Canada Beomaster 8000: Filter & Tone Control Board

The next component for restoration on the Beomaster 8000 from Canada is the filter & tone control board. The audio signal of the selected audio source goes from the preamplifier board to the filter & tone control board where the bass, treble and filter circuitry lives. This board also has the Beomaster volume control which sets the volume level of the receiver output amplifiers according to control signals from the microcomputer board. The microcomputer board also mutes and restores the volume level when the selected source is changed. Another circuit on the filter & tone control board is for the FM signal strength meter.

Here is the board before the restoration work.

The trace side of the board has screws for securing the filter & tone control plastic panel. This board was missing one mounting screw so I will supply one when I reassemble the board.

Here is the board with the control panel removed. The restoration will involve reflowing the solder connections of the board connectors, replacing the electrolytic capacitors, replacing the LF353N opamps and cleaning the bass/treble slider controls.

Here is the board after cleaning, reflowing, recapping and new LF353N devices.

There is an adjustment procedure in the service manual regarding the left and right channel DC offset for the Beomaster 8000 volume control circuit. If the offset voltage is too high it will result in a static or clicking noise in the speakers when the volume is changing (up or down).  The two trimmers on the filter and tone control board are for adjusting out that offset. There are some associated fixed resistors next to the trimmers that can be disconnected as needed to aid in the offset trimming. The board above has two of those resistors (one in each channel) disconnected. Once the Beomaster restoration is complete I will go through the volume DC offset procedure and readjust the trimmers as necessary. I have found they usually do require some sort of adjusting.

Beomaster 8000: Tone Control & Filter Board Assembly Repair

When I first opened up this Beomaster the owner had warned me to be careful of the tone control board because it had started to crack. The crack was indeed there.  Here is a picture.

Whatever force had caused the panel to crack also broke most of the mounting tabs for the underlying board that has the pushbuttons. The broken tabs made the button board loose and some of the pushbutton caps had fallen inside.

I needed to glue the board in place to compensate for the broken tabs but I didn't want to use something permanent like epoxy. Black, crafting hot glue is good for this type of repair. The glue dries right away and will hold the board in place for this application. If it is necessary to remove the board in the future the glue can be pried off fairly easy.

For the crack in the panel epoxy is a good glue to use. I do want something permanent. The crack is too thin to use glue by itself and expect the repair to last. I used two thin strips of plastic to act as a patch and applied Araldite epoxy as the adhesive.

The glued button board tabs are not pretty but they work good and this underside of the panel cannot be seen.

Beomaster 8000: Continuing the Board Recap

Three more circuit boards are now recapped...old electrolytic and tantalum capacitors replaced with new 105°C electrolytic capacitors or polyester (WIMA) capacitors where applicable.

Here is the Beomaster 8000 preamplifier board (from various angles) before recapping.

Here is the board after the recap.

Along with the capacitor replacement I also reflow the solder joints on all of the board connectors and on this board I cleaned the input trimmer contacts with contact cleaner. The solder joints on those trimmers were also reflowed.

Next to the preamplifier board are the FM and FM Interface boards so they were next to be recapped.
Here is the before picture.

Here are the after pictures.

The last board in this set of board recapping is the Tone Control and Filter board.
This is the before recapping picture.

This is probably the simplest board to recap in the Beomaster. Here is the after picture.

That leaves just the display and microcomputer boards to do before this Beomaster will be ready for a test drive.

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.