Hi Lars

Is the amp based on the TDA 2040 chip ?

BV6_Fan wrote:

First og all thank you to Beoworld for the membership.

I've salvaged a MX4000 (type 9150) speakersystem and the corresponding stereo-PCB and want to make it work as a stand alone laptop speakersystem.

I have some experience in the restoration of stereo amplifiers from the 70' and 80', but when is comes to Stereo IC like the LM1876, I lack some knowledge in making it Work.

I've been doing some searching here on the site and found two articles that came pretty close.

#1: first article was on how to salvage the speaker system of an Beovison Avant - which comes down to finding a way to keep the "mute" pin satiesfied.

#2: second article was a dutch article on how to do it for the MX7000 speakersystem - however it has an older stereo-module in the direction of "STK4122" which has slightly more pins.

So now down the MX4000 system.

The Stereo-PCB and speakers worked as they should before salvaging. (=no HW failure expected)

The connectors P31 and P32 goes towards the speakers. (everything is OK there)

The power-in connector P30 is configured as:

Pin1: VCC (A&B)

Pin2+3: Mass connection (common)

Pin4: VEE

I've here connected Pin 1 and 2 to an external power supply @ 21VDC and limited at 300mA

I've measured that the "mute" pin6 and pin11 on the LM1876TF are connected to ground.

And obviously this isn't enough because no sound comes from the speakers.

Question:

#1: How do connect the input voltage at P30 correct in order to accommodate for the -20VDC/+20VDC supply.

#2: Do I need to take some action regarding the "standby Pins" Pin9 & Pin14 - and make sure that the Stereo-power-amplifer becomes active.

any other suggestions are welcome.

Kind regards

Lars

Hello Beovox141,

Thank you for your interest and your Reply.

No it is LM1876TF which is commenly used in the newer MX4000, and all MX8000, BV3, BV6 & BV7 TV sets.

According to spec it requires min 20V and max 64V to operate proporly, it is a stereo IC which means it can handle to speakers. (the MX4000 only has 2 speakers), and it has 15 Pins.

Regards

Lars , West-Denmark

Hello Michael,

Very interesting page.

Regards Lars

Dear all,

I've pinpointed one problem - the Power input.

I've now manged to find at good stable Transformer with 2 outputs at around 20VAC. (Comes from an old AKAI BASIC-X1 Casette deck dated 1991, Type L01-3304-05, TKP0228-91)

My question is fairly simple. My Transformer has three outputs. Wenn I Connect it to the main grid power on the Primary side I can on the secondary side measure:

Between Pin1 - Pin2 = 20,53VAC

Between Pin2 - Pin3 = 20,53VAC

Now for my MX4000 amplification board (which has 4 Pins, & Pin2+3 are common ground) I Need a +20VAC Connection for PIN1 AND a -20VAC for PIN4.

Question: How do I know which one is the "+" and which is the "-" terminal (when both measure a positive VAC value) OR doesn't it matter because the Phase will Change 50 times per second anyway?

Looking forward to a Response.

Regards Lars 

I needs a split supply! Vcc is the postive e.g +20V Vee is the negative e.g -20V

The idea is the input GND/signal is in the center of the suppply (0V) witch allows the output to swing from -20v to +20V.

There is no power audio amplifier in the Beosystem 3 or Beovision 7 since they do not have internal speakers.

The LM1876 replaced the Sanyo Hybrid blocks in most B&O products. The successor was the famous LM3886 (aka the gain-clone) its more powerful, has more protection and a mute circuitry.

I dont have a schematics of a MX4000 output board only the TDA2040,- perhaps you could share a picture?

I would make assistance a lot easier... 🙂

BV6_Fan wrote:

I've pinpointed one problem - the Power input.

You need to build something like this,- note the symmetry around GND! Forget about the actual values in this schematic, but note the output at the caps  will be approximately 1.5 times higher than the AC value you are measuring at the trafo.

Hello Beovox141,

We probably just wrote/send at the same time.

I'm sorry - i dont have a schematics but I agree all the way.

Now comming back to your first sentence (mail 9:44) and my Question (mail 9:40)  - now ground is clear, but do I Need to worry About which is PLUS and/or MINUS when hooking up the Trafo to the amplification board?

Regards Lars

Hello Beovox141,

Thank you again for your Reply (Mail 9:52).

Something like that makes perfectly sence. I'll give it some thoght and make a test Setup.

Thank you for the quick Reply.

Regards Lars