Sony CMT-CP1 Micro HiFi

Upgrade to Bluetooth

My old Sony CMT-CP1 only tends to be used as a radio these days.  All my CDs have been ripped to MP3, boxed up and stashed away and, as for cassettes, I can't remember the last time I even saw one.

The stereo still looks smart and sounds great though, so I thought I'd have a go at adding a bluetooth receiver to it, giving it a new lease of life.

So below is a summary of what I did.  If you want to see more detail, I have uploaded a YouTube video here.

This is the bluetooth module that I chose, the KRC-86B V4.0.  It cost about £7.50 on eBay and I have to say I'm impressed by the general build quality.  It's pretty compact too!

It has switch inputs for track select and volume, but you don't have to connect these if you're going to control everything from your tablet or smartphone.  It also has stereo line inputs so you can feed another music source into it (eg radio, CD or whatever) and it can automatically switch its line output between the bluetooth signal (when it is paired) and the line input signal (when not paired).  It comes with a blue LED which indicates when the module is off/on/paired and a 470uF electrolytic capacitor to go across the power supply

There's very little information published on any of these bluetooth modules, but I understand that this one can run on anything between about 3V and 5V DC.  My plan was to run it at the top end of this range and, as I didn't want to run a separate power supply to it, I needed to find a suitable 5V supply from within the Sony stereo.

After a bit of digging, I came across a copy of the service manual for the CMT-CP1.  Fortunately, the manual is really detailed, with full schematics, PCB layout drawings and even quotes the various supply and signal voltages around the circuits.

I was hopeful of picking up a 5V DC supply from the power supply board, but it turns out that this board only deals with the AC side.  These are rectified to DC at various points on the main board so a bit more digging was required.

This is part of the schematic for the main board.

It shows the system controller/LCD driver chip and, luckily, this chip is fed with a 5V supply.  Now this is a pretty small surface-mount device, with even smaller pins, so making a connection to it was not going to be easy.  However, I spotted that, just next to the device, across the power rails are two smoothing capacitors.  Being conventional through-hole components, these were a much better bet for connecting the module to.


So here you can see I've soldered a pair of wires across one of the aforementioned capacitors on the main board.

However, I then discovered that the DC supplies are present throughout the stereo, even when it's in "standby".  This means that, if I were to just connect the module to the 5V supply, it would not switch off when the stereo is in standby.  This wouldn't be a showstopper, but not ideal either.

Having poured over the service manual some more, I discovered that the two amplifier chips have pins marked "STBY" (ie standby).

This must be what tells the amplifiers to "wake up" when the stereo is turned on so looked ideal as a means of controlling power to the bluetooth board too.  It's marked on the schematic as 6.9 Volts and is high impedance (via a 10k resistor) so obviously not suitable for powering the bluetooth directly, but certainly usable as a trigger signal.


I drafted the following high-side switch circuit using a couple of bi-polar transistors.  This applies the 5V supply to the bluetooth module when the STBY signal is at 6.9V and switches it off when the STBY signal is at 0V.  I'd previously measured the maximum current consumption of the bluetooth circuit (as this isn't published) and it was about 30mA when the device was paired.

To test the theory, I prototyped the circuit on a breadboard and all worked as expected.

So having proved the design, the circuit was built onto a piece of scrap veroboard and the bluetooth module was attached using a double sided sticky pad.

I drilled a 3mm hole in the rear panel to fit the LED and secured this with hot-melt adhesive.  The veroboard was perfect fit in a pair of unused mouldings inside the rear panel (lucky huh?).

I then needed to connect up the audio inputs and outputs.  I used the centre pair of phono sockets (which are marked "MD input" for minidisc) so had to desolder the phono block and snip off the centre pins.  I soldered wires directly onto the rear of the phono block, diverting the MD audio input onto the bluetooth board.  The audio output from the bluetooth board was then connected onto the main board where the phono sockets had previously been attached.  The remaining phono inputs and outputs were soldered back in their original positions so their functions remain unaffected.

With everything connected up, the circuit was refitted to the rear panel slots and the stereo reassembled.

I connected up the speakers, paired my Samsung tablet (see the lit LED below) and pressed play.

The sound quality is exceptional!!!  The frequency response is great across the board and there is zero hiss or mains hum!  I was half expecting there may have been an issue with ground loops (as the board has separate ground connections for power, audio input and audio output), but absolutely no problems at all.

I can strongly recommend the KRC-86B module for anyone wanting to upgrade their stereo to bluetooth or build a standalone bluetooth speaker.

The components for this project came to £8.02, a small price for giving a stereo a whole new lease of life.

Don't forget to check out the YouTube video for more details.