Sony KDL-42EX410 keeps rebooting - Page 1
Forum section: electronics / repair

author: DJVG
author_rank: Contributor
timestamp: October 16, 2016, 09:44:28 am
content: Hello all!To start: my knowledge about electronics is not very good but I'm eager to learn!Today I got aSony KDL-42EX410from a friend and it has the following issue:When you plug the device in the power/standby LED lights green and stays green. When you turn on the TV the backlight lits and the Sony logo shows. At that moment the screen turns of, the power/standy LED fades to almost out. Then the screen turns on again, green led to full on and it reboots again. This keeps repeating.The reason why I suspect a power problem is the fading power/standby LED (correct me if I'm wrong). I used the EEVBlog BM235 to measure some capacitors but I know it's hard/impossible to measure caps in circuit.I disconnected the backlightLED inverterand the problem persists. So I think I can make sure this board is not part of the problem.I took a high-res picture of the Sony APS-308 power supply with the capacitor values next to it:The two caps on the left top (just under the type number) are 470µF each and when I measure I get 1226µF (or sometimes even higher) and I suspect these are in parallel with some others.I also created a picture of the back:What would be the right next step to diagnose this issue?Edit:I replaced the power supply and the problem persists. With only the main board connected I see the same problem so the problem should be in there is my guess.Video of the problem:- DJVG

author: singapol
author_rank: Frequent Contributor
timestamp: October 16, 2016, 09:58:10 am
content: This is not your model but at least you can know how to troubleshoot something similar. Most likely the power supply as you mentined. Wait for "Processing" change to "Get manual" to download.http://elektrotanya.com/sony_kdl-32_40_46_55ex720_723_chassis_az2-f.pdf/download.html

author: DJVG
author_rank: Contributor
timestamp: October 16, 2016, 10:39:58 am
content: The 3.3v standby voltage is good.On the top left there are two resistors that are: green - blue - red - gold. Which should be 5.6k ? 5% if i'm right but I only measure 2.4k ? on each of them. Is it possible this value is wrong while measuring in circuit?Besides these two I measure the right values for the other resistors.@singapol:Thank you for the information! I think my knowledge is not good enough (yet, hopefully) to understand all of it but I will look at it!

author: singapol
author_rank: Frequent Contributor
timestamp: October 16, 2016, 06:15:07 pm
content: Yes it's 5.6K but since you measure in-circuit it will read a different value because it is connected to some othercomponent. The only is to desolder one end and lift it but you may damage it so better to remove out of pcb.The pictures look ok nothing suspicious. That's why it's good to read the service manual at least they tell you where to measure voltages at impoertant points of the circuit. Good luck.

author: poot36
author_rank: Frequent Contributor
timestamp: October 17, 2016, 06:04:46 am
content: Try measuring the AC ripple on all of the power supply outputs by setting your meter to AC volts and then testing all the listed DC voltages on the board. Any reading over 1V (except on the backlight circuit) should be treated as suspect.

author: DJVG
author_rank: Contributor
timestamp: October 17, 2016, 06:22:46 am
content: Quote from: poot36 on October 17, 2016, 06:04:46 amThanks! I'll test that when I get home!

author: DJVG
author_rank: Contributor
timestamp: October 19, 2016, 08:27:30 pm
content: After some testing I can tell the power supply is fine and that the problem is in the t-conn board.Here's a picture:I can see that the 3.3v (t-conn-on) is going up and down which causes the power supply to shutdown. I do see a 12v rail to be turned on on the t-conn board which means POWER ON is high on the power supply. This rail is staying on during reboot but I do see voltage drops on other parts of the board (on test points). I took off a cap that I thought was defective but it was just fine testing it standonline.I have a hard time figuring out how to proceed with testing this and I really want to learn how to diagnose this. Anyone that can point me in the right direction?Edit:Two 12v rails are going to the t-conn board. One is called REG 12V (stays on because of POWER ON high) and the other one is T-CONN. So for some reason the 3.3v (digital?) and 12v T-CONN are going up and down. I'm sure I need to look for the problem here, just don't know how, yet.

author: BMack
author_rank: Regular Contributor
timestamp: October 20, 2016, 05:11:03 am
content: Plug it in with everything plugged in, put your finger on the HDMI chip after it's powered on for a few seconds, is it hot? Tap it to tell whether it's hot, if it's bad it can get VERY hot. I've had a lot of these sets with bad HDMI chips. You can buy replacement chips from China.

author: DJVG
author_rank: Contributor
timestamp: October 20, 2016, 06:10:09 am
content: Quote from: BMack on October 20, 2016, 05:11:03 amI don't have any hot chips. Not even after running it for a long time.I made a video describing the issue:The fact that it shows the Sony logo (evne with a fade in) give me hope. Something's still processing data.

author: Srbel
author_rank: Frequent Contributor
timestamp: October 20, 2016, 06:26:54 am
content: For start, change 2,2uF 50V capacitor.

author: DJVG
author_rank: Contributor
timestamp: October 20, 2016, 07:18:53 am
content: Quote from: Srbel on October 20, 2016, 06:26:54 amThanks for your suggestion! Do you mean on the power supply? I put in a new power supply as a test and the problem persists. So the problem must be in the main board of the TV. I disconnected all other boards.

author: Srbel
author_rank: Frequent Contributor
timestamp: October 20, 2016, 11:45:55 am
content: Oh. My mistake then.Yes, I meant the PSU.But if the PSU is not the problem, and you have unstable 3,3V on the T-CON board, then check the DC-DC converter that outputs 3,3V on the T-CON board.

author: BMack
author_rank: Regular Contributor
timestamp: October 21, 2016, 05:04:57 am
content: Check the voltage on both legs of transistors Q1 and Q2 on the main board. Is it dropping out on either leg?

author: poot36
author_rank: Frequent Contributor
timestamp: October 21, 2016, 06:42:29 am
content: I would test all filter capacitors on the input and output of all the buck converters and linear regulators on the main board. I once had a PC monitor that the main micro would lock up due to a 100µF cap that was on the output of a linear regulator that powered the micro been bad. I managed to find it with my oscilloscope by looking for oscillations on the output of the regulator that had become unstable due to the bad cap.

author: DJVG
author_rank: Contributor
timestamp: October 21, 2016, 03:58:01 pm
content: Quote from: Srbel on October 20, 2016, 11:45:55 amQuote from: BMack on October 21, 2016, 05:04:57 amQuote from: poot36 on October 21, 2016, 06:42:29 amThank you al for this input! I will have some time in the coming hours to test this. Thanks again, greatly appreciated!

author: DJVG
author_rank: Contributor
timestamp: October 21, 2016, 04:56:18 pm
content: I think I messed up. While measuring Q1 and Q2 is shorted two pins on Q2 (the two pins in the green circle). And now thisDC-DC converterN-Channel Enhancement Mode MOSFET is a dead short. It needs to be replaced. I'm sure this is what they call a rookie mistake.This is what I measured so far (red probe on the pin, black probe on ground):I did found the datasheet here:http://www.anpec.com.tw/ashx_prod_file.ashx?prod_id=161&file_path=20081121131637724.pdf&original_name=APM2513NU.pdfEdit:I just ordered a new APM2513N and all the caps that are on this board. Just to be sure.

author: poot36
author_rank: Frequent Contributor
timestamp: October 22, 2016, 09:17:32 pm
content: It looks like you shorted the 5V rail to ground based on the picture. This would have killed Q1 as it is being supplied 12V from the PSU. The pin you measured 9.43V on is the gate of the MOSFET and is usually a square wave control signal. You can quickly test all the MOSFETs for shorts using your meter in diode test mode. There should be no reading from the gate pin to the drain or source pins unless there is a pulldown or pullup resistor to improve the MOSFET switching times. Between the source and drain pin you should get the diode drop voltage of the internal protection diode in one direction and in the other no reading.

author: DJVG
author_rank: Contributor
timestamp: October 23, 2016, 07:40:04 am
content: Quote from: poot36 on October 22, 2016, 09:17:32 pmThanks for your message! I think both Q1 and Q2 are dead. All pins I measure are a direct short to ground. I've ordered new parts and I need to wait for those to arrive.Edit:I will remove the parts from the board the measure better. I forgot that measuring in circuit is not the right way to do it.