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.




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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.








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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/.

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HPT4234, HPT4254,HPT4264, BN44-00161A and BN00162A power supplies repaired

If you are interested in having a BN44-00161A, BN44-00162A or any other power supply repaired professionally then email johnpreher@gmail.com

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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.

Testing HV transformers in LCD TV inverter circuits

This article is from the Preher-Tech May 2011 newsletter. In this article I show you how easy it is to test LCD TV inverter transformers.

Click here to download the article "Testing HV Transformers in LCD TVs"

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Typical electrolytic capacitor ESR values

This is a chart showing typical ESR values for good electrolytic capacitors. A faulty capacitor can often have a value that is 10X the value stated on this chart before it starts to cause problems in most circuits, of course this is not always the case and not for all of the capacitance values listed. This is the same ESR chart found on the original Dick Smith ESR meter and the also EVB ESR meter.

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LCD TV Display Failure Symptoms and Possible Causes.

This is a 6 page guide that shows LCD TV display failure symptoms such as vertical bars and lines or noise/distortion in display etc and what the possible cause of failure may be.

Click here to download PDF file of LCD TV Display Failure Symptoms and Possible Causes.

You may also be interested in our LCD TV Repair Guide. Click here to read more.



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Thermistors

A thermistor is a type of resistor whose resistance varies significantly (more than in standard resistors) with temperature. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
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Thermistors differ from resistance temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range [usually −90 °C to 130 °C].

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PTC thermistors can be used as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device.

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PTC thermistors are used as timers in the degaussing coil circuit of most CRT displays and televisions. When the display unit is initially switched on, current flows through the thermistor and degaussing coil. The coil and thermistor are intentionally sized so that the current flow will heat the thermistor to the point that the degaussing coil shuts off in under a second. For effective degaussing, it is necessary that the magnitude of the alternating magnetic field produced by the degaussing coil decreases smoothly and continuously, rather than sharply switching off or decreasing in steps; the PTC thermistor accomplishes this naturally as it heats up. A degaussing circuit using a PTC thermistor is simple, reliable (for its simplicity), and inexpensive.
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NTC thermistors are used as resistance thermometers in low-temperature measurements of the order of 10 K.
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NTC thermistors can be used as inrush-current limiting devices in power supply circuits. They present a higher resistance initially which prevents large currents from flowing at turn-on, and then heat up and become much lower resistance to allow higher current flow during normal operation. These thermistors are usually much larger than measuring type thermistors, and are purposely designed for this application.
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NTC thermistors are regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard.
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NTC thermistors can be also used to monitor the temperature of an incubator.
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Thermistors are also commonly used in modern digital thermostats and to monitor the temperature of battery packs while charging.

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