Beomaster 8000 - Workbench Unit - Investigating FM source audio in the Phono source channel

In this post I wanted to revisit Beolover's fix for reported FM audio crosstalk in the Phono channels.
I actually have been checking for that fault in my Beomaster 8000 restorations and with these new source switching IC device replacements. Unfortunately I had been unsuccessful in replicating the problem.

To review the problem...
Make sure the turntable connected to the Beomaster 8000 does not have a vinyl record ready to play.
When the Beomaster 8000 is switched on from Standby to Phono the Beomaster 8000 volume can be cranked up without hearing any music from the Phono music source.  That is good and is what is expected.

Next switch the Beomaster selected source to an FM station (preferably a strong station). Note the music that is selected. Now switch back to the Phono source (again with no record playing). Turn the Beomaster 8000 volume up and you can begin hearing (faintly) the FM station that you were just on bleeding into the Phono channel.  That is the problem.

As I mentioned, I had not been able to create this problem until this weekend when I moved my workshop Beomaster 8000 unit to a different bench and connected up a powered Terc FM antenna to the radio antenna input. This produced the strongest FM signal I had seen on the Beomaster. Retesting the FM crosstalk problem...I now had reproduced it.

Interestingly this test setup is with my new analog switch devices installed in the Beomaster 8000 preamplifier board. The preamplifier board also has TI OPA2134PA OpAmp devices for all of the analog sources.

Up until now I had reasoned that any audio crosstalk must be occurring in the analog switches the Beomaster 8000 preamp board uses. That made the most sense. However, this preamp board has the new multiplexer switching devices that have very good audio specs and low crosstalk ratings. I thought that should take care of any audio source crosstalk.

It turns out that it doesn't and that is quite surprising...and I do not understand the source of the problem at this point.

An acceptable work around for someone not wanting to make any wiring changes is to always put the Beomaster in Standby mode prior to selecting the Phono source. That will keep the FM tuner in the Beomaster muted while using the Phono source.

We want our cake and eat it too though so I decided to implement Beolover's fix for this problem.

I performed the same wiring modification Beolover came up with but I implemented the change differently.  I wanted to be able to easily switch from the original wiring to the modified wiring.  That option will allow me to keep investigating the problem.

My implementation is to put female jumper connectors on the two wiring routes related to this problem and fix.  Beolover's fix is to run the FM board audio mute control line to the preamplifier board phono source selection line. When Phono is selected on the Beomaster control panel the phono source selection line will tell the FM board to mute the FM audio. When the phono source is deselected the FM control will unmute the FM audio.

Here is the modified wiring for this on the FM board. The picture shows the FM audio mute control line connected to the default path the Beomaster 8000 originally takes.

The yellow wire from the FM board 1R37 resistor can be disconnected from the default ground path and connected to the Preamp board added connector (also on a yellow wire).

This path routes the FM audio mute control line to the preamp board Phono source selection control line.

Testing this modification in the workbench Beomaster 8000 unit I verified that I get no FM audio at all in the Phono channel with I have the FM audio mute control line connected to the preamp board.  I also verified that moving the FM audio mute control line back to the default position re-introduces the FM crosstalk problem.

Beomaster 8000 Circuit Upgrade: FM Audio Muting During Use of Phono Input

After reviving the Beomaster 8000 that came for a visit from Australia with new processor crystals, it was time to do some additional work on it. My customer was annoyed by the crosstalk between FM audio and the phono input while listening to vinyls records, and a solution to alleviate this issue was requested.

Phono inputs are especially susceptible to crosstalk since they are about 100 times more sensitive than regular high level line inputs. The result is that when the phono input is selected (with a player attached or grounded inputs) the FM audio signal can be heard weakly in the background (if a station is dialed in). And that is of course not very Beolovely. The reason for this behavior is a design flaw in the Beomaster circuit: When another input than FM is selected and previously a station was tuned in the FM tuner stays on and continues blasting its signal out to the amplifier board, even though one wants to listen to a record via the phono input.

This diagram shows the solution I developed after a bit of head scratching and downloading the data sheet for the CA3189E IF stage integrated circuit.

The CA3189E chip is responsible for shifting the audio modulation of the FM signal that is received in the front end of the tuner to a lower frequency and amplifying it before the audio signal is then extracted in the detector. The CA3189E was designed with radio applications in mind that do not have a microcontroller at their disposal. The Beomaster mutes the FM audio while tuning between stations to prevent the speakers from emitting loud static noise. It does that by extracting signal strength information from the CA3189E via pins 13 (TP22) and 12, and feeding it into the microcontroller after processing in some additional circuitry. The microcontroller then decides when to ramp the output volume down using the main volume attenuators. 

Alternatively, this chip can do this on its own via its audio mute input (pin 5). The CA3189E data sheet contains a circuit that shows how to do this if you are interested. In the Beomaster 8000 design this pin is simply grounded via R37 to permanently turn the signal output on. 

The circuit on the data sheet in combination with some measurements of the signal strength related output of pin 12 (it seems to be close to 0V between stations and at about 5V with a strong FM station dialed in) suggested that pin 5 could be controlled directly via the microcontroller signal that is fed into the two AD10/278 input selector chips on the preamplifier board. The AD10/278s have three inputs for selecting FM, Tape 2 and Phono. When one selects one of these inputs on the control panel of the Beomaster 8000, the corresponding input goes high (~4.8V), activating that particular signal path on the chips. So my idea was to simply connect the Phono selector line to the audio mute input of the CA3189E via R37 after severing the ground connection of R37 by unsoldering the resistor on the ground facing end. That way, whenever Phono is selected the FM audio muting circuit is automatically activated muting any FM audio in the system that could crosstalk with the phono input.

This shows the practical implementation of this scheme. I elected to make the AD10/278 connector directly at the ribbon cable coming from the microcontroller board. That way the additional connection can be removed from the amplifier board when it needs to be taken out.

I soldered the jumper wire to the plug contacts

and then modified the black cap with a bit of Dremeling to fit over the jumper wire:

Then I wrapped the plug with some electrical tape to keep the jumper wire from moving so it will not break off. This shows the plug plugged in next to one of the AD10/278:

I connected the other end of the jumper to the disconnected end of the R37 resistor after securing it with some shrink tubing to the capacitor located next to it. The solder spot was secured by some additional white shrink tubing:

And then it was time to turn the Beomaster back on. And it seems to work smoothly. FM still works and when Phono is selected there is no more FM audio superimposed. Beolovely!