Sunday, December 28, 2014

Wireless TV transmitter / receiver - teardown and repair

I've recently bought a broken wireless video RX/TX pair off of eBay. The audio was being received properly but there was no video.

Since the system works at 5GHz there is no way to easily figure out if it's transmitter or receiver at fault.




In the end I managed to get it working and took some pictures in the process.



The receiver is based on a unit that is well-known in the FPV community: SM206R. The transmitter has the matching module: SM206TL.


Most of the work is done by the module itself, everything else is supporting circuitry.


Some pictures of the transmitter:





Since audio was working but the video wasn't, I assumed the fault could be one of these:
- failure in the receiver video (supporting) circuitry
- failure in the transmitter video (supporting) circuitry
- failure in one of the modules (SM206R or SM206TL)
- clocking mismatch causing the modules to become out-of-sync for the (more demanding) video stream

I started with the easier path and studied the supporting circuitry for the video pins. Nothing much was happening there, not even a resistor or capacitor.
I then measured the resistance and noticed it was quite low on the transmitter side, almost like a short. I don't remember right now if it was really a short or I was just measuring the 75 ohm output impedance, but I decided to go with that.

If indeed the modules would've been broken I had to scrap the units anyway as there is no way to possibly fix those.

I desoldered the video jack and measured again: the short was still there, so the jack was not at fault.

The video input is connected to pin 3 (from the right) on the transmitter module, so it could have been a PCB production issue.

I decided to dig out the via with a small reamer tool, thereby disconnecting the 2rd pin of the main PCB from the 3rd pin on the module. I measured resistance again and noticed it was around 75 Ohms.


At this point I soldered a wire between pin 2 (module) and pin 3 (main pcb) and the unit decided to work.

I don't know if by reaming the via out I managed to remove a short or if it was something else I did that got the unit to work.
I'll just say this was a case of dumb luck.

3 comments:

  1. Nice teardown. Thanks for the close-ups! I recently took apart a (working) Monoprice 5.8Ghz Wireless Audio/Video Transmitter and found exactly the same PCB and SM206 modules. I used them for a semi-permanent outdoor link and got 500ft easily (via FPV helical antennas) before the signal got distorted at all. Even with the stock "patch" antennas I got hundreds of feet line-of-sight. Considering the performance, the current draw of each module is quite reasonable (80-100mA @ 3.7v) once you extract it from the rest of the circuitry.

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