Beomaster 6000 (2702) restoration: calibration of ultrasonic remote control Commander and receiver

The Commander has been cleaned and restored (click here).  Now it's time to calibrate both the transmitter (the Commander itself) and the ultrasonic receiver. The concept of this type of remote controller was common in the 70's. The early days of remote controllers. B&O used a AM system : Amplitude Modulated system. The principle is exactly the same as used in AM Radio transmitter/receivers: a high frequency wave is used as a carrier to travel long distances (the frequency you tune in on the radio) and this carrier wave is modulated with another frequency (the actual audible music sound).

The frequencies used here are off course are much, much lower since you only need to bridge a distance of +- 10m (the length of a room). Ultrasonic frequencies are anything above 20 KHz. This is the maximum a human being can hear when born. The older you get, the lower this frequency is. An elderly person (e.g. myself :-) should be happy if he still can hear frequencies of 10KHz.....Dogs can hear up to 30KHz (or even higher).

As you can see below, B&O used  frequencies between 34Khz and 43KHz as carriers and frequencies between 148Hz and 330Hz as modulators. In fact, they used 4 different carriers and 4 different modulator frequencies. This gives 16 different possibilities. B&O only used 14 on the Beomaster 6000. As mentioned in my post about restoring the Commander, I don't know why they did not use all the 16 options to have e.g. also the AUX2 command selection. 

I made an overview of the Commander front panel and all the corresponding values in the picture below. Every button on the Commander activates 3 circuits at the same time:

1: activating the power to the board (like an on/off switch)

2: set the correct carrier frequency

3: set the correct modulator frequency

B&O also used 100% AM modulation. In other words, the carrier signal is modulated from 0 to max level. Below an example of the 34Kz modulated with 320Hz. The frequency that you can read on the oscilloscope screen on the bottom left of 312Hz is not correct. It is difficult for an oscilloscope to measure this modulator frequency together with the carrier.

So how does one calibrate this ultrasonic system? The service manual calls for a "frequency standard" to be used. This lab device produces the exact carrier and modulator frequencies and emits them to the receiver by an ultrasonic microphone to calibrate the receiver first.  A switch is provided to turn the modulation on/off if needed. Now, guess what: I don't  have this "frequency standard" and never saw one either. So, I needed to find another solution. 

What I wanted to do was use the transmitter as the "frequency standard". The only thing I needed to do was to make sure that this transmitter was generating the correct frequencies. Should be easy since the transmitter had trimmers to adjust. Well, it turned out that whenever you put a probe on the circuit, the internal oscillator of the transmitter started to drift and frequency changed. The whole design is a capacitive sensitive oscillator and just connecting the probe (or any other metal object) influences the oscillator. The ultrasonic microphone is also basically a capacitor with a high bias voltage to improve sensitivity.  Even the ageing of this microphone influences the frequency calibration.

I decided to power up both the receiver and the transmitter at the same time and calibrate the transmitter frequencies by measuring them on the receiver side. First thing you need to do is deactivating the modulator frequencies on the transmitter to have just the carrier left (done by shorting the base and emitter of 17TR1). Otherwise it is difficult to measure. This method did work very well. Below the complete set up and an example of the incoming carrier frequency that I measured on the receiver with the f-counter. On the picture the transmitter is fully closed, but off course you need to open it to adjust the capacitive trimmers for correct frequency setting. But it's good to close, measure and open again if re-adjusting is needed because the metal keyboard may influence the calibration. Take your time for this....

 One of the 4 carrier frequencies measured on the receiver side. In this case 36.860 KHz

After calibrating the 4 carriers frequencies on the transmitter side, you need to tune the receiver to these carrier signals. The receiver has 4 identical carrier demodulators that resonate at the same frequency as soon as the corresponding signal is detected from the receiver ultrasonic microphone. The only thing left is to make sure that both signals are in phase (phase coincidence). This is done by adjusting the corresponding coils on the receiver board.

The receiver has an AGC (Automatic Gain Control) circuit to make sure that the output signal is stable in value. It is obvious that the incoming signal strength depends on the distance between transmitter and receiver. With this AGC the generated signal in the receiver is a square wave of constant amplitude that is then sent to a matrix for decoding and sending the proper on/off command to the different functions (volume, tone, balance, etc.) An early version of digital I/O binary coding one could say!

The voltage of the different square wave signals need to be set equal (max +-1dB difference around 1V). The new multi-turn trimmers that I had put in earlier (click here) made it fairly easy to calibrate this.

The low modulator  frequencies are "decoded" in basic the same way. However there is no adjusting possible here. By combining the carrier and modulator pulses, the matrix outputs 14 different signals to "Command" the Beomaster 6000 in the same way as you press the buttons on the Beomaster key panel. 

Both the Commander and receiver board are now put aside for later mounting into the main chassis.

Beogram 4004/4002: Tracking Force Adjustment and Zero Force Balancing

I noticed that the Beogram 4002 that I am currently fixing up needed a tracking force setting of 1.5g to produce an acceptable undistorted signal with its MMC20EN cartridge. At 1g the signal would be completely distorted. A clear sign that something was wrong with the tracking force adjustment (the cartridge works well on my Beogram 4004...).

It turned out that the arm needed a zero force rebalancing. The counter weight in the back of the tone arm was way too far back. I made a video about the procedure. The video shows how to do the zero force adjustment, and then how to use the tracking force gauge that comes with B&O cartridges to get a precise adjustment of the tracking weight:

It is interesting to note that the scale on the adjustment bushing at the back of the cartridge appears to be only a rough estimate of the weight acting on the needle. In my opinion, this is not surprising considering the design of the mechanism using a variable torsion spring to press the arm down.

There is another issue with this mechanism: The adjustment screw is only held in place at the back plate with a retaining clip, which gives the counter weight 0.5-1 mm play along the arm axis. This, of course, can throw off the zero balance when the turntable is moved after an adjustment. I fixed this in Beogram 4004 with a M3 counter nut and a washer. Since the nut needs to be tightened after the adjustment the weight position changes slightly once the nut is tightened. This requires multiple attempts to get it right, but I think this is worth the effort, since this will lock the adjustment in place for long-term stability. Here is a photo of the installed nut and washer:

Beogram 4004: Characterizing a MMC20EN Cartridge with an Analogue Productions Test LP

I recently bought 'The Ultimate Analogue Test LP' from Analogue Productions. I felt I needed to be able to characterize the output of my cartridges and I read about this LP. It is a heavy duty 180g high-quality vinyl that has all the relevant test tracks for basic measurements.

I connected the oscilloscope to the DIN5 connector via a DIN-to-RCA breakout and two RCA-BNC adapters. This allowed me to form a shielded signal path from the output of the Beogram 4004 to the oscilloscope. However, the measurement was still pretty noisy, so I resorted to using a low pass coupling for the trigger and averaging. This gave me usable traces. Here is a shot of the oscilloscope screen on the 1 kHz test track:

It is obvious that the MMC20EN cartridge that came with my Beogram 4002 puts out a well balanced signal on both channels. The peak to peak amplitude of the 1 kHz signal is about 15 mV. This also seems to be a pretty good way to calibrate the RPM of the platter...

Here is a shot of the 100 Hz track (timebase is 10 ms/sq):

As expected the amplitude is considerably lower (maybe 4 mV without the noise) than for the 1 kHz signal due to the RIAA pre-emphasis. This is necessary at low frequencies to keep the groove width reasonable (Faraday's law prescribes that the induced EMF is proportional to the change of the magnetic field, which in turn is proportional to the velocity of the moving magnet, iron or coil in a cartridge). I found an excellent summary that explains the background behind the RIAA curve and the historic path towards it. The article is by Gary A. Galo and can be found here.

In short, the RIAA curve lowers the amplitude progressively towards lower frequencies to reduce the groove width (otherwise long playing records would not be possible). At high frequencies the amplitude is increased to stay ahead of the noise, which, due to its high frequency content is an effective EMF generator. Hence the signal amplitude needs to be a few magnitudes larger than the noise amplitude, that in the amplifier signal + noise can be reduced together, thereby reducing the noise relative to the original signal level. This is a similar approach as is used in Dolby noise reduction systems. 

The 10 kHz track on the Analogue Productions LP produces a standardized -20 dB amplitude (i.e. 10x smaller than the other two tracks, which are at 0 dB. 0 dB apparently corresponds to a 7 cm/s stylus velocity, as suggested on the LP sleeve. Here is the (pretty noisy, disregard the Freq measurement indicator) measurement (timebase is 100 us/sq):

Without the noise the amplitude might be about 10 mV or so, i.e. it were at about 100 mV at 0 dB. That would give us about a factor 7 relative to the 1 kHz signal. 20*log(7)=16.9 dB, which, considering the noisy measurement, is reasonably close to the RIAA curve (from stereophile.com, the blue curve is the curve used on records, the red one is an idealized 6 dB/octave curve, which would be best, but cannot be implemented due to practical reasons), which suggests a pre-emphasis of about 13 dB at 10 kHz:

The 100 Hz measured amplitude above is also reasonably close to the curve: 20*log(4/15)= -11.5 dB (compared to about -13 on the curve....

All in all another interesting evening at Beolover's lab...;-)