AOC Monitor 715g2892-5-4 SMPS Clicking transformer - Page 1
Forum section: electronics / repair

author: RonaldPF
author_rank: Contributor
timestamp: January 31, 2017, 03:58:09 pm
content: Hello everyone,I am attempting to repair a AOC 2236Vwa monitor. I bought this monitor as broken, all I know is both monitors in the setup had the same problem (I only got one of them). At the beginning they suddenly turned off during use and now they just refuse to turn on.When plugged it it sometimes shows the manufacturer's logo and then shuts of after a few seconds or goes into a loop of "Power light goes on" - "Click sound & Backlight flash" - "Power light goes off".I have replicated the problem with less and less components until getting to having only the power supply board plugged into the control board (where the power button connects to, but not touching the power button).The clicking comes from the HV transformer right before the CFL connectors (which were not plugged in and I still got the clicking noise.)The power supply is a 715g2892-5-4.First thing I did was replace the bulged caps (the 5 black ones in the picture: 3x 1000u 25V, 1x 1000u 16V, 1x 470u 25V).I measured the stadby 5V line and it's ok.I measured the fuses and the values of resistors and the two diodes near the input and they're all perfect.I tried removing the HV transformer and no clicking. Any idea of what's the next step?Any help will be greatly appreciated!Here are some pictures:https://drive.google.com/drive/folders/0ByrFE4AIHGOYWE1Zd280V1ZJUmc?usp=sharing.

author: Armadillo
author_rank: Super Contributor
timestamp: January 31, 2017, 05:33:23 pm
content: Quote from: RonaldPF on January 31, 2017, 03:58:09 pmif you are able to replace or swop T801.Confirm 14.5v at F801,Confirm 5v at F901.Confirm 5.1v at F903.

author: RonaldPF
author_rank: Contributor
timestamp: January 31, 2017, 06:06:27 pm
content: After resoldering PT801, D903 and D904 the clicking sound is much softer and now the power LED won't turn on(still doesn't work, I can't understand why there is a different behaviour, I powered it without PT801. Maybe that damaged something else?).I can't find F801.I'm unsure about how to measure since F901 since it is on the input side (AC)Now F903 (which is where I measured before) is 2.7V~3.0V (looks like oscillating).I measured the 100u 450V input capacitor with the oscilloscope and it's bang on. Now just as before 300V across it.I measured the resistance of PT801 when it was removed: both input coils 0.3ohm and output coil 500ohms.

author: oldway
author_rank: Super Contributor
timestamp: January 31, 2017, 06:20:42 pm
content: Check if there is no diode in short circuit.Caps of low values don't became bulged even if they go bad , so replace them too.

author: Armadillo
author_rank: Super Contributor
timestamp: January 31, 2017, 06:37:24 pm
content: Quote from: RonaldPF on January 31, 2017, 06:06:27 pmSee F801 location attached.Remove F901, confirm if you have 5V. see schematic attached.

author: Armadillo
author_rank: Super Contributor
timestamp: January 31, 2017, 07:04:51 pm
content: if the above power rails check OK then,Check resonant wave of pin7 & pin8 for T801Check the output of T801Change T801. See attached.

author: RonaldPF
author_rank: Contributor
timestamp: January 31, 2017, 08:47:01 pm
content: Diode check:Oldway:D903, D904 0.54V fwrd voltageD907 0.01V both waysD801 0.6VZD901 1.1V reverseCap check:C604 OKC913 OKC920 770uF (instead of 680uF, not sure if that's a within tolerance)Armadillo:I think the schematic doesn't correspond to this board since I am unable to find the components layed out in the same way. (I still can't find F801)I have removed F901, 0V between pins. 0.5V on 5V rail. (I measure @ CN603, middle of the right side looking from top)Right now the output of T801 is 0V, I think it is not even being turned on. With F901 put back 5V rail @ CN603 is again 2.7V~3.0V.It looks like I goofed something else up since at the beginning of the repair T801 was clicking...All the steps I have take have been mentioned and I have carefully inspected all my soldering for proper connection or shorts, all good...

author: Agent24
author_rank: Regular Contributor
timestamp: January 31, 2017, 09:34:53 pm
content: Have you checked the output diodes? D902, D906? etc

author: RonaldPF
author_rank: Contributor
timestamp: January 31, 2017, 10:34:15 pm
content: D901 is a FMX12S Silicon rectifier diode. 0.4V on both diodes.D906 is a SP1060 Dual Schottky. 0.16V fwd voltage on both diodes.I have added a better top view to:https://drive.google.com/drive/folders/0ByrFE4AIHGOYWE1Zd280V1ZJUmc?usp=sharing

author: RonaldPF
author_rank: Contributor
timestamp: February 01, 2017, 11:26:02 am
content: Update 1:On closer inspection FB902 (which I thought was a Ferrite Bead since FB901 & another FB902 in the audio part are FBeads) is open circuit. I removed it and there is no connection between it's leads, however there is continuity across it's pads. I followed the traces and they are indeed connected. I am now confused as to why you'd want a closed loop within the same nodeEDIT:What component should I replace it with? It looks like a fuse and says F3AL250V...Update 2:Discrepancies in component labels come from my board being a 715g2892-5-4 and the manual being from a 715G2892-2-3. I will try find the service manual & schematic corresponding to this board.

author: Agent24
author_rank: Regular Contributor
timestamp: February 01, 2017, 08:57:37 pm
content: That is indeed a fuse. And if there is no continuity across it then it is blown.Most likely you have a shorted MOSFET or driver chip in the inverter section.

author: RonaldPF
author_rank: Contributor
timestamp: February 01, 2017, 09:49:27 pm
content: Why would there be a fuse in a closed loop?Switching mosfet has been checked and is OK.I managed to get my hands on working a 715G2892-2-3 to test and compare, some parts are similar. I will now proceed to test the power lines comparing to the other board... I could'nt find any schematic for my board.

author: Agent24
author_rank: Regular Contributor
timestamp: February 01, 2017, 10:29:18 pm
content: There wouldn't be a fuse in a closed loop. Either there is some measurement error, or what you are measuring is a symptom of the fault.I don't mean the main switching transistor, I mean the two SOIC-8 package MOSFETs which drive the inverter transformer.

author: james_s
author_rank: Super Contributor
timestamp: February 01, 2017, 11:56:15 pm
content: I suspect from the number that this is a 3A 250V fuse.

author: RonaldPF
author_rank: Contributor
timestamp: February 02, 2017, 08:51:27 am
content: I FOUND IT!Schematic for the 715G2892-5-4!!

author: RonaldPF
author_rank: Contributor
timestamp: February 02, 2017, 10:40:19 am
content: UPDATE 1:I followed the circuit and found some components missing from the PWM circuit. Modified circuit attached.UPDATE 2:I desoldered the two fuses before the 5V lines and the jumper that brings the +16V to the next part of the circuit, I then measured and they all have 101V to GNDFuses are OK. Soldered back the +16V jumper and still got 100VHow should I proceed knowing that there is something wrong with the power section? Should the +16V work disconnected from the rest of the circuit?NOTE 1: The schematic says that between earth and GND there is FB902 and there is a Ferrite Bead symbol, however I had a fuse (apparently blown) what's this all about? a closed loop with ferrite Beads on schematic? Fuse irl?NOTE 2: Both fuses I removed are 250V 4A

author: RonaldPF
author_rank: Contributor
timestamp: February 02, 2017, 08:14:21 pm
content: I tried disconecting the first part of the power circuit from the rest on the working 715G2892-2-3 and it worked: 5V, 5V & 16.0000V.I figured the only reason I could be getting 100V on the faulty board was because it wasn't switching, since both boards have similar spec MOSFETs i switched them and IT WORKS!The power circuit of the 715G2892-5-4 now works and I will order some power MOSFETs, but I'm afraid it's not the only thing wrong....I will now proceed to measure all the components of the rest of the circuit to be sure it's all good before putting fuses back. I'll keep you updated. Any tips are apreciated!Still completely confused about the Ferrite Bead/Fuse thing. Maybe I should post it in another topic... The 715G2892-2-3 doesn't have it.... The only thing I can think of is it's designed into the board in case you don't have the audio part (all replacement boards I can find are missing the audio part.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2017, 11:51:22 pm
content: It's pretty common to have unpopulated components on boards. Often the exact same PCB is used in multiple different models with the lower end ones simply not having those areas populated.

author: Armadillo
author_rank: Super Contributor
timestamp: February 03, 2017, 06:31:38 am
content: Quote from: RonaldPF on February 02, 2017, 08:14:21 pmyou get it to work.But I think, If it don't switch, how will you get 100v? Maybe a miss-jumper cause no feedback to IC903 that regulate the Opto-isolator which is essential feedback to the OSC IC901. Q904 needs to be in-circuit to load the circuit during operation.It's puzzling.... can you remember where you took out the jumper?

author: RonaldPF
author_rank: Contributor
timestamp: February 03, 2017, 08:34:58 am
content: Well I got the first part of the circuit to work, the part in the diagram you attached. I think the 100V where coming directly from the transformer turn ratio: 300V on one side = 100V on the other (with no switching). By replacing the MOSFET it now works so I am pretty confident that was it.The jumper I took out is the one that carries 16V to the second "page" of the schematic, right after ZD901 (TXZX18B)I am now missing the second part that goes to the HV transformer and then to the CCFLs... I don't want to connect it just yet since I do not have any replacement power MOSFETS at the moment. I've been checking diodes and there are two zeners (ZD802, ZD803) which drive the output MOSFETS, one of them measures 0.6V fwd voltage, but the other is only 0.02V... (They are both RLZ5.6B 5.6V Zeners). Maybe I have a shorted driver MOSFET and the Zener went with it....

author: Armadillo
author_rank: Super Contributor
timestamp: February 03, 2017, 09:39:49 am
content: More precisely, it should be after the 16v line that goes after supply to R194 and not before it.300V is DC, so no pulse, no induction to the secondary.Anyway, you get it to work, and that's good.You are right to suspect the diodes, I would swop it.

author: RonaldPF
author_rank: Contributor
timestamp: February 08, 2017, 12:00:03 pm
content: No, I did not get it to work yet.I got and replaced the driver MOSFETs (AO4620), zener and fuse[Q805, ZD802, FB902]. Power lines are back to17.5V&5.23V.Now it behaves in the following way:When plugged in --> orange standby light after a few seconds.When power button pressed --> (8~9 seconds from start to finish)Power light goes BlueBacklight comes on and AOC logo appears on screenScreen goes BlackOnscreen message "Input: DVI"Screen goes BlackBacklight turns off and power light goes orange[\li]I have readhttp://www.datasavantconsulting.com/roland/2sec2blk.htmland:QuoteQuoteTwo questions then:1. Is it possible that by not getting low ESR capacitors it doesn't work or should it work but fail sooner?2. How can I test my transformer? (resonant waveform?)

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2017, 12:23:19 pm
content: Quote from: Armadillo on January 31, 2017, 07:04:51 pmUse a divide by 100 HV probe, measure with oscilloscope or multi-meter. e.g. Fluke 80K-6HV Probe or Fluke 80K-40HV Probe