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Panasonic RX-DT860

I have this older terrific sounding Panasonic Boombox RX-DT860 (huge!) and the laser unit of the CD-player died and cannot replace it. The Boombox also lacks an AUX-input, so time to change it, time to mod it. New life for an older generation.

There is a guide available how to implement an AUX-input (see also zip file below) however they use the cassette input to apply the mod. This is not what I want because I prefer to replace the CD-player with Bluetooth and an input functionality. For example when you press the "CD" function/button at the front of the boombox, this module will turn on. A more nicely and integrated feature.

Modding this unit is actually pretty easy because every connection is labeled inside and you are able to find the schematics (see also included schematics below). Double checked the audio signals are analog and yes, this is because the CD-module use a D/A converter and outputs an analog signal.


Made Module mod implementation

A) Designed this module on perfboard (salvaged the bus connector from original module) and hook it up to the CD-player bus connector (CN803) at the top of the boombox. This is working fine however there is one problem: The sound, input or output is muted.

In the schematics or at the description next to bus connector there is a pin called "d.mute". Applied HIGH or LOW however does not solve the problem.

B) So I decided to take another route, hook the audio signal directly into the equalizer input at connector CN352. This is working great (mix into any audio source) however when the "CD" function is pressed, the audio is muted again! Not as muted as before however muted, no bass and low quality soft sound. Maybe this indicates that the output is muted somewhere (at a pre stage), instead of the input. What I hear now is some leakage.


Question

Is there an easy workaround to discover? Did I overlook something? What can I do to unmute the input/output? Any ideas?




Schematics

In zip/pdf format, download it here.

Quick peek connector schematics:

Tip: To enlarge the schematics, right click on picture and select "open in another tab".

connector schematics




09/26/2021 Solution, explanation

Pretty quick answer, thanks to @bobflux. After posting my question I also take a look at the CN802 connector in the same schematic he was looking at. There is the "MUTE B" pin and solder two wires to PIN5 (GND) and PIN6 ("MUTE B") of the CN802 connector to analyze what is going on. PIN6 shows 2.7 volts when muted and 0v when unmuted. When connected together (using low resistance 20 ohms) there is music! Did the same what he suggested to do and also works great. I think his variant is safer to do because the entry point is between to resistors so it is safe to connect it to ground directly. Here is a picture of how I did it:

solution

However there is still no audio (just very weak) via flat cable, this is still muted (tested on the CD line-out). Audio directly from BT-module via equalizer works fine however cannot use flat cable connection and the designed switch.

I also discover a tiny ground issue, some noise of the BT-module. Strange because I use an isolated DC/DC to avoid this. Can be a result of tapping different ground positions in the circuit. I think in need to remove the AGND of the ribbon cable.

Next is to remove the audio connections of the ribbon cable so I can use all of the module designed functions via other applied headphone socket.

Like I noticed in the picture above, there are other possibilities to get the power you need, for example CD5V and CD8V. These power points switch on and off when user switch to "CD" or other function. In fact, you don't need the ribbon cable because the audio wiring does not work.

That's all folks! Many thanks.

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Interesting project!

There is a mute circuit right before the power amplifier:

enter image description here

It is controlled by the MUTE.B pin on the connector (high level to mute) and driven by the microcontroller through Q804. Since the microcontroller also controls the CD player, I guess it mutes the output unless it knows the CD is actually playing. The same signal also goes to the tuner, so there are probably some more features like muting when the tuner is not tuned to a station, that you won't want to break.

enter image description here

If you set the base of Q802 to 5 V then it should unmute. Or maybe to 0 V if I got it inverted. There is a 22k resistor to the micro, so the micro shouldn't have to output excessive current.

I would suggest adding a bit of logic to un-mute only when the CD input is selected, no matter what the microcontroller says. That requires knowing which input is selected. There is probably a signal on the connector between the microcontroller and the CD board that tells it to turn on, and you could exploit that. Look for a signal that has a constant level (0 or 5 V) when the CD is selected, and the opposite level when it is not.

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  • \$\begingroup\$ Hi thanks for the compliments and quick answer! I think you are on the right spot. After posting this question I also take a look at the CN802 connector in the same schematic, There is the "MUTE B" pin and solder two wires to PIN5 and PIN6 to analyze what is going on. PIN6 shows 3.x volts when muted and 0v when unmuted. When connected together there is music! However there is still no music (just very weak) via flat cable, this is still muted (tested on the CD line-out). Audio directly from BT-module via equalizer works fine however cannot use flat cable connection and switch. \$\endgroup\$
    – Codebeat
    Sep 26 at 15:50
  • \$\begingroup\$ Will try your solution! Be patient ;-) Thanks. \$\endgroup\$
    – Codebeat
    Sep 26 at 15:50
  • \$\begingroup\$ See "09/26/2021 Solution, explanation" at the end of my question. Thanks ;-) \$\endgroup\$
    – Codebeat
    Sep 26 at 20:13
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    \$\begingroup\$ Happy end! Excellent ;) \$\endgroup\$
    – bobflux
    Sep 26 at 21:01

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