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Friday, February 9, 2018

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!

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