Monday, September 16, 2013

Teardown and modding a car mp3 player - part 2

In the first part I identified the problem leading to the unit overheat: a design decision consisting of dropping more than 10 Watts on a couple of 3W resistors.

I've thought of several ways to address the issue:
  • change the 5V rail to a switching design
  • put bigger resistors and mount the 7805 on a heatsink
  • have a switching or linear preregulator that drops 12V to a more manageable 7-9 Volts
I have a few switching supplies bought from ebay at around 2-4$ each based on the LM2596 reference design. However they might be noisy and I'm not sure how well they handle high loads. Should be good enough for charging the USB and anyway the 1.8V and 3.3V lines go through linear regulators first, cleaning them of noise.

The bigger resistor solution would also work but I did not have any 10W resistors at hand and I'm not sure they would fit in the small space between the CD tray and the case.

Since I have a few LM7808 and LM7809 from scrap I decided to go with the third solution. So I would basically bypass both resistors, go through the 7808 to drop 2-7V and use the 7805 to drop the remaining 3V.

I started by removing the "leaking" resistors and dead regulator. See the black stuff left behind by the droppers?

The LM7808 is prepared by soldering some heavy gauge wire and add heat-shrink tubing for insulation.

Teardown and modding a car mp3 player

I've bought a somewhat cheap car MP3 player a few years ago on the reason that it had Bluetooth support and nice amber lights to complement the design of my E39.

I understand some people will come here looking for a wiring diagram so I should start with that first:
Model: Watson CRC8060MU

I've always had the issue of not being able to charge anything from the USB port or the unit resetting with some strange USB sticks. Also, the unit lost its settings when cranking the engine with a low battery while it was playing music.

After the jump I start taking it apart to see how it works and what can be done to address the issues.