Poor Man’s Signal Generator
One of my nephews dropped this 26” Sanyo LCD TV and broke the screen. So I decided to make a signal generator out of it! I disconnected the screen and the inverter board so I could test signals on the PSU and Main Board. But the TV would not turn on, so I had to reconnect it. I have not probed around the Inverter board for this article though.
As you can see in the photo, I zip tied the boards to prevent them from moving around. I also soldered some wires onto the back of the board to a Schottky diode that is difficult to get to, and also the main capacitor since it is under the cover/ heat-sink on the power supply PCB.
The wires on the Primary Capacitor allow me to easily discharge it when I turn the voltage off to move my probe around to different test points & IC’s. This is the waveform from an N Channel Enhancement Mode MOSFET on the PSU. Part# FQP16N25C. It is singing along at 72.8 KHz.
I probed an IC #LA42205 on the Main Board and noticed that when I turned the power off, the waveform turned into a regular Sine Wave. I thought that maybe the IC was connected to the standby circuit. So I looked at the Datasheet for it and found out it is a Monolithic Linear IC for Audio Output and is a 5w x 2 channel power amplifier. I noticed that it has a Built-In Standby Function. So that is why the waveform was changing when I turned off the power. Hopefully you can see in the pictures, the needle on the Voltage Meter on my Variable Isolation Transformer, and how the waveform changes.
I also probed an IC #LA5774. It’s a Separately-excited Step-down Switching Regulator (Variable Type). It has a built-in Time Base Generator (160KHz) and Thermal Shutdown Circuit. You can see in the picture the nice Rectangle Waveform it sends from Pin 2 that goes to the LVDS power.
It is fun probing the IC’s and learning about them and checking the Datasheets. Even though I don’t get to repair very many devices, I can still learn about them by studying my poor man’sSignal Generator.
Please DO NOT try this without an Isolation Transformer and only if you are Qualified to service high voltage devices or you can get hurt badly and possibly even Killed!!
This article was prepared for you by Robert Calk Jr. from USA and he is a Hobbyist. He’s been learning Electronics for a year now and really enjoy it. He welcomes any comments and suggestions you care to contribute.
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