Question about serial cable for ln40b630n1f

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Hello. I have a dead ln40b630 that I got for almost nothing. It looks like the main board switches itself on/off--maybe due to firmware corruption. I would like to try to get access to the uboot to reflash it if possible. I removed the ps_on pin from the power connector so the main board stays powered up.

I made an exlink cable as shown in the wiki for the b-series. I dusted off an old pc and connected the cable to a real serial port. When I switch the tv on, I see voltages of -3 to +5 on the rx line, but I can't get anything to appear in putty (serial console). Loopback works fine, so I know the computer's port is working. Not sure if these voltages are conventional rs232 or ttl serial, but my understanding is that ttl serial doesn't swing negative.

Here's what I have tried:
1. swapping rx/tx--that didn't help
2. changing baud rates--that didn't help (seems like 9600-8-n-1 should be ok from the wiki)
3. tried a ftdi-based usb serial cable--that didn't work either though I did see a period on the console every second or so (".......")
4. looked for a debug switch or pads on the main board--can't find anything

Any thoughts on this? Main board could just be bad and serial not working but it seems strange that I get some kind of voltage changes on the exlink port.

[crosstalk]

I did some additional checking on the old serial cable that I used for this. It looks like one mistake might be wiring this up with an old cellphone cable--there is probably some kind of IC inside the molded part of the cable. I will have to look again at my spare parts for something better.

In the meantime, am I wasting my time trying to reflash this main board? Is this even possible on this tv?

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I built a new serial cable and it still doesn't work. It doesn't seem like the signal voltages are quite right anyway. I guess it's time for a new main board--any thoughts out there on this? Has anyone had a main board fail with no apparent hardware problems?