Beomaster 8000: Filter & Tone Control Board Rework

The Filter & Tone Control Board for this Beomaster had some problems. The first thing was that it wasn't doing its part in the volume control process. The microcomputer tells the Filter & Tone Control Board what volume level to send to the output amplifier. This was not functioning and full volume level was being used.

That problem turned out to be with the Analog Devices Digitally Controlled Audio Attenuator IC (AD7110). This was unfortunate as it is very hard to find this chip anymore. I had to pull two of these (one for the left channel and one for the right channel) from a spare Filter & Tone Control board. But then that is why we have spare boards.

Here is the circuit for the Beomaster 8000 volume control.

When I put the spare board next the the one I am working on I could immediately see a difference in the boards. The spare board was a later version of the board than the board from this Beomaster (serial number 2251030).

The older serial number Beomaster has extra components.

The Beomaster 8000 service manual has a section in the back that describes this circuit as a spike suppression circuit that was removed on later Beomasters.

Thank you B&O for providing good documentation.
After replacing the AD7110 chips the Beomaster volume control was working again. However, now I had some popping noises in the output when I switched the tone filter (bass & treble) on and off. I'm not talking about the slider controls which will need Deoxit treatment. The noise was happening when the Filter button was pressed.

At first I suspected the AD10/192 analog switch (also very difficult to find) so I swapped that IC with the one from my spare filter board but that didn't fix the problem. The filter circuitry uses three OpAmps that are either TL072 or LF343 type OpAmps. The Filter & Tone Control board has five of these in the signal path so I changed them out with OPA2134PA (TI) OpAmps. The OPA2134PA is a high quality audio OpAmp and is what I keep in stock for replacing older audio OpAmps.

That did the trick. No more noise in the Filter & Tone Control board. Here is a picture of the reworked board. The replaced OpAmps are outlined in the picture.

Of course I ran the Beomaster for a while with the Filter & Tone Control board installed. This time I ran the iPod Nano source through TP1 while I started work on the Beomaster FM boards.

Just three more boards to go.

Beomaster 8000 Volume Attenuator Repair Left Channel

Before the Holidays a Beomaster 8000 decided to not be able to adjust the volume on the left channel anymore. The right worked fine, but the left channel had a constant, fairly high (maybe 3.0) output, no matter what the volume encoder wheel tried to do. I opened it up and hoped for bad contacts on the plug on the cable that connects the 6 control bits to the microcontrollers as it happened before in a different 8000 (see: http://beolover.blogspot.com/2012/12/beomaster-8000-left-channel-volume-cuts.html), but no luck: All 6 bits came through to the pins of the volume attenuator (board 4, IC102, this is the board under the control panel). I also checked the supply voltages, and they were all there. This meant the chip most likely died. Very unfortunate, since this Analog Devices AD7110 is not made anymore. Of course it would be possible to replace it with a modern chip, but this would have required some circuit design and most likely the fabrication of an adapter circuit board. Luckily I was able to find a handful of NOS AD7110KN chips on AliExpress. It took about two weeks to get them. I replaced the defect chip with a DIP16 socket and plugged one of the NOS chips in. And it worked again! I think this is yet another reminder to put these old machines on a quality uninterruptible power supply, which are usually pretty good at filtering power spikes and lightning related voltage sparks. They do not have much in terms of transient voltage suppression on their power supply board. The usually only way silicon dies is by too high voltage. The really only way this can happen in a Beomaster 8000 is through grid voltage spikes .
Here is a pic of the replacement:

It is strange that the original chips have different markings: AD13/002 8401, while the circuit diagram shows them correctly as AD7110. The replacements show the real names. Maybe 'in the day' they did not want anyone to know what hot state-of-the-art chips they put in there to slow reverse engineering. I guess we will never know...;-).