Viewsonic VX912 LCD monitor - power supply repair  - Page 1
Forum section: electronics / repair

author: Richard_L
author_rank: Contributor
timestamp: November 11, 2018, 05:56:07 pm
content: I'm trying to repair an old Viewsonic VX912 LCD monitor and would like some advice about a burnt resistor and the extent of the possible damageI've repaired this monitor once before - a classic monitor capacitor repair, but this time it has failed in a different manner, albeit on the same power supply PCB.I've established that the FET in the circuit that takes the mains AC and converts it into a 12V DC output for the separate PCB that handles all the fancy display stuff has failed short and caused the combustion of several SMD resistors on the way back to the IC responsible for controlling the AC to DC conversion process.The FET is an N-channel MOSFET - CET CEF04N6http://web2.cet-mos.com/PDF/CET-MOS/TO-220-263-N/CET_CEP04N6(F).PDFThe IC that controls the AC/DC conversion is a System General SG6841S Green Mode PWM Controllerwww.sg.com.tw/semigp/data/6841/iro33.0001.b5_sg6841v2.1.pdfI have two main questions:1. How likely is it that the damage extends to the IC controlling the voltage conversion? Not critically important as I've found a few available but it's a choice between £10 for 1 in UK, 10 for £1 in China! But if it has failed then I either pay a lot or wait a long time for the part! Or do these types of failure tend to damage components much further away?2. Can anyone make an educated guess at the value of this resistor (see attached photo). In the circuit, it's between pin 8 of the SG6841S IC and the gate of the failed MOSFET. Unhelpfully the datasheet for the IC has a reference circuit that doesn't include any resistors in this position, yet the actual circuit has two. The burnt resistor between pin 8 (driver output) and the MOSFET's gate and an apparently undamaged 10k resistor between the MOSFET's gate and source. I have my own idea of what the markings might say but I'm not going to reveal it as I don't want to bias anyone else's interpretation of the charred remains.many thanks,Richard

author: Rasz
author_rank: Super Contributor
timestamp: November 12, 2018, 12:40:41 am
content: I can make an educated guess at the value of this >15 year old TN monitor, and its somewhere in 0-10 pound range. Just duct tape another 12V power supply to the back and move on, its not worth even debugging the issue.

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 12, 2018, 12:54:20 am
content: With a gate resistor in that condition, sure you can change the PWM too.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 13, 2018, 02:18:22 am
content: On many applications schematics i've seen an direct connection pin 8 of the ic to the gate of the mosfethere's a schematics with an dfferent aproach : there is an resistor in serie with an diode and a resistor in parallel

author: Richard_L
author_rank: Contributor
timestamp: November 13, 2018, 03:59:28 pm
content: Thanks for that schematic, it seems to confirm that I wasn't too far out with my guess of the burnt resistor's value. I'd guessed150 Ohmsfrom the markings and the schematic suggests 150 Ohms is at the upper range of what's normal in this situation.

author: coromonadalix
author_rank: Super Contributor
timestamp: November 13, 2018, 09:27:57 pm
content: I hope your smps ic is not toast ...