I’m in the process of restoring a BeoMaster 2400-2. This is what I’ve done so far:

Replaced ALL the electrolytic caps, including the two large filter caps with low ESR 105Deg units. 

Replaced the no load trimmer resistors and adjusted no load to 12mV as per SM.

Replaced all the transistors on Board 3 (Volume Control).

Replaced D48, 12V Zener Diode on board 2.

Replaced two failed volume indicator bulbs. Here I use 12V 1 Watt units.

Cleaned the Balance adjust variable resistor as this had poor contact.

However, there is still a fault that I’m struggling to resolve and I’m hoping someone can help with:

The volume control touch pads are unresponsive ie. The only way I can change the volume is by changing the volume preset switch.

All other touch pads respond and function as they should. I’ve checked all the other bulbs and they appear ok.

It's a bit cliché.

Upgrade your membership to silver or gold and download the onsite manual of the 2400.

Just changing parts without measuring isn't the solution, sorry.

My first impression if all the other touchpads are working, look for bad connections between the volume and mainboard.

And start as far as possible (you might need a scope) measuring the voltages.

A very nice system (also my first B&O master), but tricky electronics inside.

Sounds like you already have the manual. Have you replaced the bulbs inside the white box that regulate the volume? (I am not that technical!!)

Yes, I have the service manual. I've not taken the little aluminium box apart but from what I can see on the SM there is only one bulb inside. I've tested the external points with a meter and this indicates continuity.

Hi MartinM

A few points for you to consider

The small volume control PCB for the 2400-2 is not the same as the 2400.  It is more complicated and provides 128 adjustment steps instead of the 16 steps on the 1900/2400 models.  I couldn’t make any sense of the circuit diagram in the 2400 manual until I got the updated volume circuit for the 2400-2.  Have you got the correct circuit diagram for the volume control board?

In my 2400-2, my problem was that the volume would not increase, it would only decrease.  The same as you, I could adjust the volume using the preset switch but the problem started after that.  Since the volume is clearly being controlled by the presets, there should be nothing wrong with the LDR controller and the amplifier circuits.  Do not fiddle with the LDR box if it isn’t necessary.

Your problem could be related to a failed IC but that is only a wild guess.  In my case, IC2 was only half working and had to be replaced.  That fixed my fault.  You will need to do some voltage measuring on the volume control board to get a better idea of what is wrong.

First check the collector voltage outputs from the touch controllers.  TR23 and 24 for down and TR26 and 25 for up.  If they are functioning properly then you will have to search for your problem on that small board.

Hope that helps

Regards

Geoff

Hi MartinM

Some extra information for you from my records.

To test the action of the touch pads, measure the DC voltage on the collector of 3TR25 and 3TR24.  When the pads are not being touched, the voltage is around -10V.  When you touch the respective pads, the voltage jumps sharply to around +12V.  The two test points are the large silver pads in the lower middle part of the PCB3.

If the board is working properly, the output is a variable DC voltage which can be measured on the base of 3TR27.  At high volume, the voltage is above +10V and at lowest volume is around +3.5V.  If you use the different preset conditions, you should see a difference in the voltage and this signal is being sent to the LDR regulation circuit for the amplifier.  If you touch the volume pads and this voltage doesn't change, then your problem is most likely on PCB3 and you will have to investigate further. 

Have fun

Regards

Geoff