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 - Workbench Unit - Testing a replacement device for the AD 10/192 Multiplex Switch on the preamplifier board

To go together with the previous Beomaster 8000 replacement part for the preamplifier source switching device (AD 10/278 - 3IC102/3IC202), I have completed the replacement part for the other switching device on the preamplifier board.

The second source switching device is 3IC4. The original device is an AD 10/192 multiplexer and like the other switching ICs, this device is no longer available.

Here is the schematic diagram again that shows the two source switching type devices in the Beomaster 8000 preamplifier.

The 3IC102 and 3IC202 replacements were completed earlier. Now it is time for the 3IC4 device.
Like the the other replacement I first built a prototype and tested it in my Beomaster 8000 workshop test unit. It worked as good as I expected so I had an adapter board made.

Here is the replacement switching device on the new adapter board next to my prototype.

...and here is the first replacement device unit installed in the Beomaster 8000 preamplifier.

The Beomaster 8000 preamplifier works great with the source multiplexing devices replacements.
Now to the fun listening test phase.

Beomaster 8000 - Workbench Unit - Testing a replacement device for the AD 10/278 Multiplex Switch on the preamplifier board

One of my first projects utilizing my new, dedicated workshop Beomaster 8000 unit is to test out a replacement device I created for the BM8000 preamplifier board's input source switching ICs - IC102 and IC202. They are the no-longer-available AD 10/278 multiplexer device.

Here is the circuit diagram of the Beomaster 8000 preamplifier source select switching.

As you can see there are actually two types of analog switch devices. 3IC102 and 3IC202 are AD 10/278 devices for selecting between FM, Phono and Tape 2 left and right channel sources. 3IC4 is an AD 10/192 device that selects between Tape 1 or whichever source is selected with 3IC102 and 3IC202. Selection control of course comes from the Beomaster 8000 Microcomputer board (via 3P14).

Once in a while AD 10/192 devices can be found for sale but I have never been able to find NOS AD 10/278 ICs. Concerned that a failed AD 10/278 can bring an entire Beomaster 8000 restoration project to a grinding halt I decided to look into making a modern replacement. I was also motivated by reports from some Beomaster 8000 owners who complained about crosstalk between the FM source and Phono source. I felt that if that were to occur it would most likely be due to a problem with the 3IC102 and 3IC202 analog switch. However, I must confess, after quite a number of Beomaster 8000 restorations I have yet to experience any crosstalk problems between input sources myself. Still...another reason to create a replacement device.

To follow Beolover's lead in creating replacement parts for the various Bang & Olufsen restorations we have done, a key goal is that the new part be a direct plug and play replacement. No modifications to the Bang & Olufsen component to use it. So in the case of the AD 10/278 replacement I needed a design that would result in the old 3IC102 and/or 3IC202 desoldered for removal...and the replacement inserted and soldered into place.

I believe I have accomplished that with a custom adapter board and a modern three channel SMD multiplexer device designed for audio switching.

I started with a prototype using the replacement device in a standard DIP package layout and a breadboard circuit where I could test inputting test signals and selecting channels just like the Beomaster 8000. Once I had that perfected I sent my design off to have my adapter boards made.

Here is my breadboard tester for the AD 10/278 with the various test devices.

Confirming that the prototype and first replacement units function just like the original devices I installed the first two replacement units in a Beomaster 8000 preamplifier board.

Then the preamp board into my Beomaster workshop unit.

The results are as good as I expected. The source selection works just as it should and I cannot hear any crosstalk between selected sources.

I need to run some additional testing of course but I am thrilled with these initial results.

Beomaster 8000 - Workbench Unit

After restoring quite a few Beomaster 8000 receiver units I have developed several bench testing aids. I have some jigs for testing the rebuilding of displays and I have a test setup for testing the Beomaster 8000 output amplifier assemblies.

During various Beomaster 8000 projects I have also restored a number of Beomaster 8000 spare circuit boards. To checkout my spares and to aid in Beomaster 8000 diagnostic trouble-shooting I have been wanting to built up a dedicated, workbench unit. This would be a test unit I could swap boards in and out of easily for testing purposes. I had a couple of spare Beomaster 8000 chassis laying around so the time finally arrived to implement this workbench Beomaster 8000 unit.

Here is the completed workbench Beomaster 8000. It will remain in its service position and never be completely reassembled. I want it always available for testing.

Even the startup circuit is left open for measurements and to test the startup relays. This unit has two brand new 7RL1 and 7RL2 relay devices installed. It also has replacement 7R1 and 7R2 5.6Ω current limiting resistors.

I also replaced the four ±55VDC power supply reservoir capacitors with the Beolover Beomaster 8000 reservoir capacitor kit.

A bonus side benefit of having this workbench Beomaster 8000 is that I can also listen to it while working on various Beolover Blog projects :-)

In the upcoming weeks I will be doing some Beomaster 8000 board restoration and testing. I will utilize this test unit for those tasks and detail the work here on the Beolover Blog.