if these limits aren't more than maxed out.

[PlainBill]

Re: Viewsonic VA 903m shutting off/on

Good pictures, that helps, thanks.

The problem may be U101, but a few rudimentary tests should be done first. First, are both output voltages present and in the expected range? It appears the main output is 5.2V, the inverter supply is probably 12V. Check the heat sink for the output diodes.

Next on page 13 of the datasheet there is a schematic for a typical power supply. C104, D101, and R103 on the datasheet are critical, check their equivalents (maybe C120, R124, D???) on the board.

PlainBill

[Panther]

Re: Viewsonic VA 903m shutting off/on

Thanks for the help guys.

PlainBill, I'm getting 5.15v and 11.5v on the outputs.
R124 measures approx 16k as far as I can tell.
C120 I don't seem to be able to measure properly. If I stick the meter on one way it reads about 1.5nF, but put the probes on reversed I get no reading at all.

Sorry, not sure what you mean by check the heat sink for the output diodes.

Dgtech, I checked Vcc on pin 3. According to the specs it should have a max of 20v, my measurement was 14.4v with the ccfl's on, 13.8v with them off.

Now this next bit doesn't make any sense. While I was checking the voltages and the thing was doing it's on/off cycle, I checked the temp of U101 again and it was only getting to around 35C and there was no overheating smell.
I then removed the board and decided to replace two of the caps again with better quality ones just to check. It still did it's on/off cycle, but U101 was overheating again with temps over 110C and the overheating smell was back.

I don't know what's going on here, but I wonder if it's worth the effort to continue with this one given it's very odd and changing behavior.

[PlainBill]

Re: Viewsonic VA 903m shutting off/on

Thanks for the help guys.

PlainBill, I'm getting 5.15v and 11.5v on the outputs.
R124 measures approx 16k as far as I can tell.
C120 I don't seem to be able to measure properly. If I stick the meter on one way it reads about 1.5nF, but put the probes on reversed I get no reading at all.

Sorry, not sure what you mean by check the heat sink for the output diodes.

Dgtech, I checked Vcc on pin 3. According to the specs it should have a max of 20v, my measurement was 14.4v with the ccfl's on, 13.8v with them off.

Now this next bit doesn't make any sense. While I was checking the voltages and the thing was doing it's on/off cycle, I checked the temp of U101 again and it was only getting to around 35C and there was no overheating smell.
I then removed the board and decided to replace two of the caps again with better quality ones just to check. It still did it's on/off cycle, but U101 was overheating again with temps over 110C and the overheating smell was back.

I don't know what's going on here, but I wonder if it's worth the effort to continue with this one given it's very odd and changing behavior.

In general there are only a few reasons U101 would be overheating.

1. Excessive output load.
2. Defective SMPS transformer (VERY unlikely)
3. Problem with timing components
4. Problem with IC itself.

What I find most interesting is that changing a cap would cause the problem. I could see that several high ESR caps MIGHT cause problems, but replacing one with a better one????

The only thing I can think of is bad solder joints. I'd resolder all of them on the primary side of the power supply and retest. If you still have the same problem I'd have to say either replace U101, find another power supply / inverter board, or use the monitor as a source of parts.

PlainBill

[Panther]

Re: Viewsonic VA 903m shutting off/on

In general there are only a few reasons U101 would be overheating.

1. Excessive output load.
2. Defective SMPS transformer (VERY unlikely)
3. Problem with timing components
4. Problem with IC itself.

What I find most interesting is that changing a cap would cause the problem. I could see that several high ESR caps MIGHT cause problems, but replacing one with a better one????

The only thing I can think of is bad solder joints. I'd resolder all of them on the primary side of the power supply and retest. If you still have the same problem I'd have to say either replace U101, find another power supply / inverter board, or use the monitor as a source of parts.

PlainBill

Thanks for your valuable help.

I'll try resoldering as suggested and if it works I'll let you know. If not I'll use it for parts. These monitors just aren't worth enough these days to put a whole lot of time into fixing them.