I have found two additional causes of no video or no sound, besides the causes I described in my earlier posting on this thread.
1. If you have an LC III with no speaker sound, but sound does come through the headphones, then check continuity from pin 8 of U6 to the speaker pins. If neither speaker pin connects to pin 8, then a corroded via between C13 and the crystal could be the cause.
Drilling a hole through the via and connecting it on both sides with tinned wire should fix it.
2. If your LC has no video
and no sound, then check for 5V at pin 4 on U22 while power is turned on. That chip provides the clock signal that goes to the CPU (among other things). Obviously, if this chip does not have power, you'll get no CPU clock, and thus no video or sound.
The cause is likely a leak from capacitor C18 (replaced by a ceramic capacitor in the image below) that damaged the nearby via. This via connects this +12V trace to the input of VR1. VR1 provides the power to the clock chip.
I drilled out the via using a #80 (0.0135 inch) drill bit using just my fingers. Use a carbon steel or high speed steel bit (
https://amzn.to/3J6CXR5), because carbide CNC PCB drill bits are brittle and may break in the hole. Guess how I determined that!
In the picture below, note the crumbly gray chips beside the fresh shiny stringy chips. The gray material is corroded, followed by uncorroded metal.
You also need to scrape off some solder mask to expose a clean connection to the trace. Then you can solder a bare tinned wire through the hole to reconnect the top and bottom of the board.
Earlier I said you needed to test 5V on pin 4 with power turned on. That's because the clock chip is sourced from it's own voltage regulator, not the main +5V from the power supply. This is likely to keep the clock steady even as the standard +5V power supply sags and spikes due to computer and accessory usage. Therefore, do NOT test for continuity between pin 4 and standard +5V, and do NOT wire a bodge wire to standard +5V, because that is not the correct circuit.
Hope this helps,
David