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Beogram 4002: Restoration of DC Motor Video Published - Check It Out!

By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...

Saturday, February 17, 2018

Beomaster 8000: Repaired Rotary Volume Sensor

Swapping the volume rotary sensor module on this Beomaster with a spare got the volume control working again. That doesn't mean the original sensor is a throw away and cannot be repaired though. The repair is pretty easy. The sensor module is easy to get to and the infrared emitter and photo sensors can be de-soldered and replaced.

Here is the original, defective rotary sensor module with the replacement parts.

It is difficult tell the emitter device from the sensor just by looking at them. The leads on the emitter are different from photo sensor but to be safe I keep them in their packaging until I am ready to solder them in place.

Just that simple and the original rotary sensor is working in the Beomaster again. My spare sensor assembly can go back in my spare parts bin.

Now for that pesky left channel DC offset problem.

Thursday, February 15, 2018

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.

Beomaster 6000 (2702) restoration: restoring the Commander - remote controller

This Beomaster 6000 quad came with the original remote control, called "Beomaster 6000 Commander" ! In the past I have seen different names printed on this device however: like "Beomaster 6000 control module". It is an ultrasonic remote control device that allows some basic functions: volume up/down, balance up/down/left/right, FM presets P1 to P5, input selection for Phone4 & Tape4 and stand-by. No idea why they did not include the AUX2 for example. Technically there is no excuse for not doing it.

The Commander did not look like it was heavily used. Just the usual small marks and scratches, but nothing uncommon.  After opening it, the same picture inside. Very little dust or dirt. Off course, the contacts needed to be cleaned and the board recapped (only 2 capacitors). And the trimmer replaced.

There is only one good way in cleaning the contacts: take them out! And that turned out to be more difficult than expected. The (4) bars with contacts are soldered onto the board. But even after desoldering everything, they did not want to come out. Closer (very closer...) inspection revealed that these bars have tiny retainers that keep the bars in the plastic "frame". You need to straighten these retainers in order to get the bars out and that was microscopic work.

Once out, it was just a matter of cleaning with a fiber pen and further cleaning & coating with Deoxit Gold. I noticed some small scratches on the contact bars. Probably from an earlier attempt to try to clean them with sand paper and without taking them out. 

After cleaning the bars they were soldered back in place. The gap between the bars and the small gold-plated "dots/contacts" is about 1 mm.  And yes, while your in there, clean the battery compartment contacts as well!

The whole board is fitted with only one bolt in the middle. Strange. But it does allow for some adjustment to bring the board closer or further away from the key panel. That is why the nut on the bolt is sealed with red paint.

A good looking Beomaster 6000 Commander I must say! Time to recalibrate this transmitter and the receiver.

Beomaster 6000 (2702) restoration: testing the main power supply

A healthy body starts with a healthy heart ! This Beomaster's heart is a massive 4kg toroidal transformer with several different windings. The primary windings have the usual connections for 110/120/220/240 volts. The secondary windings have a 27 and 42 volt winding.

The 42V AC goes into a bridge rectifier (mounted just next to the transformer) with a 10.000 µF capacitor, is not stabilised and used for the main power output stages. This 60V DC is further "filtered" through a 100 ohm resistor and 50 µF capacitor to be stabilised into 20V and 21,5V for the electronic switch board.

The same 42V AC is also used to create the high bias voltage needed for the ultrasonic micro receiver. This is done by a simple 3 stage multiplier using diodes and capacitors. Since the ultrasonic micro only needs a static high voltage charge, and not real power/current, this multiplier is sufficient to create a high voltage of about 180V DC. More on the ultrasonic emitter and receiver in a next post.

The 27V AC goes into another bridge rectifier (mounted on the power supply PCB 5) followed by 2 stabilisation circuits to give the required +18V DC and -5V DC. Both these voltages are adjustable through a trimmer on PCB 5.

The transformer was cleaned up and provided with new foam rubbers to fit between the transformer and chassis.

During the first evaluation of this Beomaster (click here), I already noticed some slight  buldging of the 10.000 µF capacitor, so time to check. And, no surprise, the capacitor was leaking inside!!

A new one was fitted with the same diameter.

The smaller 3000 µF capacitor for the 18V and -5V was also replaced off course (while this one was not leaking however). 

Some Beomaster 6000 quad have an additional fuse of 4A, mounted on top of the bridge holding this 3000 µF capacitor and connected in series with the 27V AC winding. This Beomaster did not have it. It is also not mentioned on any of the electric schematics that I have. I'm thinking of adding this fuse for an extra safety.

The following picture is taken of another Beomaster fitted with this additional fuse. Looking at this picture, it's obvious this was an add-on that was not originally supposed to be there. The B&O engineers must have had some feedback from the service centers after the first series of units were sold........

Time now to connect to the variac and slowly increase the mains supply to see if all is good. Well, hmm, I only got 58,6 V DC out....Should be at more than 60V DC unloaded.

Then I realised that the voltage selector was set on 240V. In the past, my country (Belgium) had 220V on the grid (the UK had 240V), but nowadays Europe is standardised on 230V.  But what I have in my house is more close to 240V (the officially allowed tolerances are +10% and -6%). So, since the voltage selector was set on 240V, I needed to up the variac to  the same 240V. And now I got the required voltage!

With the new trimmers fitted earlier on the power supply PCB, it was now easy the set the stabilised outputs at +18V and -5V.

With this fully operational power supply, I'm now ready to start testing the other boards of this Beomaster.

Wednesday, February 14, 2018

Beomaster 8000: Power Testing the Receiver

Time to catch up with the status of the Beomaster 8000 restoration. I left off with the Beomaster reassembled and ready for its initial power up test.

Plugging the Beomaster into the AC power outlet for the first time is always filled with a little nervous excitement but it was all anticlimactic. Nothing visually happened. Mainly there was no red dot on the display board showing that the Beomaster was in standby mode. As I unplugged the Beomaster I did hear one of the power relays click so that was good to hear.

When this sort of thing happens the first thing to investigate is what is going on with the power supply. The 120 VAC, 60 Hz line voltage was there at the transformer (my house is actually 125 VAC). The 5 VDC regulator was measuring 5 volts. The ±15 VDC regulators and the ±55 VDC rail voltages were not present...but that is to be expected on initial start up if the unit never gets past the standby mode.

I opened the lid to the processor board and verified +5 VDC to the processor chips. That wasn't so welcoming of news. It means a problem on the microcomputer board.

I have three spare Beomaster 8000 microcomputer boards for this type of scenario. I swapped in the first one and tried power on the receiver again.

This time the standby LED illuminated.

Now I could start exercising some Beomaster 8000 operations. A recheck of the voltages showed me the ±15 VDC on the +15 and -15 voltage regulators. The large reservoir capacitors for the 55 VDC rails measure ±56 VDC.

During those above tests I also tried to increase the volume level but it was stuck on zero.
The problem with that could be anything from the rotary volume wheel sensor (or cable) and the microcomputer IC.

I tried a second spare microcomputer board with the same result so the problem is most likely the sensor or sensor cable. I have a spare sensor to swap with so I tried it.

There it is. The original rotary volume wheel sensor assembly has a problem. The replacement sensor assembly allows the microcomputer board to adjust the volume level up and down.

That left me with some decisions to make regarding this Beomaster microcomputer board. I have spare boards ready to go but I also have one set of Beomaster 8000 master and slave processor chips. The set is used and I don't know 100% if they are good but I decided I might as well exhaust all of the options I have. I installed the spare processor ICs in the original microcomputer board I took out of this Beomaster and gave it a try.

What do you know, the replacement processors bring the Beomaster into standby mode and then to on mode. The volume control works but now there are missing segments from the display. I know the display board itself is fine as I ran a 24 hour burn-in test on those display modules earlier.  So is the segment problem due to the processor chips or are there additional problems on this microcomputer board?

I decided to reflow the solder joints of the signal paths for the display segment drivers along with trying a couple replacement driver ICs (SN74247) I have for IC1 and IC2. The results were not successful and the problem got worse.

Eventually I got to a point where the microcomputer board would not function again. Not totally surprising as I wasn't confident that the replacement ICs I have available for testing this microcomputer board were good so I decided to go ahead and use one of my spare, working microcomputer boards as a replacement for this project. It is the safest and more reliable direction to go. I considered moving parts from the spare board to the original board out of this Beomaster but that would be an unwise risk. The less handling of the processor chips the better (even with wearing anti-static straps).

Spare board installed...These are much better results.

Moving on I found another problem with the Beomaster.  After the original bench testing of the output amplifier boards where I set the initial no-load idle current, I rechecked the left and right output amplifiers again now that they are re-installed.

The idle current check still measures good so I moved on to the DC offset voltage adjustments (for the left and right channels).  The right channel easily adjusted to the specificied tolerances of the service manual : 0.0 ± 5mVDC.  I am at 0.0 - 0.6mVDC on the right channel.

On the left channel adjustment I can only adjust the DC offset down to around 0.0 ± 50mVDC.

It isn't a high enough value to prevent me from test playing the Beomaster. I have been playing the radio and an ipod connected to TP2 for a few hours now.  However, it is not satisfactory to leave it like this. I will have to pull out the left channel output board and investigate the problem with the offset.

Beogram 4000: Restoration of Arm Lowering and Tracking Systems

I started working on the Beogram 4000 from Germany. As usual, I restored the arm lowering and tracking mechanisms first. This shows the arm lowering solenoid and damper assembly:
I took the linkages and the solenoid out for cleaning and lubricating:
Once everything was back together clean and well-lubricated, it was time to take the sensor arm out to get to the damper-to-arm linkage, which is often stuck due to hardened lubricants. This shows the back of the arms:
Taking out the two screws at the bottom of the sensor arm assembly released the arm:
After removing of the locking washer the linkage can be taken out:
I cleaned the pivot bearing and put a bit of synthetic grease on the pin and then put everything back together. The next step was to align the arms that they are parallel to each other and orthogonal to the carriage rods:
The next step was to replace the cracked carriage pulley. This shows the original plastic pulley
and this the machined aluminum replacement:
Beolovely! Send me an email or use the contact form on the right if you are interested in getting such a pulley for your own Beogram. I'll be happy to get you in touch with Nick who provides them to the B&O community.
Finally, it was time to replace the original incandescent light bulb in the tracking sensor with a Beolover LED assembly. This guarantees long term stability. The bulbs often break and they are not available anymore. This shows the original setup:
The bulb is in the square black housing in the center of the photo. Taking it out reveals the tracking sensor aperture:
This shows the original bulb assembly in comparison to the LED replacement:
The LED sits in a spot close to the location of the filament of the light bulb. The blue trimmer that is integrated with the LED assembly allows the adjustment of the LED intensity, which is very practical when adjusting the tracking sensor feedback sensitivity. This shows the sensor implanted:
The LED light source is available to other B&O enthusiasts. Just send an email or use the contact form if you are interested.
The final act of this part of the restoration was to replace the old grimy sheet metal screw that clamps the aperture assembly to the tonearm assembly. This shows the original screw:
These are notoriously difficult to tighten and it is a great idea to replace them with a modern hex driven M2-12 mm bolt and a M2 nut, which can be tightened easily without changing the aperture alignment while doing so. On to rebuilding the electronics.