Sony BRAVIA KDL-55EX500, no power, APS-256(CH) power supply repair

Sony BRAVIA KDL-55EX500

Failure symptom: No power

Upon diagnosis of the power supply I found that  power MOSFETs Q6304 and Q6305 were shorted between source and drain and fusible resistor R6350 was open. Power MOSFETs Q6304 and Q6305 were 2SK4096 , 8 amp, 500 volt, N channel MOSFETs and were replaced with substitutes, FQPF9N50CF 9 amp, 500 volt MOSFETs. Resistor R6350 was a 1/2 watt 0.47 ohm fusible resistor.

After replacing the faulty components and installing the power supply the TV turned on and was completely functional.

 KDL-55EX500, APS-256(CH) Professional Repair:

If you are interested in having an APS-256(CH) or any other power supply repaired professionally contact johnpreher@gmail.com

Thank you for visiting our blog and remember to also visit www.preherservices.com

Viewsonic VG1930WM no power, repaired

I just finished repairing a Viewsonic VG1930WM that had no power, no standby LED, completely dead. I opened up the monitor and checked the output voltages from the power supply and both were present and stable. I verified that the 5V was still present at the main board. Next I checked the output from the 3.3V and 1.8V linear voltage regulator ICs on the main board and the 3.3V regulator's output was considerably low around 2.2V, the output from the 1.8V regulator was also very low but it gets its input from the 3.3V regulator so it makes sense that the 1.8V regulators voltage was low. I replaced the 3.3V regulator after verifying it was faulty and the monitor was still dead when I plugged it in to check voltages again, the 3.3V regulator's output was now within tolerance so I again checked the output from the 1.8V regulator and it was still low about 1.2V, so I then replaced the 1.8V regulator and when I plugged the monitor in to check the output from the 1.8V regulator I saw the standby light turn on, the voltage output from the 1.8V regulator was now within tolerance and the monitor turned on and functioned flawlessly.




Also don't forget to visit www.preherservices.com

Viewsonic VX910 no power, repaired

I recently did a repair on a Viewsonic VX910 that had no power. Upon opening the monitor I could see that someone had already replaced all the electrolytic capacitors on the power supply secondary side. I plugged the monitor in and attempted to power on to check for any output voltages from the PSU and I could here a faint chirping coming from the power supply, this is always a good indication of a shorted output diode or faulty components in the feedback circuitry, the power supply is continually trying to start up and then shutting down,over and over. As I suspected there was no output from the power supply. I removed the power supply and the first thing I checked was the dual schotttky diode D660 and one of the diodes in the dual package was shorted. The original diode was a MBR10100CT dual schottky diode, I replaced it with a B20100G dual schottky diode. After replacing the dual diode the monitor powered on and worked once again.








Learn how to repair any power supply...

Also don't forget to visit www.preherservices.com

Ask about our 3 day power supply repair course.

Philips 37MF321D/37, 37MF331D/37, 715T2056-1 Power Supply Repair

We have been receiving quite a few 715T2056-1 power supplies for repair from Philips 37MF321D/37 and 37MF331D/37 LCD TVs. The failure symptom is typically that the green standby/power indicator led blinks constantly and the TV will not turn on. The most common reason this particular power suppy fails is the 2200uF 25V capacitor at location 2074 has puffed and vented or has a significantly high ESR, some of the other electrolytic filter capacitors on the secondary side of the power supply may have a high ESR as well. The 9.1V 0.5W zener diode(BZX79C9V1 is used for replacement) at location 6079 is commonly shorted, but may also be open as well. The SMD 2.2 ohm 0.125W resistor at location 3045 has opened and is usually burned often damaging the copper trace on the bottom of the PCB. Parts are available from http://www.mouser.com/.

Remember to visit www.preherservices.com

Samsung HPT5044 (HPT5044X/XAA) BN44-00162A power supply repaired.

To learn how I repaired this power supply email johnpreher@gmail.com

HPT4254X/XAA, BN44-00162A Professional Repair:
If you are interested in having a BN44-00161A, BN44-00162A or any other power supply repaired professionally visit http://www.preherservices.com.

Make Your Own Ring Tester

In this post I will share with you all the info you will need to make a homemade ring tester also called a FBT/LOPT tester, note FBT= Fly Back Transformer, LOPT= Line Output Transformer.

Although originally designed to test flyback transformers this tool is used more often today for checking the primary winding of SMPS transformers for shorted windings and also the primary and secondary windings of high voltage transformers found in inverter circuits along with other high Q inductive components.

Click here for the assembly manual for the original dick smith ring tester which is no longer in production. This manual includes a schematic and a parts list.

Below are some pictures of the beautiful ring tester my good friend Behzad made using the schematic and parts list from the included assembly manual.

Remember to visit www.preherservices

Gateway 2100 repaired

I felt it was time to post another case history.

Gateway 2100 LCD monitor, the monitor would turn on screen would flicker and then go black. After opening the monitor I found 1 capacitor, 47uF @16V with an ESR of 10 ohms(obviously bad) in the secondary side of the SMPS section of the SMPS/Inverter board. Looking at the inverter section I noticed two puffed and vented electrolytic capacitors both 220uF @16V, locations C301 and C302 that were obviously bad as well. Sorry but I did not record the location of the 47uF capacitor, but you can see where all the capacitors were located in the photos. I also noticed some darkened spots on the component side of the PCB, on the other side, the solder side of the PCB you could see that where the dark spots were is where the inverter circuit MOSFETs were located, U301, U302, U304 and U305.After testing the MOSFETs for shorts between gate to drain and gate to source to my surprise they tested good, but they did have major solder connection problems, the solder on their connections had completely broken down from heat. I re-soldered all the connections and replaced all the bad electrolytic capacitors and checked for any other problems with the monitor. After putting the monitor back together hooking up signal and turning it on, it worked like new.









If you are wondering why the transistors are identified with a U instead of a Q, as normally transistors are labeled with a Q and ICs are labeled with a U, well this is the case on some PCBs the designers will label transistor locations with U instead of Q and instead of labeling ICs as U they will just print IC.

Mail in repair service:
We offer mail in repair service on LCD monitors. We do the repair on the PSU/inverter board from the monitor in this post for $75.00 plus return shipping and handling. Click here to see our mail in repair service page for more information.

Sylvania LD320SS8 LCD TV, Repaired

A Sylvania LD320SS8 LCD TV came into the shop that would turn on for a few seconds and then turn off and go back into standby mode. This type of failure is often caused in LCD TVs by the inverter board failing or a bad CCFL. After testing the CCFLs and the components in the inverter section of the inverter/PSU board I found a HV transformer in the inverter section location T1100, with an open secondary coil. Watch the videos on youtube for more info. Have a great day and remember you can always email me john@preher-tech.com with any questions.

 Picture of HVT in inverter/PSU board

Remember to visit www.preherservices.com and learn how to repair any power supply.

VIZIO L32 HDTV, No Backlight, Repaired

A VIZIO L32 HDTV came into the shop that would power on, the standby/power LED would go from amber to green and TV had audio but no picture. The back lights were not coming on, not even briefly. I opened the TV and powered it on. Next I checked all my secondary voltages on the SMPS to see if they were all present and within tolerance. Immediately it was apparent one of my voltages was missing, and it was the voltage to the inverter board. I disconnected the cable that connects the SMPS board to the inverter board to see if the voltage returned with the inverter board disconnected, so that I could verify it was the SMPS and not the inverter board that was failing. After disconnecting the cable and turning the TV back on the secondary output was still measuring 0V. I removed the power supply and checked the corresponding components for that output on the SMPS. I found the .027uF 630V MKP "bootstrap" capacitor location C9 was swollen and obviously bad. I check the rest of the surrounding components, for instance secondary diodes and filter capacitors and the Power MOSFETs on the primary side that are connected to the bootstrap capacitor. Everything was fine so I replaced the MKP capacitor with a .022uF 630V MKP from another board I had and put the TV back together. Upon turning the TV on it powered up the backlights came on and the TV is working great.

Bootstrap Capacitor Defined:

A N-MOSFET/IGBT needs a significantly positive charge (VGS > Vth) applied to the gate in order to turn on. Using only N-channel MOSFET/IGBT devices is a common cost reduction method due largely to die size reduction (there are other benefits as well). However, using nMOS devices means that a voltage higher than the power rail supply (V+) is needed in order to saturate the transistor and thus avoid significant heat loss.

A bootstrap capacitor is connected from the supply rail (V+) to the output voltage. If the capacitor is polarized then the orientation of the capacitor is as follows: Anode(marked with ‘+’)→(V+) and Cathode (marked with ‘-’)→Output. In other words, the capacitor should be between the output (source of an N-MOSFET) and (V+). Usually the source terminal of the N-MOSFET is connected to the cathode of a recirculation diode allowing for efficient management of stored energy in the typically inductive load (See Flyback diode). Due to the charge storage characteristics of a capacitor, the bootstrap voltage will rise above (V+) providing the needed gate drive voltage.

Hope you enjoyed this repair tip if you have any questions you can always email me john@preher-tech.com. Have a great day.

Professional Repair: If you are interested in having your power supply board repaired professionally then you may be interested in our mail in repair services. We can do the repair on this particular power supply for $150 plus the cost of return shipping and handling. Click here to see our mail in repair service page. You can also email contactus@preher-tech.com for more information.

Troubleshooting and Repairing LCD TVs. This 195 page easy to follow repair guide is only $20 dollars. Read more by clicking this link, LCD TV Repair Guide.