Beomaster 8000: Step Six - Rebuilding the uProcessor Board

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ACHTUNG: Please, note that the crystal exchange procedure described here can damage the microprocessor chips on the uProcessor board. It is recommended to remove the chips before replacing the crystals and the capacitors. Due to the inherent capacitance of these devices, high voltages can be present between their terminals, which upon release, can burn out the gate that forms the oscillator together with the crystal. Make sure you short circuit the leads of the parts before installing them.

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After completing the display board rebuild with new SMD LEDs in the displays themselves, as well as replacing the incandescent light bulbs in the indicator cabinets with SMD LED based assemblies, it was time to do the uProcessor board.

The uProcessor PCB is the main source of operational trouble due to a few shortcomings in its design. In the 80s double sided PCBs were only beginning being standard in consumer electronics, and the earlier 8000s have boards without through plated vias. This made it necessary to insert metal plugs into the vias and solder them. Many 8000s have startup trouble due to these solder joints cracking. Luckily this Beomaster 8000 already had an updated board where the via plugs were replaced by soldered in colorful wire jumpers. This shows the processors, the jumpers and the crystals:

Another source of trouble are crystals that stop working reliably. This can cause severe malfunctions in the start-up circuit potentially damaging the main transformer. I started replacing them as a standard restoration item since I had a malfunction in one of my own Beomaster 8000s that caused the uninterruptible power supply (UPS) I run this Beomaster on to go in protection mode. This indicated that very high currents were flowing in the main transformer while the malfunction occurred. More here about this phenomenon. When the exact same condition happened in another Beomaster that I restored a few years ago, I took it as a sign that this is an issue that may be prevalent in many 8000s.

So here we go. This shows the old big crystals replaced with smaller modern 2MHz units. 

You can pretty much use any 2MHz crystal, as long as you also install the correct capacitors. The original crystals use 12pF capacitors to resonate, while modern units usually have 18-22pF. If you look closely, you will see that I also exchanged the two small capacitors directly underneath the crystals. They are now 18pF. 

I also replaced the two electrolytic capacitors on the board with high quality 105C grade Japanese units.

Here are some more photos: This is a detail shot of crystal X1

and a picture showing the solder side:

This shows the solder side of X2 for reference:

After this I plugged everything back together, and it was time for a test run! This shows the display board in all its glory:

Beautiful! Very Beolovely! By the way: One can 'provoke' the clipping light (left most) to come on, too, by setting the tuner to a station and then cranking up the volume. Above about 5.0 the clipping light usually comes on (make sure you do not have speakers connected during this test...;-).

On to replacing the speaker switches!

Testing B&O MMC Cartridges with a QuantAsylum QA400 and Test Records

A while ago I agreed to try and qualify a few B&O cartridges. I received a MMC20EN, a MMC4000, and a MMC6000 in the mail. For comparison I added a MMC20CL to the test line-up for comparison. The CL came along with the Beogram 8000 that I just restored. In fact, I used this 8000 for this test. The setup was as follows:

I connected the Beogram 8000 directly to the inputs of a QuantAsylum QA400 audio analyzer which was hooked up to a PC. The QA400 can reliably work with signals down to -130dBV, which is an ample range for measuring moving magnet cartridges, which typically emit a signal in the -50dBV (~3-4mV) range at the highest volume of a vinyl record.

I used a Analogue Audio Test LP for the channel separation, direct frequency response values and the pink noise spectra. For measuring the frequency response at higher frequencies I employed a "TheFisher Test LP" for CD-4 set-ups. This LP has some test tracks that play the -70dBV 30kHz carrier tone for the rear channels of CD-4 records. This allowed the measurement of at least one solid datapoint beyond the 20kHz range (where the Analogue record seems to end).

This graph shows the pink noise spectrum of the cartridges:

The amazing item about this graph is that all four cartridges yielded pretty similar spectra. The steep cutoffs at 20kHz are a result of the fact that the pink noise track of the Analogue Audio record seems to cut out at 20kHz, i.e. the pink noise does not contain frequencies above 20kHz. The only difference between the cartridges is that all of them except the MMC 20 CL showed a ~2-3dB difference between the left and right channels (which I believe is a difference that is hard to discern when listening to music).

These are the spectra measured with the TheFisher LP:

The narrow peak on the right end of the graphs is the 30kHz carrier peak of the CD-4 signal. CD-4 essentially uses frequency modulation (FM) of this carrier signal to encode the back channels. This can be seen directly when the rear signal contains an audio signal. In this case the narrow peak widens reflecting the modulated frequency spectrum. The spectra shown here were measured on a track that only contained the back channels, and during a quiet moment of that channel (so only the carrier frequency was present).

Again, the amazing item is that the 20CL, the EN and the 6000 show a very similar behavior. All peaks come in at about -70 dBV, which is very reasonable, considering that the carrier is supposed to be -20 dBV (=1/10th of the voltage of the regular 0VU signal of the front channels) below the front channels at full volume (0 VU = -50dBV).

While these spectra are helpful to decide whether a cartridge is in good condition or not, they are not equivalent to a 'real' frequency response curve. Unfortunately, there seems to be no way to do such a measurement with the QA400 since it cannot detect an external input frequency and then measure the transfer function of that signal. This means that the frequency sweeps on the Analogue Test LP must be used with manual readout if a glimpse is sought into the real frequency dependent output levels of a cartridge.

This table shows the measurements I made:

the 1 kHz response was measured using the first track on the first side of the record. It simply plays a 1 kHz tone at full 0 VU. This yielded values of about 3-4mV (corresponding to dBV values around -50 dBV). The separation was measured on Tracks 2/3 and the values represent the dBV difference between the on and the off channel.

Then I used Tracks 6 and 7 which are a 1-20kHz sweep in 1kHz steps at -20 dBV relative to 0VU and a 1 kHz - 100Hz sweep in 100 Hz steps at 0VU (the 100Hz values are adjusted to the -20dBV scale to match the other values). The dBV values are the amplitudes of the main peak at these frequencies. It is obvious that all cartridges have a stronger response at higher frequencies than at lower ones. Considering that the pink noise spectrum varies in the opposite way by about -20dBV from 20 -20000 Hz, the (relatively flat) pink noise spectra roughly agree with these values.

In conclusion, it appears that all four cartridges have a very similar frequency response. This is somewhat surprising in the light of how these cartridges were marketed for different applications and listening tasks.

Beomaster 8000: Step Five (2) - Rebuilding the Displays and Indicator Lights with SMD LEDs

This is a continuation of this post about rebuilding the display board of a Beomaster 8000:

After 24 hours the LEDs were all still in business, and so I decided to put the displays back together. My current method is to press the display covers onto the PCBs with carpenter clamps and then use black hot glue to seal the sides of the display units. This keeps them together reliably, while it is still reversible - with some effort the glue can be peeled off to open them again. I like solutions like this since no one (and no electronic component) is 100% perfect, and so it is nice if one can repair things (again) at a later date if necessary.

After putting the displays back together I usually run them for another 24 hours to make sure that the assembly process did not inflict damages on the SMD LEDs:

While this test was running, I did the rest of the display PCB. I replaced the two electrolytic capacitors with 105C grade Japanese units and then I went on to replace the four incandescent light bulbs that illuminate the indicators (clipping, filters, mono, and manual tuning). This shows the right two bulbs in their reflector boxes:

The original bulbs usually have very weak leads since they corrode over time. The slightest motion of the bulbs can break them off. Therefore, I developed a SMD LED based solution to replace them. This shows them in place of the bulbs with the reflector box removed:

There need to be two versions of these SMD LED boards since the reflector cases are mirrored for the left side, i.e. the polarity is reversed (which plays a role for LEDs while bulbs are impervious to the current direction): 

And here with the reflectors put in place:

After the 24 hrs test I soldered the displays back into the PCB:

Before I test this board in situ I will restore the microcontroller board...more about this in the next post of this project.

Beomaster 8000: Step Five - Rebuilding the Displays and Indicator Lights with SMD LEDs

No Beomaster 8000 restoration is complete without rebuilding the displays. They will all fail eventually, and most in the near future. And since those beautiful large LED displays are a main design feature of this lovely receiver, nothing is more frustrating than having a few missing segments. No matter how good the condition of the enclosure, when the displays are not complete it just looks sad! Not beolovely at all!

Luckily the displays are large enough that one can update them with 0603 packaged SMD LEDs, which can still soldered fairly well by hand. For this to happen one needs to extract the displays from the display board:

Once liberated, the next step is to open them up and scrape off the original LEDs, which are directly bonded to the PCB...truly a 1980s design!:

Once the pads are prepared it is time to line up the SMD LEDs and solder them into the places of the original LEDs:

The critical element here is to place the SMD LEDs precisely into the areas that are underneath the light guides in the LED assemblies (white parts with long slots in the picture above) when assembled. If they are not centered, the LEDs will get damaged during re-assembling the displays since the light guides will exert lateral pressure on the LED packages, which can result in them breaking off their pads. I do this by soldering the LEDs on only one side, then checking where the LEDs come close to the light guide slot walls, and then I correct the position of those that are a bit off. Once all are properly in place, I solder them on the other side.

After soldering all the LEDs in, it was time to give the boards their first 24 hrs test. For this I have a breadboard setup that mimics the wiring on the display PCB:

Let's see if they are still all on tomorrow!

Beomaster 8000: Step Four - Rebuilding the FM Tuner, Preamplifier and Control Panel PCBs

After completing the power supply restoration of the Beomaster 8000 that I am rebuilding right now, it was time to do the remaining boards. While it is relatively rare to find a dead capacitor on the FM tuner, preamplifier and control panel PCBs (they run relatively cool), it is a great idea to replace them anyway while one is in there. That brings everything on the same 'capacitor clock' (for another 30 years countdown...;-). It is also a great idea to re-solder all the wire to board headers since they tend to develop bad solder joints over time. Here are a few impressions of my progress:

This shows the board under the control panel switches and sliders in its original condition:

And after replacing the capacitors:

On to the preamplifier board:

In original condition:

and after rebuilding it:

The FM tuner has two boards, the front end and the detector part:

This shows them after exchanging the capacitors:

On to the display and uComputer boards!

Beomaster 8000: Step Three - Replacing the Main Reservoir Caps

No power supply restoration of a Beomaster 8000 is complete without also replacing the four big cans in the back. They are the reservoir capacitors for the output amplifier ±54V power rails. One 10000 uF capacitor per polarity and channel - massive! That gives the Beomaster 8000 its excellent output stability at 150W per channel. Awesome!

Here are a few impressions of my process:

This shows the original capacitors on the right side:

I unsoldered and pulled them out and replaced them with modern 105C grade Japanese units. As usual, modern capacitors are a bit more compact than 1980s types. That is why I use 3D printed adapters to fit them into the capacitor bays of the Beomaster 8000. This picture shows the original capacitors and the modern replacements with their 3D printed adapters:

The adapters and capacitors are available as a kit, just send me a message using the contact form on the right

After the installation it turned out that I broke off one of the resistors at the thermal switch for the right side:

This happens frequently when pulling out the capacitors, since the resistors are in the way, and they cannot take much bending. I replaced both resistors with new ones:

Then it was time to turn this 8000 around and do the left side:

After replacement:

After this I measured the capacitance of the original caps and of course they were out of spec (they usually are at this point):

4000uF is less than half of the specced 10000uF, but this would still have worked at moderate volume settings. But of course, the caps would have failed in the near future. So it was the right thing to replace them now for piece of mind. 

An then it was finally time to turn this Beomaster 8000 on for the first time:

And of course, one of the segments in the input display was a no-show. No problem, since I usually rebuild these displays anyway with SMD LEDs to ensure stability in the years to come. These displays all will fail in the near future, i.e. it is almost better to buy a Beomaster with failed display for less money and then fix it. That way one has a Beomaster with displays that will likely last another 30 years.

On to recapping the rest of the unit!

Beogram 8000: Installation of a Grounding Switch, Repair of a Broken Off PCB Post and, Finally!, Test Drive with Gabor Szabo!

I gave the Beogram 8000 that I am restoring right now a couple final touches:

1) As usual, I installed a switch that makes a connection between system and signal grounds. Such a connection usually extinguishes any hum that may arise from incompatible connections between turntable and amplifier. In my view there are no adverse effects resulting from connecting the two grounds, but the original B&O set-up is to keep them separately until they meet in the amplifier, so I put in a switch to make the connection easily reversible, if desired down the road.

This shows the installed switch on the output relay board:

2) I needed to reattach a broken off PCB mounting post. The post that holds the rubber bumper that presses the main PCB into its mounting brackets broke off when I removed the PCB...this old plastic can sometimes be a bit brittle. Luckily, there was enough 'meat' that allowed me to drill a small hole through the post and the underlying plastic with my Dremel tool. Then I bolted it down with a 2-56 screw and this worked very nicely:

All good now under the hood!

I closed it up and then performed a calibration of the tracking weight:

And then it was finally time for a test drive! I had just received a new vinyl from Sweden: Gabor Szabo's "Small World" from 1972. This is one of those he recorded in Sweden. I got the original Bellaphon release (Made in West Germany...;-)

What a great record (especially after ultrasonically cleaning it with my CleanerVinyl Pro..;-)! And his awesome 1970's purple outfit! Those were the days! The album contains another version of 'people', which is one of my favorite tunes ever. And Szabo brings it to life like no other. Too sad that his life was short and things did not end that well for him. Very unfortunate! I am very grateful, though, he left us a bunch of wonderful albums to enjoy! More achievement than most of us will ever be able to claim when we go meet our Maker for that final exit interview...;-)! This is Beolove!