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1995 S500 Coupe
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Discussion Starter · #1 ·
I am running into the well discussed subject of irritating noise coming from several channels of my audio system, originating from the Bose Amplifier in the trunk. I searched the forum for the schematics but was unable to locate it. Can somebody help me with that? It is for my 1995 S500 coupe.
Thanks!
 

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1994 S600 Coupe, 1995 S600 Coupe
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I assume you are talking about internal schematics. Never seen any. I think MAVA has repaired some of these, so he may be able to help. Changing ALL of the electrolytic caps is the most common fix.
 

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93 SL500, 95 SL320, 96 S320, 98 S500, 2002 E320 4Matic Wagon & A little 91 5.0 FORD Mustang
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Their are no "exact" schematics, but only channel schematics that apply to the general repair to these BOSE amps. The rest is technician knowledge. I have been fixing these amps for a while, and it just comes down to dried or leaking capacitors, and one can not just go change the "high resistance one's". One has to replace ALL the electrolytic capacitors. Then asses if an actual "BOSE chip" is damaged.

Before you go nuts fixing the BOSE amp. Hook-up speakers to the output channels of the radio as they will fail you too, so "garbage in garbage out". I had one with this condition. The customer was very adamant on the amp, and it was the radio output channel capacitors were toast... On 92/93 W140 this is more of a problem...

Here are two threads here on this info on this, and it applies to all these BOSE 90's amps.

See post 12, but read the whole thing:

https://www.benzworld.org/forums/w140-s-class/1705705-w140-bose-amp-bench-testing.html

See post 34, but read the whole thing:

https://www.benzworld.org/forums/r129-sl-class/1947330-diy-amp-repair.html#post8149306

Hope it helps,

Martin
 

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1995 S500 Coupe
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Discussion Starter · #5 ·
Jon, Martin

thanks for that. I already replaced the capacitors that I had readily available and wait for the remaining to be delivered. Looking at the document that you provided it seems that my unit has different "Bose" chips than what was referred to in the document. Martin: Can you confirm that this is indeed the same circuit (I have the S500 coupe)? Do you think this is the same unit as used in the R129?
Great comment to check the radio:I did - And it works fine on all channels.

Stephan
 

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Discussion Starter · #6 ·
I take that back: Further research confirmed that the static noise is coming from the Radio unit (BE 1492), left rear output channel. Interesting enough only with engine running and after a few seconds of operation, sometimes triggered while operating the rear electric window switches. This is not a HF noise suppressing issue. Does anyone have the schematics for that model?
 

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93 SL500, 95 SL320, 96 S320, 98 S500, 2002 E320 4Matic Wagon & A little 91 5.0 FORD Mustang
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Cannot help you with the radio schematics.

Some of these radios have a 1/4" spade lug terminal bolted to the center of the rear black heat sink, and a female spade loose ground cable that attaches there. Look for the ground. Then again you plant a new ground as their is a nest of ground ring terminals below the ash-tray on the 3"- square crash-sensor there..

At the same time, one of your soldered ground terminals in the radio might be broken, and may need to re-soldered(re flowed). I believe the center outside case has a tab that solders to the bottom circuit board, and something is touching the tuners too. Just remove the bottom and top cases. you will see it there.

Martin
 

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Discussion Starter · #8 ·
Thanks for this input - will check the ground connections, although a first look did not reveal any obvious problem there. As only the Rear Left channel is affected how probable is it that there is a bad capacitor in the amplifier section. It also looks like that the amplifier board can be detached from the main board. Is that correct?
 

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Power amp section and pre amp section are interconnected with a large soldered ribbon cable.

Martin
 

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Discussion Starter · #10 ·
Thanks again. After having both radio and amp on my bench I am back with concluding that the problem is with the Bose amp. Sounds confusing but the moment the amp does not get any signal from the radio it stops the static noise. Here I got mixed up thinking that the radio provided the static noise signal. Again, only the LR channel is affected. I introduced a different radio channel to the LR input of the amp and get the same noise. So I am back in replacing the capacitors of the amp circuit and see where that gets me. Confusing to me is the fact that this noise only appears with the engine running. Without, the LR channel is silent/dead. Indicates a "dirty" supply voltage to the channel (bad capacitor)?
 

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You are welcome,

What I can say is your amp is in the transistion-years, and the 92/93 used a amp which resembles a house hold brick, 94 to maybe 95/96 were fully transistor output, and 95/96 had a blend with some late 96 were all integrated outputs with the woofer channel being discrete transistor output.

The brown(long skinny 820uf to 1000uf 16v) capacitors are switch mode low-ESR capacitors, so only replace with switch mode power supply low ESR types, or you will blow-up the drive transistors, and then take out the BOSE chip for that channel. The integrated channels will have a big short stubby 2200 right near it.

The pre amp board may be your problem too with leaky signal electrolytic's. See the R129 post near the end for leaky capacitor.

This is why i just shotgun-the-repair, and replace all the capacitors, and make sure you use 105c rated capacitors, so they last, and you will not comeback to the repair 5 years later...

Check corroded connections. reflow some connections too...

Martin
 

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Discussion Starter · #12 ·
Thanks Martin. I ordered a full set of capacitors from digikey as per the list (R129). Right now I am struggling to find the path from the LR pre-amp board to the associated components on the power board. I identified the front channels, but so far have not reached a positive conclusion on the rear channels. Reason:
Re the version I have: Mostly discreet Transistors on the heatsink, but with three additional integrated circuits attached to the heatsink as well. I assume the integrated circuits re used for the two woofer channels and the center fill. Correct? Or are they perhaps used for the rear channels?
 

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93 SL500, 95 SL320, 96 S320, 98 S500, 2002 E320 4Matic Wagon & A little 91 5.0 FORD Mustang
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You are welcome, then it goes like this:

Looking the component side of the power amp section with the end-connector on the left side.

From bottom left to bottom right:

Integrated amp = right front door

next

four transistors = left woofer

next

four transistor = right woofer

Next

Integrated amp = left rear door

Top left to top right

Four transistors = left front door

next

Four transistors = right rear door

next

Integrated amp = center channel

Martin
 

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Discussion Starter · #17 ·
So I replaced all caps and found indeed several bad ones. I also found, that some individual strains of the two flat cables connecting both boards were broken, so I replaced both cables. I am still fighting the noise on the left rear door channel though. Interesting enough this noise can be heard as well (although muted) through one of the bass channels in the rear. Having replaced all caps I have to get back to the beginning to find the root cause. As two channels seem to be involved I can rule out channel-specific hardware (IC, Power amp).

BTW and FWIW: My amplifier (140 820 23 89) has a different channel setup than you sent me:

Looking at the component side of the amp with the end-connector on the left side:
From bottom left to bottom right:

right rear door
left front door
right front door
left rear door

From top left to top right:

left rear woofer
right rear woofer
center fill

There are Bose ICs used that are different from Gary James' write-up: Bose 1661R88A894114.

Stephan
 

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So I replaced all caps and found indeed several bad ones. I also found, that some individual strains of the two flat cables connecting both boards were broken, so I replaced both cables. I am still fighting the noise on the left rear door channel though. Interesting enough this noise can be heard as well (although muted) through one of the bass channels in the rear. Having replaced all caps I have to get back to the beginning to find the root cause. As two channels seem to be involved I can rule out channel-specific hardware (IC, Power amp).
Then you have buy a amplifier from another W140 or W202 90's cars to get the BOSE chip from the junk yard. That is how I do it. I had found some in china, but it was easier for me to just get one from a another car to fix the one I have...

Here is the junk yard list...

Row52 | Mercedes-Benz

https://www.lkqpickyourpart.com/locations/

At the time I was fixing a few years back. I had found the chips online, but now no more :crying

https://www.aliexpress.com/store/pr...stock-car-chip-QXDDZ/1459899_32286721563.html

What is the chip you need? Check with aliexpress

Martin
 

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1996 s320
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I did the same on mine. I had to replace one of the integrated driver chips and I took it from an amp from an e-class (w210) in the junkyard. The amps are different in the e-class but they used the same output driver chip. I rarely see any w140s in the junkyards around me.

I had verified the problem was the integrated driver chip by bench testing it. I used a sine wave tone played from a CD and checked the input and output of the driver chip. The output was clipped while the input was ok.
 

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Discussion Starter · #20 ·
Thank you for this suggestion. Do you recall the chip number? The one I need is mentioned in my previous post.My car is a '95 and I recall that the chips Martin mentioned were different to the ones I need
 
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