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Friday, October 30, 2020

Beomaster 2400 Type 2902 - Canada Project Volume Muting Circuit When Source Selection Is Made

Every now and then there are some discussions on Beoworld regarding the Beomaster 1900 and 2400 receivers' volume muting during selection of a music source. It is something that I never have really paid much attention to so the discussion caught my interest and I decided to measure the volume muting activity to see it in action.

Here is the Beomaster 1900 and 2400 source selection muting circuit.
One thing to note...I have a few service manuals for both the BM1900 and 2400 but neither show this circuitry for later model PCB3 Volume Control boards. The circuit didn't change but other things on the Volume Control board did. As a result, the later model boards have different reference designators and the location of the components changed some. I found that the new reference designator for the old 3TR6 transistor to be 3TR29. Since the documentation didn't change I will continue referring to the transistor as 3TR6 though.

Here is the trace side of the newer BM1900/2400 Volume Control Board (PCB3).  I have attached three wires (black, yellow and blue) for my oscilloscope probes to connect to.

Here is the source switching volume muting description from the Bang & Olufsen BM1900 service manual.

Now for a look at the signals on the oscilloscope.  For these measurements I am still using the Beomaster 2400 with its original lamps.

The first photo is of the signals being monitored (3TR6-collector, 3TR6-base, 3TR6-emitter and 5R3)...when one source has been selected (Beomaster 2400 "On").

The voltage measurements are pretty much like the service manual states...except the value of 5R3 was expected to be higher than the voltage at 3TR6-base. I will measure these again when I replace the lamps in the PCB 5 lamp board.

Here are the same signals when a source switching event occurs.

As the service manual described, the 3TR6 collector goes to about -12 VDC. That should mute the Beomaster 2400 volume. The duration of the volume muting event I measured is 324msec. The service manual says it should last about 500msec. So I will check the duration after I replace the lamps.

I performed another series of tests using a different PCB 5 lamp board. This board I replaced the source indicator lamps with LEDs and 1KΩ load resistors (to limit the current through the LED to its rated value).  I have used this type of lamp replacement before and it didn't cause any problem with source selection and the display of the selected source. However, I suspect that it does affect the short, ~500msec volume muting circuit.

Sure enough, the LED lamps negate the volume mute circuit during source switching.  The LED assemblies don't use much current compared to the incandescent lamps and the act of switching the selected source doesn't excite the base of 3TR6 to engage the volume muting.

I could experiment with changing the 5R3 resistor and adding some bypass resistors across the LED assemblies to come up with a combination that excites the muting circuit. I would have to be careful because too much current could result in the volume muting always being on.

For now I believe I will stick with incandescent lamps for these source indicator lamps. That way the volume muting during source switching will be functional. This time I will spend a little more time seeing if I can actually hear a difference during the Beomaster 2400 source switching events.

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