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Becker Cascade BE7944 need help!

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zero123456789

Hello to everyone!
Thanks in advance if someone can help)
I have a problem with replacement of malfunctioned final amplifier at Cascade BE7944.
It is play sounds through front loudspeakers with distortion. 
It is not a problem with wires (I have tested it them with ohmmeter) and loudspeakers (brand new).    
I have replaced with Traffic pro and it worked fine.
Radioengineer has announced the verdict - problem with final amplifier and it is must be replaced, and preferably with service manual or schematic, which I do not have. And I did not allow him to unassemble my Becker just "for to watch how it is looks from inside")  
The problem is following:
1. Do someone have and can share with me with service manual or schematic of Becker Cascade BE7944.
2. As far as I understand BE7944 is further development of schematic Audio 10 ECE Typ 6011 (or 4011), which service manual is available at Internet - can I use it as general background for BE7944?
3. At present time unfortunately there is no possibility to find original MLT TDA 7454, only CHN or unnamed versions. I do not want to install that, but according to datasheet for TDA 7454 it can be replaced with TDA 738 *. And TDA738 * can be raplaced with TDA 7560 or TDA 7850 (in my case TDA 7851 L). If it is possible to replace TDA 7454 with TDA 7851 L and with which modifications? And will I not lose the “Backer surround” function? 
4. Any other suggestions? Please withought "sell this dinosaur"
 I have written to Harman / Becker with this problem but still no answer ...   
Thanks to all!
 

Comments
bluemoon

Hi,

as first you need to measure voltage on speaker pins, of course without speakers and any signal.  On the pins you should measure 1/2 of supply voltage. If voltages are correct, you dont have a problem with output amps, else you must dissassebly the whole radio and you will finde the correct signe on hybrid IC. It is usuali not hard to do. If the working points are correct whitout scope you can't find this error. Usualy, if out amps are OK the sound processor i damaged. This is usuali end of servicing possibility because it is hard to finde this part.

best regards

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zero123456789

Thank a lot! I hope that problems with out amps)

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tomirevesz
tomirevesz's picture

Hi!

Next step: check the Zobel network of the amplifier output. Maybe it causes excitation.

https://www.wavecor.com/html/zobel_networks.html

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zero123456789

Thanks a lot! I will check that too.

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zero123456789

Hello to everyone!
I have found an experienced radio engineer and he has replaced TDA7454 for TDA7560.
It also compatible with TDA7850 or TDA7851L. But TDA7560 sounds better.
All functions works fine, include becker sorround.
25-th pin is not important at it is can be not used at all.
The same to 16-th pin.
Just use datasheets for both amplifiers
and make necessary modifications according to the manufactures recommendations.
It is justifiably for all backers. Thanks to them)
Best for all)

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