PCB 3, TR2 transistor base would be one to monitor (B92 to C92).  Or PCB 4, IC1 pin 38.
As you have discovered it isn't a real mechanical switch.  It is a pushbutton that the IC1 on PCB 4 has to detect (along with other control panel/remote control pushbuttons).  IC1 controls the sending of the mute signal.

Since the mute isn't working from the front panel and the remote then start from the mute pin out of IC1 pin 38 and trace it to PCB 3 TR2.  

-sonavor

Thanks for the tip Sonavor.

Looking at PCB3 and the base of TR2, i read no voltage and I see no effect when pressing the mute button on either the front panel or MCP remote.  It does jump to ~3.5V or so when the unit is turned off (and is in stand-by mode).

So I started to look at PCB4.  Pin38 also reads 0V and is steady when either mute method is attempted.

Then I looked at the mute signal coming into PCB4 from the outside world.  This comes in in pin 24 of IC1.  This pin reads 5 volts, but when I press the mute button on the front panel, I see a 2ms pulse to 0V and then back to 5V.  So a digital signal is being sent to IC1.

Since I have a parts unit, i tried simply swapping PCB4 and the unit behaves exactly the same.  Remote works well as far as I can tell except for the mute function. Pin 24 of IC1 will display the same 2ms pulse to 0V when you press it the front panel mute.  Pin 38 (and the base of TR2) both show 0V and no effect when pressing mute.

Both PCB4s have now been recapped, but they showed the problem before and after.

I'm stumped!

Along with Mute pin 24 handles FM Mono, Open and Select.  Do those other functions work?

I haven't had to dive into that area of a Beomaster 5000 I'm afraid.
Most of the time problems like this are related to a connector or wiring problem.  Especially when two different PCB4 units have the exact same problem.

Look at P26 and check that it is good.

-sonavor

pin 24 seems to be doing its thing.  Open works, select works and so does FM Mono.   Only the mute function fails.  Also took a look at P26 and it looks good.  Also, i was doing all my measurements from the back side of P26 where the solder bumps live on PCB 4.

The only thing I can think of is that my digital pulse to tell IC1 to mute may not be long enough for some reason.  I have no idea what the spec is for this, but my pulse seems to be about 2ms long as measured on my scope (this is the pulse when it drops from 5V to 0V on pin 24).  Maybe the IC needs a longer pulse to pass along a mute toggle?

Looking at this a bit more, if I press and hold the "mute" function, a series of digital pulses can be observed on pin 24 and they shut the unit off, which is the way the thing is supposed to behave.

So is it IC1 that has the smarts to decide if I'm holding the button down briefly (maybe only one or a few digital pulses) or hold it down for enough pulses to shut down?

If "yes", then it is most strange that I have two PCB4 boards that behave the same.

And if anyone knows, what is the series of events the MCP should make happen when toggling "mute" ?

It is difficult to say for sure as I haven't ever measured the audio mute signal of the Beomaster.  The service manual does show a 0V to 3.8V to 0V pulse of 300ms on what appears to be the P12-3 (C92 reference on Diagram B) to 3R6.  That is the audio mute signal to PCB 3.  The other end of the cable (reference B92 on Diagram C) goes to 4IC1-38 volume mute.  

Check the muting pulse on those points using both of your PCB 4 boards and see what you see there.

-sonavor

I think I've finally figured out why I can't get the "Mute" function to work.   I tried a different pre-amp board (which has other problems due to a dose of DC current) and it worked with the mute logic.

So the problem I was seeing was on the pre-amp board.

I eventually figured out that the mute signal is sent into IC5 through the serial data line on IC5 pin 22.  I can put that on the scope and see the change when you press and release the mute button.  So the signal is working and getting into the integrated circuit.  I see this on both the working and non-working pre-amp boards.

The mute signal to the relay is supposed to be a voltage change that comes out of IC5 on pin 39 going from something like .1V to .8V (hard to read on the schematic).  The working IC5 does this and you can measure the voltage that gets sent to the base of TR7.  The non-working pre-amp board does nothing with the mute signal on pin 39.  Voltage is zero either way.  So I think the issue I have is with IC5.

Unfortunately, this is a 40-pin integrated circuit and is not socketed.  I don't think I'm brave enough to move the working IC5 from one pre-amp board to the other to get a fully functional Beomaster 5000...  I somehow think the most likely outcome of the attempt is that I go from a almost completely working Beomaster 5000 to a non-working Beomaster 5000!

Fun excercise.  Would have been more fun if i could have finished it!  Thanks for the encouragement and help!

Send me a PM via the forum and we can arrange something.  If you send me the boards I can swap the IC for you.

-sonavor

Thanks for the offer Sonavor.  I decided to repair the pre-amp board with the working IC5 and I managed to get a fully functional Beomaster 5000.  Sounded great.  Played it for a few hours.  Was happy to be done with it.

But the story doesn't have a happy ending.  The next day, after being plugged in all night and on standby (like you would normally do with it), I tried to start it with the remote again.  Nothing.  Nothing from the control panel either.  And the heat sink, particularly the right-hand side of things was very, very hot.

So something is haywire on the amp board and i'll have to start chasing voltages all over again!  If I never see another one of these, it will be too soon!