Monday, August 04, 2008

Belkin Tunecast 3 FM transmitter disassembly

I just disassembled my Belkin Tunecast 3 FM transmitter. It's pretty easy to open. I first removed the battery cover from the back by sliding it down. (That cover comes off far too easily!) Then I pried the black front outward from the white inner part to disengage the plastic teeth holding the two together. You can see what I mean in this photo:

Here are the closeups of both sides of the circuit board. You can click on any of the images to see a higher resolution version.


The ICs are marked, but it was hard to take a good photo of all the markings. The largest (rectangular) IC on the bottom is a BH1418F FM stereo transmitter. The small square chip is a Cypress CY8C21434 mixed signal array, which contains a microcontroller and configurable analog and digital circuitry. (I have no idea why they couldn't just use a microcontroller.) The small rectangular IC is labelled L640 6V17W. I haven't been able to identify it.

The top of the board has three rows of interesting holes with labels:
SDA SCLK X GND +
GND_D GND ? MISO MOSI
VCC_3.3V HLD(inverted) CS(inverted)
The labels remind me of I2C and SPI.

The antenna is the red wire seen along the top of the last photo. The attachment near the bottom is the actual connection to the antenna. Near the top it seems like it might connect to ground but it does not.

In the case of some devices, a lot of information may be obtained through FCC ID search. However, the information available for this device (ID: K7SF8M010) is quite limited because the interesting parts are confidential and not available from the FCC (see the summary list). The only part I found interesting is the operational description. Some interesting points there:
  • The device is powered via 3.75VDC from an onboard SMPS
  • The audio signal is filtered and attenuated before being passed to the FM modulator IC
  • The radio signal is attenuated before being passed to the antenna

3 comments:

Poul said...

Very interesting! :) Would it be possible to expand the antenna for a better range?

Goebish said...

Hi, I know this article is dated but I still ask...
I've one of those device, FM transmission works but it shutdown after 30s, just as if no audio input was detected, any idea of where I should look at ?

Thanks :)

Boris Gjenero said...

Try turning up the volume on the device supplying the audio signal. I would turn up until I hear distortion, and then back off until distortion disappears plus a tiny bit more. I expect the auto shutdown "feature" is managed by the microcontroller.