MB 140 7 channel BOSE amp

[MBfreak]

Need some help.
I am trying to repair a MB W 140 ( 500SEL -93) that has a fancy Becker radio and then a 7 channel power amp. Sits in the boot.
The type is BOSE, it has a MB number 140 820 00 89 and is available from MB Old Timer center in Stuttgart for about half the price of a well maintained W140.
The amp has 4 channels with BOSE integrated circuits for PWM and 3 channels with standard 30 W analog 11 pin IC type TDA 7256.
HOWEVER in this module is also a 20 pin DIL IC , make BOSE , number 121 661 R88 HB 9231.
I believe this is a 4 channel opamp.
The unit was designes and manufactured i Taiwan by a company making PWM units for smaller electrical motors. They made this amp for BOSE/MB exclusively for 2 years. Company was closed down around -96.
NO documentation is available. Contacts with BOSE only states company closed down and never published any available documentation.

The analog module in attached pic is ill behaved, from time to time is starts making an awful noise ( imagine a 300 A MIG welder) and then goes back to normal.

Can anybody help wit documentation or direct me where it may be available.

Best regards

Ola

 

[RPH]

MB,
This will be a tough one. Find chips doesn’t recognize it and digging through Newark’s. I think you stumbled upon a golden chip!
Any old chassis’s to be found?

 

[RPH]

Not sure this will help. People do complain about this system, I learned that!

 

[Hohn]

Back when I dabbled in car audio, the general consensus was that any Bose system issue or modification meant removing it entirely as a system and replacing with something else. They are proprietary and very difficult to integrate with anything else.

 

[MBfreak]

RPH
Thanks a lot for the link.
Unfortunately the doc you kindly enclosed is not valid for my wonderful MB W 140 7 ch amp. Some good hints though.
Hohn. The Taiwan made BOSE stuff are disasters.

I have removed the analog amp plug in unit and will use it to make a schematic.
The much toted BOSE differential inputs are close to useless. They have very different input impedance ( + around Z 120 k, - around Z 54 k.

Ola

 

[MBfreak]

Finally located the area that disturbs channel D, left read door speaker.
It is the filter setup feeding the amp module that causes the noise. After 2 min it stops and all works OK

Ola

 

[MBfreak]

Now repaired. Must have taken me 20 hours. It was a 22 nF surface mount capacitor ( 2x2,8 mm) that had a bad solder joint on one end
This caused the bandpass filter op amp go crazy when Zfeedback from output to inverting input rose to VERY high value.
However, repairing a BOSE W140 7 channel strange design made with unobtanium documents can only be done by crazy people, of which I am one.
The rational solution is to dump it!

Ola

 

[RPH]

Persistence solves most problems. Good job!
Rational hasn’t anything to do with it.
Guys like you and me it becomes about the hunt and I will win this.
Good traits for electrical work.

 

[Metal-Marc]

Persistence solves most problems. Good job!

A big hammer does the same sometimes.

 

[RPH]

A big hammer does the same sometimes.

In electronics it’s called a surge. It’s been known to happen by the maximum smoke test.