Samsung LM46M87BD - 2secs to black

[tn245]

Actually it's more like 1 second or less. There's a brief flash of the backlight, some buzzing and then it shuts down.

Ohms tested the inverter transformers - none seem bad. The prot-p pin (feedback from inverter transformers) goes up to almost 12V so I'm thinking bad bulb. Tried with the link cable between right and left inverters disconnected and prot-p on the left stays low and TV stays on with partial backlight. Right inverter obviously isn't getting the on signal with the cable disconnected but by elimination looks to be where the problem is.

Have tested AC volts on the feedback pins of each transformer. Most go up to about 0.4V, but several go up to about 1.2-1.4V before shutdown. I was going to try norcal's trick of bypassing the transformer and eliminating one bad tube, but as none stand out as bad, but several test slightly high I'm not sure where to go next.

I'm thinking I need to get in and look at the bulbs to see what's going on but I'm not actually sure how. Do I need to take out the LCD panel from the front? I'm a little scared of doing that with a 46" panel.

Also, what are these caps on the PSU? Someone's been in this TV before me and also done a repair on the main board involving a resistor and some hot glue. Are the caps any good? Should I leave them in or change them?

Thanks for any help!

[tn245]

Re: Samsung LM46M87BD - 2secs to black

I'm back inside this TV and it's got me stumped! Just to update, it turned out the original problem was that someone had pulled and then badly refitted the balance boards and in doing so bent the ends of several of the CFL bulbs and broke one. I fixed it by bending them back and replacing the broken bulb with a slightly longer salvaged one. It worked great for almost a year but I came back from holiday and now when the set is cold it buzzes and goes black again.

All the tubes light up briefly, with one going much brighter and sparking at the end inside the tube. The sparking tube can be in one of two locations - 9th from top or 10th from bottom, but one more often than the other. I have tried swapping around various bulbs so have ruled out the bulbs as the problem. It seems to be something with the balance boards.

If I warm up the balance boards everything works great, but when they are cold it just buzzes and cuts out. I have checked all the connections and I don't think that's the issue.

Any ideas what might be going on with these balance boards? Thanks for any help!

[tn245]

Re: Samsung LM46M87BD - 2secs to black

Well mystery solved I think. I didn't know that transformers can become thermally sensitive, but according to another forum I came across they can. After a bit of precise heat gunning I've isolated a couple of the small transformers that seem to be problematic. Have ordered a used board with the same transformers from a 40" set. Will steal replacements from it and hopefully all should be good.

[tn245]

Re: Samsung LM46M87BD - 2secs to black

Yup, that was it. Replaced a few of the transformers in the end and all is good.