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!

Beogram 4002 (5513): Replacement of the Sensor Arm Bulb with an LED and Improvement of the Record Detection Circuit

On the way to an all-LED Beogram 4002 the replacement of the sensor arm LED is usually my final step. And so, after restoring the DC motor, it was time to implant my recently developed flex-PCB based LED assembly to replace the incandescent light bulb. This shows the sensor arm with pulled out detector assembly:

If you click on the picture to get the full resolution and magnify it a bit on your screen, you will see that there is already a small black spot on the glass bulb, indicating that significant tungsten evaporation has taken place from the filament. This points to a likely near term demise of this bulb, which would incapacitate the Beogram. The installation of a LED promises a much better long term stability, and this is the main reason for LED upgrades outside the scarcity of fitting small light bulbs - they are hard to find these days.

The challenge with this particular LED upgrade is of course the small space and the need to replicate the power draw of the light bulb to not confuse the record detection circuit (see below).

My LED replacement therefore contains an additional resistor that compensates for the much lower power draw of the LED (which is a high brightness low color temperature model running at a very low emission level). This shows the assembly:

It is built based on a flex-PCB that can fold into the bulb compartment:

The glued on 3D printed red wedge on top assures that the LED sits in a place comparable to the bulb filament.

This shows the assembly in action:

Note the accurate color of the B&O logo, which is a result of the use of a low color temperature LED which has a high red component.

Any light source replacement in the sensor arm should be accompanied by measuring the sensor response when the arm is over a spinning empty platter to verify that the signal is strong enough to precisely and reliably trigger the protection circuit that prevents arm lowering in absence of a record. This is the trace I measured on the collector of TR3 after installing the LED:

On first glance it looks textbook, the dips going all the way until they bottom out at 0V. However, closer examination yields that the amplitude of the signal is only about 3.7V, while the circuit diagram prescribes something closer to 6V. This amplitude is too small to reliably trigger the protection circuit.

This shows the pertinent part of the circuit diagram:

This is how it works: The base of TR3 is biased by the voltage divider formed by R26 and the BE-diode in the transistor. The signal from the photocell in the sensor arm OPH1 is coupled in via C12. When there is no signal (like over a record), the voltage at the base is such that the collector of TR3 is at 4V. This means that the transistor is slightly ON creating a voltage divider with R27 (and R30). When OPH1 delivers its AC signal of about 20 mV amplitude, the voltage at the TR3 collector starts oscillating between 6 V and 0 V, i.e. TR3 operates as an amplifier. This 6Vpp signal is then fed into the base of TR4 via C16, which takes out the DC component. Since the base of TR4 is pulled up to 21V via R32 and protected against negative voltage (relative to its emitter) by D15, the resulting signal at the TR4 base is an oscillation between 21.6V and 20.4V. While at 20.4V the transistor is on, and C18 charges. This in turn biases it towards 21V. This pulls up the base of TR6 via R34 as current limiter and the collector of TR6 is pulled to ground. This signal then prevents the arm lowering circuit to work and the arm stays up.

There is a second way to pull up the base of TR6, which is via the D18/R37 link. This connects to the collector of TR5 via the Zener diode D17. This is the circuitry that can invoke the 'no record present' signal at the collector of TR6 if the light bulb is broken. If the light bulb is open circuit, no current flows in R36, causing the voltage at the collector of TR5 to go up. This pulls up the base of TR6 and the collector of TR6 goes down and the arm cannot be lowered anymore under any circumstance. This is the reason that the LED replacement needs to be designed in a way that the current through TR5 is similar to the current caused by the original light bulb (~50-60mA).

Back to the signal on the collector of TR3, which measured to be less than 6Vpp. I long ago noticed that some Beograms have issues with the detector circuit and that sometimes they do not recognize that there is no record and happily lower the arm on the platter even though everything seems to be in working condition. If that happens the stylus' life depends on the proper adjustment of the arm lowering limit keeping it from hitting the platter ribs.

I addressed this problem in the past by soldering in a 10M resistor between the base of TR4 and GND. This biased TR4 slightly on and even a weaker signal from TR3 would be enough to stop the arm lowering circuit. At that point I did not understand the root-cause of the problem, but rather discovered the fix by accident: The circuit tended to work whenever I connected my oscilloscope to the base of TR4 and I realized that the internal resistance of the scope fixed the problem...a bit of trial and error yielded that a 10M resistor to GND tugged enough on the base of TR4 to keep things working after disconnecting the oscilloscope.

But as I know now, there is a much better solution: Double R26 to 2M. This causes a reduction of the current through the BE diode of TR3, and so the transistor is less on, and the voltage at the collector goes up to the prescribed 4V. This fixed the problem in this Beogram:

The amplitude of the oscillation is now close to 6V and can trigger TR4 reliably.

An interesting question is: Why did B&O let Beograms leave the factory with 1M resistors in place and a too low TR3 collector voltage?

One of the issues with the design around TR3 is that the collector voltage depends on the DC current gain of the particular transistor used as TR3. The current gain is the most variable parameter of any transistor series, i.e. even within one production run it can vary considerably by up to a factor two or three. It would have been better to bias the base with a stiff enough voltage divider to GND, and to implement a feedback for setting the gain of the amplification. With the design as it is, every Beogram has in essence a different TR3 working point depending on the particular BC182 that is soldered in. With a new light bulb in place in the sensor arm this variation may not have been a problem due to the high initial light intensity making enough signal even with a non-spec working point of TR3. But as the bulbs aged the intensity gradually went down due to deposited Tungsten on the bulb glass. And after a while the signal became weaker and at some point the mechanism stopped working. So they may not have noticed back then that there is a problem when they manufactured these Beograms.

Interesting stuff (at least to the Beolover...;-)!

Beogram 8002: Adjusting the Tracking Sensor Aperture

During my test drive of the Beogram 8002 that I just brought back to life I was pondering Grover Washington, Jr's artistic output while listening to 'Knucklehead' on his seminal 'Mister Magic' record, when I realized that the tonearm was not tracking properly. It ran at a much to large angle relative to the sensor arm. So it was time to put on my least liked record, some free jazz by Sam Rivers, and do the tracking sensor adjustment procedure! This shows the arm with the original misadjusted sensor. It is not parallel with the sensor arm:

Luckily one can adjust the tracking sensor aperture without putting the 8002 into service position. All that is necessary is to remove the black panel that is under the arms in their home position and then the screw that controls the aperture can be accessed:

It is a good idea to drive the carriage a bit towards the center of the record. That allows to fit a small screwdriver into the enclosure for better fit into the screw head:

The service manual prescribes to block the platter with your hand to do the adjustment (one needs to achieve a setting of the aperture that the carriage starts moving after 2±1 turns of the platter after the needle touches down). This seemed a bit brutal to me, and I settled for simply letting the platter spin and then pressing 'play' to engage the needle. Then I counted the rotations while watching the spindle that drives the carriage. A few small adjustment steps forth an back and I was able to achieve the 2±1 spec. Here is an impression of the arm when the aperture is properly adjusted:

The arm is now virtually parallel with the sensor arm while the record is playing. This is Beolove! Now away with Sam Rivers and back to Grover Washington, Jr.! A much smoother sound!!..;-).

Beogram 4002 (5513): Rebuilding the Arm Lowering Mechanism and Upgrade of the Tracking System with a LED Based Light Source

Now that the DC motor is performing within specifications again, it is time for the remaining tasks of the functional restoration of this Beogram 4002 (5513). The next step was the rebuilding of the arm lowering mechanism and the upgrade of the tracking feedback system with a LED based light source and an adjustment of its sensitivity.

This shows the solenoid and the damper of the arm lowering mechanism:

I took out the damper and the linkages for cleaning and lubricating:

Here are the parts spread out on my bench:

This video shows how to rebuild the tone arm mechanism in detail. After putting everything back together it was time to do the same for the linkage that connects the damper with the tonearm. I made a video that shows how to get to the damper to arm linkage and re-lubricate it. This shows the arms from the back:

I took out the sensor arm to get to the linkage:

And then removed the retaining washer and the spring to remove the linkage:

After cleaning the old lubricants and putting some synthetic grease on the pin that holds the linkage I put everything back together and adjusted the arms to be parallel with each other and the keypad edge:

The arm lowering limit will be adjusted once the platter height is up to spec.

On to the tracking system. This shows the original bulb housing in place:

I replaced it with my custom designed SMD LED based replacement that comes with its own fitting 3D printed enclosure. It fits exactly into the place of the original housing:

This shows it installed:

The blue part is a trimmer with which one can adjust the intensity of the LED. This is very convenient to do some fine adjustment to the tracking feedback sensitivity, alleviating the coarseness of the mechanical adjustment via the excenter that allows the moving of the light sensor relative to the light source.

Then it was time for my least favorite record (Sam Rivers playing free jazz) and a dented MMC20 EN from an ebay purchase that went DOA to do the tracking sensitivity adjustment. It is a great idea to not use a pristine cartridge or a good record, since it is absolutely possible that the needle gets dragged laterally across the record if one is not careful.

This shows the adjustment of the LED intensity as final step of the adjustment process:

There is a video that shows the tracking sensor adjustment procedure in detail:

Beogram 4002 (5513): Replacement of the Tracking Sensor Light Bulb with an SMD LED Light Source and Adjustment of the Feedback Sensisitivy

This particular Beogram 4002 had a bent tracking sensor aperture impeding the lateral arm movement. This mechanically prevented any reasonable feedback response from the tracking system disabling the mechanism. This shows the original setup with the incandescent bulb light source:

After taking off the bulb housing I saw that the aperture was bent down, chafing on the optical sensor housing:

I bent the aperture back to a reasonable position and then I replaced the bulb assembly with my most recent design of the SMD LED based light source that integrates a potentiometer allowing the adjustment of the light intensity. A great feature for precise adjustment of the tracking sensor feedback sensitivity. This shows an original bulb assembly and the SMD LED based replacement in comparison:

This assembly is plug and play, i.e. all that is needed is to take the old assembly out and replace it with the same form factor replacement:

After installing it the tracking sensor sensitivity had to be calibrated to specifications. Time for using my least favorite record, a free jazz album by Sam Rivers. It is also a great idea to use a cartridge that is not pristine, since there is a real danger of damaging the tip if the tracking system 'goes wild' during the adjustment process. I have a special dented MMC20EN cartridge that suffered some bad ebay fate for that purpose. This shows when I did the coarse adjustment of the mechanical position of the tracking sensor assembly using the excenter that allows to move it left or right:

If you are interested in doing this yourself, I recently made a video that shows the tracking sensor adjustment procedure in detail (it uses the prototype for the current assembly, but the process is the same):

Beogram 4002 (5513): Replacing the Tracking Sensor Light Bulb with a Plug-In Ready SMD LED Assembly

Yesterday, I installed my latest version of the SMD LED light source for the tracking sensor of the Beogram 4002 (5513) that I am currently restoring.

A while ago I developed a drop-in ready SMD LED based assembly for replacing the incandescent bulb fixture. While this worked great, it still required the application of the inconvenient original tracking sensor sensitivity adjustment process. The mechanical positioning of the light sensors relative to the light source is quite tricky, and it is always chore to get this adjustment precisely right. The reason for this is that the mechanism is quite sensitive, while the adjustment mechanism is pretty coarse, requiring a lot of trial and error to get the adjustment right.

A recent beolove-affair with a Beogram 4000 taught me a better way to get this adjustment right: Add a trimmer potentiometer to the LED circuit, which allows the adjustment of the intensity of the LED. This gives an additional adjustment parameter that enables a straight forward fine tuning of the tracking response. I liked this a lot in the 4000, so I adapted the approach to the 4002's higher bulb voltage.

I made a short video about the installation and the tracking response adjustment process for this new design:

Here are a couple high-res pictures of the new LED assembly. I will be happy to provide this part to other enthusiasts!. This shows the added trimmer:

And this is a shot of the bottom side. The LED is in the same spot as the filament of the original light bulb assembly. The body of the replacement part is 3D printed and has the same shape as the lower part of the original fixture. This ensures a precise fit onto the light sensor assembly below the aperture:

This shows the part installed: