Monitor repair advice

[kbs1]

Hello, I finally had some time to check the monitor once again. I was trying to measure the inverter transformer input using my oscilloscope to see if it's really 13.18V flat line DC or if it has some sort of saw tooth wave and my DMM is aproximating.

I've found out that my oscilloscope is very unstable (unusable) if I do not connect the probe's ground. The A-B mode does not work at all, I just get a lot of noise on the display. I tested this measuring a simple USB adapter that outputs 5.16V DC. I only get the proper display (flat line DC) if I connect the probe's ground, that is in single channel mode. But the adapter is floating, so that's no problem.

Now trying to measure the inverter input, once again there is a question where to put the scope ground lead. Dave had an excellent video on this topic, the way I understood it is that I have to connect the scope ground after the transformer, that is after T901 (main transformer) on my board, and hence a good spot seems to be on the top left side of the board (the screw hole) [see attached picture]. Is this ground spot safe?

I'd like to measure IC201 and inverter input correctly once I get my scope showing the right data.

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On a different note, the board makes a high pitch noise (but very quiet) all the time. When I turn it on, the noise becomes louder for ~2 seconds and then quiets down again, but never absolutely silent. On one YouTube video I saw that even caps could make this whining noise. Even through they all measured good and are not bulged, do you think it's worth replacing them? Or is it just the transformer whining? I'll try to isolate the noise but it's cery unclear which part really makes it.

Thanks a lot for your help, step by step we'll definitely find out what's wrong with this monitor (and from my point of view) learn a lot of new stuff on the way.

[kbs1]

Only pin 19 is outputting some saw tooth wave output at all. Pins 11, 12 and 20 are always exactly 0 volts. Pin 19 is active when the backlight should be on (with lamps disconnected). I can not know for sure if it's a saw tooth wave, but it looks like it, because my scope just draws 2 concurrent horizontal lines when measuring it regardless of any setting (resolution, AC/DC, stability, etc).

"peak to peak" value of this saw tooth wave (that is number of volts between the two shown lines on the scope) is ~4.3V. I tried measuring pin19 at max 2MHZ, and never seen anything drawn between the two horizontal lines, but I thinks it's the scope's fault (it's really old, circa 1977).

So if only pin 19 is active, that is "P-MOSFET gate drive output", it means the other pair of mosfets is not turned on at all. I also measured voltage directly at inverter transformers, and it's ~13V DC, no ripple (just straight line, no AC).

So according to toshiba.pdf page 30, only Q204 and Q209 P-mosfets are driven

Also according to toshiba.pdf page 41, "with led, no backlight" troubleshooting step 4

"Check the pin1 of U201 have saw tooth wave"

I don't know if this is a typo and they meant pin 11 or not, but pin 1 "Timing capacitor to provide striking time and timing resistor to provide shutdown delay time" is also 0 volts at any time (according to scope).

I also measured pin 5 of IC201 (toshiba.pdf page 41), step "Check U201 pin5=6V". It's only 4.80V. But then again, IC201 manual says that VDDA recommended value is 4.6V to 5.5V so that should be also OK.

Does this mean that IC201 is blown for sure or can it be the surrounding circuitry's fault? For example some bad resistors or capacitors along the path (toshiba.pdf page 30). If it's blown, where can I get another one? Hope they sell it a piece.

[kbs1]

Thank you very much anyway. I'll try to study these things and see if I can come up with a solution. In the meantime any other advice from anyone else is highly appreciated. Since this is a beginner forum (and sure I am one), I'll post some unable to crack mind dilemmas (questions) here if I won't find the answer anywhere else on the internet. Thanks a lot for your effort!

[G7PSK]

One trick you could use when probing with your oscilloscope. Use a resistor clipped onto the earth lead to limit current if you are not sure of where the earth is on a board, you wont of course get the correct voltage readings and at higher frequency it might not work correctly either but it will help you find things without blowing up the scope or board. I am sure that the purists out there will be cringing with horror at this suggestion, but there is nothing like a little protection until you are sure of things.   

[kbs1]

Thanks, I tried that and it worked OK for touching the spot on the board for the first time if unsure. However, no one including me did know what was the real issue with this monitor, but it's now repaired. So just to finish this thread off, I'm posting how I eventually (and to this date) got the back light working again.

I bought one of those new cheap universal CCFL inverters that have recently sprung up all over ebay (about time!). I bought this one: http://www.ebay.com/itm/110944897770 It's actually made (or at least sold) in Czech Republic, an adjacent country of mine. So shipping was lighting fast and cheap (2 days and it was here).

Hooking the inverter up was a piece of cake, it only needed ground, +10V to +30V input voltage, on off signal and DIM signal - all of this is easy to find and signals were at correct levels on my board. So I soldered ground to cold ground spot on board, Vin after fuse F902 (+11.93V after fuse for protection, draws about 0.8A), and similary soldered the ON/OFF (0V or 5V) and DIM signals (0V to 5V).

Plugged the monitor in and viola, backlight works! The picture looks BRILLIANT. Very happy with the repair. Cramming the thing all together was a different challenge, took me 3 hours to find one possible (still very tight) fit in the back panel case, but some ceramic capacitors, resistors and both of the original inverter transformers had to be desoldered in order to make room for the new board.

Attached are some pictures of the repair before closing the case up. The monitor works flawlessly to this very day, does not heat up at all, perfect. One "issue" that is present is that the brightness actually decreases when turining it up in OSD and vice versa, but I don't care. At 0% brightness it is bright as 75% of the previous original state, which is perfectly bright and ideal. At 100%, it is too dim (~50% of original brightness). So the range is not full, and flipped.

In conclusion, maybe it's not a real repait or an ideal one (more of a hack) buy hey, it works. So that's that, and thanks for the wonderful help from you guys. Learned a lot of new stuff on the way.

Cheers!