sony_ep30_bluetooth
hacking a Sony EP30 compact stereo combo to support bluetooth
parts
- Sony EP30 compact stereo
- KRC-86B bluetooth module
- blue LED (included)
- 47uF cap (included)
- LM7805
- 0.33uF foil cap (or ceramic)
- 0.1uF foil cap (or ceramic)
- some piece of prototyping board
resources
- Sony EP30 service manual http://diagramas.diagramasde.com/audio/HCD-EP30%20EP40%20sm.pdf
- https://hackaday.io/project/10794-portable-stereo-speaker
- http://www.ebay.de/itm/401223081706
goals
- bluetooth audio playing as long as bluetooth is connected
- else: tape audio passthrough
- bluetooth module is powered only powered if the system is powered (using internal power supply)
build log
stage 1: audio test with external supply voltage
- supply via 5V breadboard supply + 9V wall wart
- simply soldering the L/R/GND to PL/PR/GND on the back side of the tape connector (labeled CASSETE BOARD/CN201)
- audio does only play while the hardware tape play button is pressed
- caveats: tape motor is running
stage 2: using the CD audio input
- soldering the L/R/GND to back side of the CD connector (labeled CN203)
- could not get the audio to play at all
- I think the output is muted (CN302/MUTE) as long as no CD data is read
stage 3: back to tape
- soldering the L/R/GND to back side of the tape connector (labeled CASSETE BOARD/CN201)
- unplugging the tape play switch and tape motor (labeled CN202)
- shorting the S201 un-muted the output, bluetooth audio is playing
stage 4: power supply
- LM7805 constant voltage supply circuit is shown in the datasheet
- input voltage is +9V, soldered to the back side of the CN202 connector (V+)
- ground soldered to CN202 (M-)
- shorted S201
- +5V output is connected to KRC-86B VIN
- GND is connected to KRC-86B GND
- bt-audio is working but there is noise, KRC-86B powers on when the audio system is turned on
- putting everything back together
stage 5: audio troubleshooting
resources:
there are two types of noise present:
- a solid lower frequency humming, by comparison it sound like 50Hz triangle, according to the fourier transform of the triangle wave it contains 50Hz*(2n+1)
- high pitched noise if bluetooth is in pairing mode
solutions:
- removing KRC-86B AGND -> decreasing low-frequency humming volume, but still present without sound playing
- the system uses a common ground for both audio and V+, so disconnecting AGND removed a grounding loop
final thoughts for this stage of the project:
- moving the grounding to the central GND point of the systems power board could prove beneficial in a star grounding ansatz
- the high-frequency noise could be induced by the unshielded bluetooth circuit, but since it is all on one board, putting the circuit into a shielding box would not work
- I would like to look at the L/R and especially GND line with an oscilloscope and maybe spectrum analyzer (kinda overkill here) to perform a frequency analysis
- regaining tape functionality could be achieved by breaking the solder bridge (made by me) on S201 and plugging the connector CN202 back in
- closing SN201 sets COTR on CN201 high (+9V) (traces to the muting switch control), so VIN and GND should be moved to CN201 and a transistor could pull COTR to +9V if EN on KRC-86B is high (+3.3V) (high if bluetooth device is paired); no damage should be done since the IC seems to be protected by a diode from the COTR signal