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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...

Friday, December 7, 2012

Beomaster 8000: Left Channel Volume Cuts Out Above 2.9

Here we go again. My second Beomaster 8000 lived happy in the living room for about 8 months. We used it everyday, and it performed like new. After all, it had a full recap and restoration and also looks basically like new! Well, now it sat on the bench again. The issue: When cranking it up above 2.9 the left channel suddenly cut out, while the right one behaved normally. I swapped it out with BM8000 #1 and we partied on on that afternoon. It is like with classic cars: You need at least two if you want continuous service...;-). Today I opened it up. The problem was quickly identified:

The volume is digitally adjusted via a 6 bit attenuator made by Analog Devices. This AD7110 chip is able to attenuate the input signal in 1.5 dB steps based on a 6 bit digital input. This gives it 64 attenuation steps, which would amount to a total of 96 dB attenuation. However, the design of the chip assigned the highest four numbers (1111XX) to an infinite attenuation. With 000000 being fully open, this gives us an attenuation range of 88.5 dB in 59 steps. This explains the 0.0-6.0 volume scale of the Beomaster 8000. The beauty of this design is that the volume display directly correlates to the binary information sent to the chip. Each ±0.1 step corresponds to a ±1 change of the binary number sent to the chip. Form follows function!

Knowing the above, the fact that the volume cut out at 2.9 immediately suggests that the most significant bit (MSB) of the 6 bit data bus was not connected anymore. A glance on the circuit diagram of board 4 shows that pin 3 (the MSB) is pulled up with a 220 Ohm resistor to 5V. According to the circuit description this is a precaution to prevent the AD7110 from opening up fully if the data bus is disconnected. In this case MSB would be set to 1 through the resistor, effectively causing an attenuation of 48 dB, preventing potentially disastrous effects should a speaker be connected in this circumstance. I concluded that the MSB must have gone open circuit, resulting in a permanent MSB=1 setting due to the resistor. Hence, after passing from 2.9 to 3.0 the attenuation reverted back to 88.5 dB. In presence of the right channel blasting loudly, this appeared as a complete loss of the left channel. In fact, after disconnecting the right speaker, the volume on the left channel recovered to the 3.0-equivalent level when increasing the volume from 3.0 to 6.0, confirming the hypothesis.

The problem was easily fixed by cleaning the pin header contacts at the left channel AD7110. The location of the chip is shown here:




The two ribbon cables connect directly to the AD7110 for each channel. The right plug is next to the AD7110 regulating the left channel. This picture shows the chip and the 6 headers connected to the bus pins of the chip:



It is strange that the chip is not labeled AD7110, but rather AD13/002 8201. But the pinout seems to be exactly that of the AD7110. Anyway, after cleaning the pins of the header (I also bent them a bit alternatingly up and down to increase the tension in the female connector), everything was fine, and the Beomaster is back in the living room!
It is interesting to note that this chip was apparently $10 in 1981(see Analog Dialogue 14(2), pp. 7, 1980 - http://www.analog.com/library/analogdialogue/cd/vol14n2.pdf) if purchased in the "100s", i.e. a single chip would have run about $20. Adjusting for inflation this corresponds to maybe $50 in todays dollars. This helps explaining why these Beomasters were quite expensive back then (but also quite advanced for the time!).



Monday, October 15, 2012

Beomaster 8000 Suddenly Goes into Standby Mode

The Beomaster 8000 is on the bench again. After about two weeks of happy service in the living room after 'completing' my restoration, out of a sudden it went into standby. Turned it on again...half an hour later: Standby! Not great. Tried it a few more times....Standby! And again!. I swapped it out with Beomaster 8000-2 (it is like with classic cars...one best has two if one is supposed to run at all times...;-), and opened it up. It turned out that the standby pin (#14 on IC4 - the microprocessor responsible for the radio, volume and startup) occasionally disconnected from the power supply causing the Beomaster to go into standby. This becomes evident by a larger-than-0V voltage (~3.5-4V) at pin 7 on P76 on the microprocessor board (#9). I traced the problem to a bad via on the board. Resoldering the via between P76-7 and pin 14 of the processor solved the problem. Here is a picture:



I resoldered all three vias above the microprocessor (to the right of the C97 label) for good measure. This is done best by simply putting a dab of flux paste on each via, and briefly reheating them with the soldering iron (340C). Use the usual precautions against ESD if you do this in a dry-air environment...the microprocessors are hard to find...;-).

Sunday, September 23, 2012

Beomaster 8000 Keyboard Restoration/Not Working Buttons

The final step to restore my Beomaster 8000 to its original glory was to fix a keyboard issue. I noticed earlier that the TP2 button only worked intermittently. I thought that this was a problem originating from corroded keyboard contacts since wiggling the button seemed to do the trick once in a while.... Hence, I took the keyboard apart to clean the contacts. While this was an interesting exercise, it turned out that this did not fix the problem. But more about that below. First a few pictures of the keyboard innards:

Removing the retaining washers turned out to be a pain. Since B&O used one-time use washers that need to be destroyed for removal I had to cut them off with a wire cutter:


This photo shows the buttons from below after removing the PCB and a plastic foil:

Here a shot of the PCB before cleaning:


For cleaning the contact pads have to be removed:

These contacts were quite oxidized (or probably rather sulfidized, assuming that the pads are silver coated)

Using a small head Eurotool fiber brush (I am grateful to Gunther at Beoworld.org for this excellent tip!!) I cleaned the contacts:

The above picture shows the PCB after reassembly.

I reinstalled the board using 3 mm external retaining rings from Grainger (113828) and #4 nylon washers. This is shown here:


So far so good. Plugging the keyboard back into the Beomaster and turning it on immediately revealed that this procedure did not fix the problem.

After playing a bit around I realized that the P4 button also not worked whenever TP2 was out. A look at the circuit diagram for the processor board immediately revealed that TP2 and P4 use the same input on the slave processor (it distinguishes between the two buttons via the strobe mechanism that also drives the LEDs of the display in three "phases"). The oscilloscope showed that after pressing the TP2 or P4 buttons the voltage at P81-7 (7th pin on plug 81 that goes to the microcontroller) was higher than on other pins connected to working buttons. This led to the conclusion that the connection between switch and microcontroller must be interrupted whenever the buttons ceased operation.

Hooking up an Ohmmeter between pin 7 and R103 indeed revealed a broken PCB trace, with an intermittent contact depending on the flex of the PCB in the vicinity of P81. I installed a bypass (pink wire in picture):

This seems to have done the trick. TP2 and P4 seem now stable.

After this I finally put the Beomaster back together. Tomorrow, I will swap it with the one I currently use in the living room and the next months will show if this rebuild is stable. At this point all features seem to work correctly. On to the next B&O!





Saturday, September 22, 2012

Beomaster 8000 Display Repair - The (Hopefully...;-) Final Solution

I finally had some time to get back to the Beomaster 8000. All that was left is to rebuild the display boards with SMD LEDs and to fix the keyboard (the TP2 button only worked intermittently, and that is the important button in the days of the iPhone as it allows to select the only input not used for its magnificent siblings, the Beocord 9000 and the Beogram 8000). Anyway, here a few pictures from my latest display repair process. I posted several times about displays in recent months, and if you care to pull them up, you can clearly see my progress in this matter. Here now, what I finally consider an acceptable solution to this fairly tricky issue with these units:

I used the same process to remove the red covers and white 'light conduits' (for lack of a better word) that I discussed earlier (I basically cut off the small plastic discs at the bottom of the PCBs with a scalpel, which immediately allows to remove the covers). Here a picture of the frequency display board after scraping the old LEDs off:


This picture shows the input selector board. Compared to the first two Beomaster 8000s that I refurbished this one shows a soldermask (green). Another 'advance' of this board generation is that the solder pads are now coated with another metal. Unfortunately, this metal coating did not survive the test of time too well, and it flaked off in several places when I removed the old LEDs. This is shown in the detail shot below.



I scraped the flaking parts off as good as possible to create a stable surface to attach the SMD LEDs to.

Here some pictures of the boards with soldered LEDs:




Nothing really new here, except that practicing improves skill...;-). I used MG Chemicals No Clean Flux Paste (8341-10ML) and standard Kester .4 mm solder wire (SN63PB37 #66/44). While it says "no clean" these boards defintely should be cleaned from residue in an Ethanol bath.

Now to the assembly process. This is the part of the process that did not satisfy me when I refurbished the first two Beomaster 8000s. Since there is no other way to get into the displays than to cut off the plastic discs that hold the red covers in place, the reverse process needs to inadvertently involve some kind of glue. Since I wanted to be sure that I could open the displays another time in case some of my solder joints would fail etc...I previously elected to attach the covers with Scotch tape. However, this did not look pretty enough in my opinion.
Another issue is that the 'light conduits' need to sit tightly on the PCB to eliminate optical crosstalk between individual LEDs. This is especially annoying when the Beomaster is off in a dark room. In this situation the single lit standby LED has a halo in the adjacent LED segments if the light conduit is not attached perfectly (o.k. I admit here that this type of concern is a sign of a severe B&O addiction, but what can one do??)
Here is what I cam up with for this set of displays:
This picture shows the light conduit of the volume display in place with a skirt of 5% window tint foil applied to stop light emission into the display-surrounding space


Seating the red cover squeezes the foil into place forming a tight light seal around the display block:


The last part of the process is to somehow fix the cover in place. Instead of taping it, this time I put dabs of super glue gel on the tabs of the covers where they emerge on the bottom side of the PCB. Then I put small squares of transparency foil on these glue areas. this has the effect that the glue is squeezed a bit into the surrounding space, similar to the plastic discs that I had to cut off to remove the covers. This is shown here:


Once assembled I pressed the entire thing together between two thick cardboard pieces using two carpenter's clamps as shown here:


After about 2 hours I removed the clamps and the displays were ready. Here some shots when I ran them on the breadboard set-up for a final test before reinstalling them in the Beomaster 8000:


Note the absolute darkness around the displays...no more halos. I should add that I also blackened the backs of the PCBs to stop light emission there, too.

After testing for 24 hrs I reinstalled the displays in the Beomaster. Before reinstallation I adjusted the display voltage to reduce the intensity to a level similar to the original displays like I described earlier here: 

http://beolover.blogspot.com/2012/05/beomaster-8000-display-brightness.html

This is a nice and fast method to get the intensity right without having to change all the current limiting resistors of the LEDs. 

On to the keyboard!








Wednesday, September 5, 2012

Beovox 4500 Re-Assembled

I re-assembled the Beovox 4500s...below is a picture of the right speaker. Beautiful! Communication with the Beosound 9000 through Power Link works also very well. I used "Din 8 (Large Din) 8 pin Male-Male" cables for the Power Link connection. The so-called original (overpriced) Power Link cables that are available on ebay are nothing else. This is the beauty of this set-up. The speakers are connected to the receiver by a data bus and 2V audio signals only, i.e. all that is needed is a shielded 8-pin small signal cable. The amplifiers run on their own power - each speaker has its own power supply and an integrated amplifier. I connected my ipad on the 9000's auxiliary input (selectable through "ATape" on the Beo 4 remote) for testing. Need to get a 75 Ohm FM antenna to get the tuner working. And burn some CDs that I can use the CD changer...how very vintage!...;-)

Monday, September 3, 2012

Beolab 4500: Panel Polishing (2)

Today, I did the vinyl panel of the right Beovox 4500 speaker. Experience matters. Yersterday, I did not dry the panel after each step before inspection. This resulted in some missed scratches that consequently were not polished out in subsequent steps. Today I made sure to dry the panel with a soft cloth after each step and then give it a thorough inspection. This resulted in a more consistent polish. Here are some detail pictures before and after each polishing step. One can clearly see the alternation between horizontal and vertical patterns until the 4000 grit Micro-Mesh, after which a uniform shine develops.

1) Damage detail (these scratches were easily felt with a finger tip):



2) 220 grit


3) 400 grit


4) Micromesh 1500

5) Micromesh 1800

6) Micromesh 2400

7) Micromesh 3600

8) Micromesh 4000

9) Micromesh 6000

10) Microgloss polish paste - done!

After this I thought I were done, but unfortunately I had to discover that the transformer in one of the amplifiers sheared off its mounts...this probably happened during transport. The fairly heavy transformers are only held in place with four fairly small screws bolted into thin extruded plastic tubes attached to the amplifier housing...another example for B&O's sometimes weak internal designs...let's see what some superglue can do (luckily the broken off parts were all present...).


Beolab 4500 Speakers: Polishing of Plastic Panels

No soldering today. I thought it is finally time to work a bit on my recently obtained Beolab 4500 active speakers. They came from ebay where they were listed as non-working. While they turned out to be working after all, they had badly scratched plastic panels and one significant scratch on one of the aluminum panels. While I am not sure what to do about the aluminum panel, the plastic panels can be easily fixed. Last year I polished the dust cover of a Beogram 4004 to a pretty new looking state. I used the amazing Micro-Mesh KR-70 polishing set, which consists of a rubbery polishing block and a series of fine grit sandpaper. Following the instructions that came with the kit allowed me to fully restore the 4004 cover.

Today, I used this kit on the Beolab 4500 panels. Taking the panels off from the amplifier part of the speakers is pretty simple, but one needs to pay attention that some pretty thin and short cables that are attached between the panel and the amplifier chassis. Hence, after removing the torx screws, carefully lifting off the panel and removing of the plug on the display board and the complete input switch board are required to get the panel off.

Before polishing I also removed the display board from the panel. Since the polishing process involves the use of water, it is best to protect the display board. I also covered the brushed aluminum coated part of the panel with bubble wrap to protect its surface during the polishing process. Here is a pic:


This pic shows a close-up of one damaged area:

When the damage is so deep that one can feel it while moving a finger across, the Micro-Mesh manual prescribes to start with standard 220 grit sand paper from a hardware store. Now, this type of paper is common for wood working, i.e. it is pretty coarse. When I did this the first time I was very reluctant to put this grit to work. Essentially, it dulls the surface completely - a scary moment for the Beolover...;-). Here is a picture after I worked the panel with the 220:


Looks pretty 'woody' doesn't it? This step is probably the most important step of the entire polishing process. With the 220 basically the surface is removed to a depth of the deepest damage. In other words the 220 needs to be worked in an even parallel stroke across the surface (and sides of the panel) until the only scratches present are those from the 220 grit. This means all scratches need to be in the direction of the stroke, and the entire surface needs to be completely homogeneously scratched.

The following steps are basically to use ever finer grit, and apply each grit level with strokes that are 90deg off relative to the stroke used for the previous grit. This allows to see the moment when the scratches from the previous grit are replaced by scratches from the current (finer) grit (dry the surface and look at it against a light source - if you see 90deg off scratches keep at it until they are gone). Successively, with each step, the scratches get more shallow, until they become invisible. That is the moment when the surface is fully reflective and shiny.

Here are a series of pics from this process:

After 400 grit:

After Micro-Mesh 1800:


After Micro-Mesh 2400:

After Micro-Mesh 3600:

After Micro-Mesh 4000 (it starts getting shiny):


After Micro-Mesh 6000 (very nice! almost as new!):


After Micro-Gloss polishing fluid:

That's it!! 
In summary, the most important aspect is that after each step all scratches from the previous level must be gone, and that only scratches from the current grit are left. If the previous scratches are not completely removed, it will be impossible to get them during subsequent steps, since the grit gets ever finer, and does not have enough 'power' to remove deeper scratches from coarser, previous, grits.
One more panel to go! Can't wait to hook these elegant beauties up to my Beosound 9000!!


Sunday, September 2, 2012

Beomaster 8000 Main Reservoir Caps Replacement

Time to go fully under the hood! Like for my second Beomaster 8000 I replaced the four main 10000uF capacitors with the 105C ECES1JA103EZ Panasonic type, which have almost the same size. Here is a shot comparing old with new (one of the new ones is already wrapped with some cardboard to compensate for the slightly smaller diameter...):
I measured the ESR of the old ones. These were in pretty good shape with all four measuring only 0.08-0.09 Ohm (pretty much identical to the new ones).
Here some pictures taken during the installation.
After taking the right heat sink off:

This time I changed my routine a bit. For easier access on the right side, I also unbolted the right bridge rectifier. This made it easier to get to the solder tabs of the caps. Last time I ended up ripping off a wire from the rectifier when removing the back cap.  This picture shows the rectifier dangling and the cap below already unsoldered.

Unfortunately, it seems almost impossible to get large caps with big solder tabs. The new ones have much smaller tabs. Here a picture showing the smaller tabs bent to clamp the wires in place before soldering:



This picture shows the caps on the left side fully installed:






Friday, August 31, 2012

Beomaster 8000 Left Output Stage Rebuild

The summer is coming to an end. Back to B&O! Today, I finally recapped the left output stage and also fitted it with a 12x precision trimmer for the no-lad current. As stated earlier, the original low-cost trimmers are an Achilles heel of these beauties. Corrosion in combination with vibration can easily result in the destruction of all TIP transistors in the output stage. It is a multi-hour repair effort if that happens. Hence, whenever a recap is done, one should also immediately replace these trimmers!
Here is a picture of the left channel output before (power rail plugs already pulled - Some advice here: make sure that the big reservoir caps are completely discharged before messing with these plugs. One thing is that there are almost 110V DC between them, which can be painful, the other is that touching components with the unplugged cables during rebuilding the board can easily result in damage, especially if the circuit is still grounded. To discharge the caps I usually put a 10R power resistor across them for maybe 10-20 sec - measuring the voltage at the terminals confirms completely discharge):

The picture above shows one of the two100uF caps of this board...cracks are visible. A sure sign that this capacitor would have soon died.

This is how the board looked after restoration: