BMW CM5905 teardown + technical info thread

[ZeKahr]

My Alpine radio was busted so I decided to tear it apart and see how it works. I have a basic understanding of audio circuits but not enough to reverse engineer the entire thing. If anyone else wants to share their knowledge on the low-level workings of these old radios or on the parts shown in the thread, feel free to post it here. I figured I'd help the community out on radio related projects/fixes by sharing what I found. This thread will be updated whenever I get the time.

This is the volume potentiometer. It is a 20 kOhm 2-channel unit (hence the 20KAX2 designation) made by a Japanese Alps (maybe that's the same for Alpine?). The unit is mated to a PCB which contains two diodes and a light. The board contains a 5-pin connector and two 3-pin connectors.


The connections for the 5-pin connector are labeled as follows on the motherboard, from left to right respective of the image with wire colors in parentheses: L+ (brown), L- (red), IG (orange), PWR (yellow), SE (green). I believe L+ and L- are to power the light, PWR is to power the potentiometer, IG is a ground for the potentiometer. Not sure what SE is. I presume both 3-pin connectors are to relay a stereo signal through the potentiometer. One of the two 3-pin connectors routes to the motherboard while the other one connects to the tone control circuit.




This is the tone control circuit (AKA your bass and treble sliders). It is a set of 20 kOhm 2-channel slider potentiometers (also made by Alps) mounted on a PCB with 4 capacitors and a bunch of resistors. The black/white/pink wire connection connects to one of the 3-pin connectors on the volume potentiometer and the brown/red/orange connection connects to the motherboard.

[ZeKahr]

So since there is a 5 attachment limit, I'll have to split this thread into multiple posts... Anyways, here is the backside of the tone control.



This is the motherboard. As you can see, it is a multiplexed mess of wires, capacitors, resistors, diodes, and ICs. Most of this stuff is probably AM/FM modulation logic and interfacing with the tape deck. Given that we've moved on from tapes and AM/FM to aux input and bluetooth streaming, 95% of this logic can be eliminated and be replaced with a much simpler circuit to handle audio amplification. The 4 boxes to the right appear to be some sort of amplifier circuits - I believe the wattage is 15W nominal and about 8W RMS per channel. The heart of the motherboard is an IC known as the Alpine 51T71686F02, which I'd wager has been long out of production.



Alright, so it turns out the amps are actually at the back of the radio. They are a pair of Fujitsu MB3731 units rated at 18W peak. They are glued to a black heat sink which forms a part of the rear casing of the radio. A datasheet for this unit can be found at https://datasheetspdf.com/pdf-file/5...jitsu/MB3731/

[bmwman91]

Good times, it is always fin to tear this old stuff down and see how they did it. Thanks for posting the pics.