Beomaster 6000 (2702) restoration: finishing re-assembling without chassis + testing

All the PCB's have been reconnected and time to check if all is working. I know, it looks a bit messy, but at least I got it all together on my piece of wood. I checked and rechecked all the wiring, looked for possible short-circuits, etc. 

Let's see if we can bring this mess to life ! Plugged it into my variac and slowly brought up the power. I saw lights coming on.........and the power relay kicking in. Yes !!!! No smoke or bad smell either :-)

I was a  happy man, but not for long sadly enough........Two major events happened. The first one: I could not select anything. The volume/tone/balance control was working. Motor and clutches running. I also did not get any bias current on the output stages (I had connected my multimeter before powering up to check). Hmmm.. No 60V DC rail either. I had checked the power supply before and all was working fine with the correct voltages. The 20V and 21,5V DC for the electronic switch are derived from the 60V DC, so that is why the switch is not working and no selection can be made (FM presets and input selection). Turned out the power relay did not make good contact to bring the 60V DC to the PCB's! The other voltages from the power supply PCB (18V DC, -5V DC and 180V DC) are not going through the power relay. I took the relay out and cleaned the contacts (again). But then decided to put in a spare relay that I knew was working fine. Powered up again and YES, not only lights but also switch board working and bias current. Measured over the output darlington emitter resistors and got about 4mV. Should be 14.4mV DC but I had set the multi-turn trimmers to almost minimum to be safe ! Plugged in 4 speakers and got music on all 4 channels! Time to relax a bit and take a drink...

And now the second event: when I came back a few minutes later (with my drink...) the music was still playing and I wanted to raise the display a bit because I could not see it very well. A big electrostatic discharge between my fingers and the display and 2 channels out with big hum. Disaster arrived....It's freezing cold over here, heating turned up, dry air, rubber shoes, synthetic sweater, chair with synthetics and rubber wheels,...The perfect "ingredients" to get my body fully charged with static electricity!! Should have known better. Half an hour later and 5 transistors replaced (all in the output stage) and everything was back to normal.

Now it was time to finish my drink.....

Beogram 4000: Restoration of the PCBs and the AC Platter Motor

After rebuilding the arm lowering and tracking systems of the Beogram 4000 that is on my bench right now, it was time to replace the electrolytic capacitors and the RPM control relay and trimmers as well as the AC platter motor. This picture shows the original reservoir and motor capacitor setup together with the AC platter motor:

I removed the caps and the motor,

and assembled the new capacitors into the 3D printed holder that I designed to fit the strap that held the original capacitors in place:

Then I took the motor apart for doing an oil infusion under vacuum. This shows the extracted motor:

and opened up after drilling out the two rivets that hold it together:

This shows the rotor with the spring and washers as they are placed on the shaft:

I put the enclosure parts including the bearings into motor oil and pulled a vacuum:

While I let this sit for 24 hrs I focused on rebuilding the PCBs. This shows the main PCB in its original condition flipped out that I was able to access the components:

I replaced all electrolytic capacitors as well as the RPM switch relay and the RPM trimmers that allow to tune the speed:

This shows the RPM section in detail:

The two blue 25 turn trimmers were installed in a way that their adjustment screws are accessible from the solder side of the board:

This is important that one can adjust the RPM while the board is installed. The next step was to rebuild the small power supply board. This shows the original condition:

The two red dots are the only electrolytic capacitors on this board. When I flipped the board around after taking out the three screws that hold it in place the fuse that is located in the yellow tubing in the back came apart:

This seems to be a frequent issue with these fuses, and I think it is time to add replacing them to the standard Beogram 4000 restoration procedure. I installed a new fuse and secured it with some shrink tubing and then installed the board again:

After the bubbling stopped in the vacuum jar I extracted the motor enclosure parts and put the motor back together. Since the rivets that need to be drilled out to open it up also doubled as M3 nuts to hold the tilt adjustment screws of the motor, I used 3D printed parts that I designed a while back to hold M3 nuts in place for the adjustment screws (which I usually replace with hex head stainless screws since they allow much easier adjustment when the platter is installed and running):

Then I implanted the motor together with the capacitor assembly:

This shows the 3D printed parts on the motor in some more detail:

The motor housing is held together by the other two screws that bolt it to the enclosure.

This shows the two 2200uF caps behind the main motor and reservoir caps:

I usually measure the big capacitors after I remove them. Indeed one of them was completely dead showing just a few pF instead of 2000uF:

It is always a good idea to replace all the electrolytic capacitors at this point in time since frequently they are dried out or leaking. Sometimes they even short circuit, which can cause them to burst...not a beolovely sight. After this measurement, I plugged the deck in and started it up. It showed life signs but there still seems to be an issue in the control system since the arm spontaneously lowered when pressing one of the << or >> keys.

Beomaster 8000: Left Channel Output Amplifier DC Offset Resolved

This Beomaster 8000 is almost fully functional. Actually it does function and play music but it doesn't completely pass the service manual DC Offset adjustment procedure. It is very close but no cigar.

The right channel adjusts perfectly. With the Beomaster on and warmed up I have the right channel output amplifier adjusted so the DC Offset measures 0.0V ± 0.1mV.

The left channel could only be adjusted to around 0.040V (40mV). Here is the differential amplifier circuit where this adjustment is made.

The adjustment trimmer (R200) adjusts the current to even out the differences in hFE between TR201 and TR202 (BC546B NPN transistors). As I said, the left channel cannot be adjusted to the 0.0V ± 5mV range specified by the Beomaster 8000 service manual.

I pulled out TR201 and TR202 to measure them with my transistor tester. Sure enough, their hFE measured quite a bit different. TR201 measures 146 and TR202 measures 318.

I checked a few other BC546B transistors and they are all close to hFE = 300. So TR201 appears to be the culprit. I went ahead and replaced both TR201 and TR202 with BC546B transistors as close of an hFE match as I could find.

That did it. I rechecked the no-load current adjustment first then performed the DC Offset adjustment. Now I can get the left channel output amplifier down to a good DC Offset value.

This Beomaster is feeling like it is into the home stretch now. Time to start closing up the cabinet and doing some real play testing.

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.

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.