Negatoscope  - Page 1
Forum section: electronics / repair

author: vidarr
author_rank: Frequent Contributor
timestamp: March 07, 2020, 08:25:53 pm
content: I have a broken LED(?) computer monitor that I want to make into a negatoscope for my wife. When it is finished, it needs to look professional for her office.There is a problem with the driver board in the middle that it is making some noises and gets very hot. I was going to try to troubleshoot the problem, but I am still very much a beginner with electronics.Narrowed down, there a three options:Someone help me troubleshoot the middle board to fix it.Create a new power board driver circuit from scratch. (Would require a circuit diagram/schematic that I cannot find searching online.)Use the existing power board, ditch the little board with the power button and replace with a switch on the power board. Also, get rid of the board in the middle, and the unnecessary parts on the power board. This option is a good one, because I don't need to build from scratch and having just the power board, it will be easy to box it and have a switch on it.All my wife needs this to do is turn on/off and not look like a child built it. Any input would be appreciated.Thank You.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2020, 08:36:49 pm
content: If the backlight still runs, one could measure the required current and voltage for the LEDs. With some luck this could be compatible with LEDs panels used for room lights and one could use such a supply to power the backlight.For the combined backlight and monitor supply board, this one may not like to run without the monitor part. Those supplies may not be fully independent. In addition changes are the monitor is broken because of the supply part.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 07, 2020, 09:01:38 pm
content: The backlight still works. The monitor electrical was fine, it was just the screen that broke while in luggage. I removed that piece, so it is just layers of white film now.I tried to measure the voltage and the electricity went through the test leads and shocked me -- not bad, but not something I want to repeat. Still looking for my other leads that can handle higher volts. I don't usually work with anything over 12 volts.There was no way to test with just the single power board because in order to turn it on, it needs the small button board, which connects through the monitor plugin board that is in the middle. Ugh.If you had to guess the voltage, what do you think it would be? Could it actually be 12 volts? That would be great because I have a very good, brand new, 12V power supply.Thank You for your reply!

author: Kleinstein
author_rank: Super Contributor
timestamp: March 07, 2020, 09:51:40 pm
content: Looking at the board picture in more detail, the supply very much looks like a cold cathode supply to power 4 tubes, so some higher frequency higher voltage (some 200 V ?) converter.In this case chances are slim to find a suitable stand alone supply. Without much knowledge it would be hard and dangerous to strip down the supply. In some cases the backlight is also coupled with the rest of the supply. So it is not sure one can separate the two parts.A picture of the back side of the power supply board could give some hints if a separation is feasible.

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 07, 2020, 11:00:26 pm
content: The CCFL inverter section will be powered by 12V (or 24V) from the other transformer.If you look at the IC used to drive the inverter datasheet or maybe the markings near the bunch of coloured wires, you should get a clue on what needs flipping high/low to enable/disable the backlight.

author: kjr18
author_rank: Regular Contributor
timestamp: March 08, 2020, 11:32:27 am
content: I think there should be two wires to be aware of, one is for enabling backlight, and other one for brightness. Clear picture of back of the board should help us with providing answer. And be careful when handling this, half of the board in basically mains, that could very well kill you. And one fourth can give you a nasty shock.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 08, 2020, 03:40:35 pm
content: Thanks for the replies. Here are better pics of the boards. I don't like working with things over 12V. This is really helping me out a lot. Thank you.A long, long time ago, I got stuck to a covered outlet while cleaning in a restaurant It was the power line to the giant, commercial floor mixer and the bare wire was in contact with the box cover and wall. I was completely frozen stuck. a coworker knocked me free with a wooden pizza peel. It was very fast, maybe 1-2 seconds. I will never forget the feeling of being fully frozen -- it was more terrifying than the actual pain of it. If my coworker wasn't there I would be dead.

author: kjr18
author_rank: Regular Contributor
timestamp: March 08, 2020, 09:13:41 pm
content: You can see on controller board that this one socket has two pins called BKL ADJ and BKL EN. First one is to adjust brightness, second one is for enabling and disabling backlight driver. Try to measure voltage on this pin when backlight is enabled and disabled, you probably can disable and enable backlight by pressing some buttons on button board.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2020, 09:57:29 pm
content: The circuit has the backlight powered from the isolated supply. So there is no easy way to reduce the circuit. So the converter in the center of the board is required. So no easy solution.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 08, 2020, 10:57:08 pm
content: Quote from: Kleinstein on March 08, 2020, 09:57:29 pmOK, having to use all three boards is not ideal, but building a lager box to fit them in isn't too much of a setback. The only problem is, the monitor shuts of if there is not signal for some amount of seconds, I think about 15-20 seconds. How can I defeat that?Thank You!

author: vidarr
author_rank: Frequent Contributor
timestamp: March 08, 2020, 10:58:27 pm
content: OK, Will do. Thank you!

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 08, 2020, 11:16:51 pm
content: You can get rid of all boards except the power one once you have figured out the BKL_EN, basically all you'll need is a toggle switch to set it to 5V or 0V.Better still: Tie it to always on and put a switch on the mains inlet.I'm not sure if the BKL_ADJ left open will make the backlight run 100% or 50% brightness but usually it won't stop the backlight working when left open.You may consider a potentiometer between 0-5V on this if you have any use in dimming the brightness.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 08, 2020, 11:57:33 pm
content: Quote from: shakalnokturn on March 08, 2020, 11:16:51 pmNot sure if I understand this fully. When I figure out what the voltage of BKL_EN and BKL_ADJ is, I do what? And how do I tie it to always be on?This other stuff I understand well:Putting the switch on the mains inlet is the best idea. That will make it very easy for me.There is no need for the potentiometer. It just needs to be turned on and off.Thank You Very Much.

author: kjr18
author_rank: Regular Contributor
timestamp: March 09, 2020, 05:59:38 am
content: For BKL_EN just verify if when display is enabled it's voltage reads 5V or 0V. If 5V, then connect this to either 5V or GND (directly could be enough as there are in series resistors R101 and R136), depending on what voltage level you see.About BKL_ADJ it's either voltage or PWM based, depending on a backlight chip. I don't know how it will behave when flapping around in the breeze, but I guess connecting it to 5V could give you full brightness.Display turns off because there is no data incoming, if you get rid of image processing board it will work all the time.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 09, 2020, 06:06:01 pm
content: OK, tested and got:BKL_EN 3.3VBKL_ADJ 5.20V (It kind of fluctuates from 5.0 - 5.2, but it could be my very cheap multi meter.)When it goes to standby, BKL_EN goes to zero and BKL_ADJ stays at 5.20. I am ready for whatever the next step is.And Thank You Very Much!!!

author: JKKDev
author_rank: Regular Contributor
timestamp: March 09, 2020, 06:47:52 pm
content: IMO the easiest solution at this point would be to jumper the 5V rail on the PSU board to the BKL_ADJ and build a voltage divider between the 5V rail and GND to get the 3.3V needed for BKL_EN. You could use a voltage regulator to do the same but I don't see an enable pin sinking that much current that it would matter. At this point, the only required board should be the PSU one.Also, your DMM might be able to tell you if there is a PWM signal on the BKL_ADJ pin if you use the frequency setting. Anything above a few kHz should indicate it's using PWM. 0 Hz and the voltage is stable.

author: kjr18
author_rank: Regular Contributor
timestamp: March 12, 2020, 05:55:18 am
content: Your measurement on BKL_ADJ pin kind of suggests that is pwm based controller.Can you see what markings are on I101 chip? If there are any, this might help finding datasheet.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 23, 2020, 10:38:13 pm
content: Quote from: JKKDev on March 09, 2020, 06:47:52 pmI made the connections you suggested and they are working properly. The 3.3V to BKL_EN is good. The 5V to BKL_ADJ is good.The "monitor screen" comes on briefly at initial turn on, but then shuts off. I tried several times and left it on for several minutes and it doesn't turn back on.Do you have any other suggestions I could try?Thank You

author: vidarr
author_rank: Frequent Contributor
timestamp: March 24, 2020, 05:18:42 pm
content: There are two lines labelled "VDD" and when I touch them to GND and then release them, the light turns on. The measured voltage from each of the VDD's to GND is 0V. If I touch and release the connection fast enough (human powered PWM), the light stays on. Connecting the two VDD's together does nothing. Connecting two VDD's together with GND does nothing until released (turns ON).When I just about touch a VDD wire to GND, just very close, it sparks, even though when measured it is 0V. If I put a small "pull down" resistor between VDD and GND, would that work?I am so close. There has got to be a way to connect these wires somehow to get a permanent ON, without having to build another circuit. Well, I hope anyway.Thank You

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 24, 2020, 08:15:14 pm
content: A photo of your work in progress?

author: vidarr
author_rank: Frequent Contributor
timestamp: March 24, 2020, 09:06:49 pm
content: OK, here it is. It is nothing too exciting to look at.The BROWN wire is the 3.3V.YELLOW and GREEN are 5VGRAY and PURPLE are GND

author: vidarr
author_rank: Frequent Contributor
timestamp: March 25, 2020, 07:44:02 pm
content: I put a 1k "pull down" resistor from the GREEN wire to the PURPLE GND and the light stayed on for about one minute (without this, the light stays on just one second, or less).So, I was able to get it to stay on longer. It seems there needs to be a signal turning on and off to keep the light on.I found this really simple "flashing" circuit and was thinking something like that might work.Does anyone have any other options, or advice, of what to try next?Thank You

author: JKKDev
author_rank: Regular Contributor
timestamp: March 26, 2020, 03:10:16 pm
content: Please measure the voltage of the divider. Does it stay constant at 3.3V? The 2k/1k voltage divider might have a bit too much resistance/not enough current capability to keep the BKL_EN on.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 27, 2020, 05:03:12 pm
content: Quote from: JKKDev on March 26, 2020, 03:10:16 pmHere is what I got. See pics.5.27 V in3.49 V at the voltage divider.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 28, 2020, 03:55:08 pm
content: What would happen if I send 5V to BKL_EN, instead of 3.3V? I want to try it, but also don't want to fry it.I am running out of things to do with this ridiculous COVID-19 situation.

author: JKKDev
author_rank: Regular Contributor
timestamp: March 29, 2020, 06:13:01 am
content: If it was originally 3.3V it should be fine where it is now. You could trace the enable pin. It probably goes to one of the fets on the heatsinks. Then you could check how the output of the fet changes depending on the 3.3V applied. Also if you can, read the identification off of the fet.With the fet identified and it's datasheet as well as the configuration arround the fet we should be able to figure out exactly what it needs to stay permanently on.Another thing you could try before all of this is to manually connect and disconnect the BKL_ADJ pin to simulate slow PWM. Maybe it wants that present.

author: vidarr
author_rank: Frequent Contributor
timestamp: March 30, 2020, 02:20:56 pm
content: There is glue over the "fets" except one:URF1020CBut, that probably is not the right one."Faking" the PWM works, but I would have to build some kind of simple "blinking" circuit. I posted a picture of one a few posts ago. I don't know if it will work though.THANK YOU!

author: ogden
author_rank: Super Contributor
timestamp: March 30, 2020, 02:32:04 pm
content: Consider to LED-retrofit it. There are "monitor backlight repair / LED retrofit kits" available. Other option would be DIY-install common LED strips. Your "DIY-mileage" may vary depending on monitor. Then you can power LED backlight using widely available 12V "black brick" power supply.

author: JKKDev
author_rank: Regular Contributor
timestamp: March 30, 2020, 06:42:04 pm
content: Quote from: ogden on March 30, 2020, 02:32:04 pmAt that point, all he needs is some ground glass and a picture frame and he doesn't need the monitor at all (which might not be that bad of an idea when it comes to reliability tbh).

author: vidarr
author_rank: Frequent Contributor
timestamp: March 30, 2020, 08:48:04 pm
content: I can't believe this -- I mean it is no one's fault here, of course, I just can't believe turning this back light on is so annoyingly complicated.Thought this was going to take five minutes.I think I'll just start playing with the wires; if I fry it, it doesn't matter because if it isn't going to work, it goes to the e-waste recycle bin and I will build one with milk glass and LEDs (all of which I have already for something else, including a PSU).Thank You Guys!

author: ogden
author_rank: Super Contributor
timestamp: March 31, 2020, 02:08:13 am
content: Quote from: JKKDev on March 30, 2020, 06:42:04 pmNah. Backlight of LCD monitors and TV's have much more even illumination which is key requirement here.