Beomaster 8000: Repair of a Chipped Veneer Corner

One more thing needed to get tidied up before the Beomaster 8000 that I just finished restoring could join its friends, the Beogram 8000 and the Beocord 9000 in our living room: The front right corner of the front veneer panel was damaged:

Unfortunately, it is quite difficult, maybe even impossible, to find this type of rosewood veneer these days. So I decided to give it a try to fix this corner instead of replacing the entire veneer strip. I still had some teak veneer from earlier experiments with rebuilding the veneer panels of a Beomaster 1900 that I was given at some point with all veneer strips missing. I did some 'microsurgery' using this veneer after I thinned it down to the approximate thickness of the 8000 veneer strips. I cut appropriate small pieces and glued them into the veneer gap. After that I used drawing pencils to color the wood in a rosewood-similar hue:

The outcome: Not 100% perfect, but pretty good in my opinion. One of those flaws that people will only see after one points it out to them. Definitely much better than before!

Beomaster 8000: Replacement of a Damaged Control Panel Lid Linkage with a 3D Printed Reproduction

The Beomaster 8000 that I am restoring right now is close to being embedded into my Beosystem 8000 for some real life testing in our living room. I usually do that to make sure that there are no intermittent issues before I ship the unit out. Today I put it back together and when I was about to bolt the control panel lid back in, I realized that the linkage that connects it to the damper was installed in reverse orientation, and that one of the receptacles of the linkage had been bent open. When I took it apart it already felt that there was something wrong since the linkage came loose very easily:

Most likely the 'expert' who worked on this unit previously did not know that the linkage can easily be dislocated by turning the damper arm vertical and then sliding the linkage to the right off the hook that holds it in place. He instead used the hot air blower of his soldering station to soften the plexiglass and then removed the linkage...This is the fourth Beomaster 8000 with a damaged linkage that I am working on. When I encountered this issue the first time, I designed a 3D printed reproduction of the linkage. Luckily I printed a few more, so it was an easy task to replace the broken linkage with a working one:

For the record, here is a picture that shows how to position the damper arm to slide the linkage on or off:

Allright! After this I buttoned the Beomaster up and tested it. Everything seems to work now including the Terminal remote. Another 8000 back in service!

Beomaster 8000: Volume Encoder Damping Restoration - The Final Answer

The Beomaster 8000 that is currently on my bench had lost all damping of the volume rotary encoder wheel. This issue is more than a cosmetic one, since damage can occur to the speakers if they are not dimensioned properly if someone accidentally spins the volume encoder up to 6.0. Without damping this can easily happen if attention is not being paid. Up to now I used a method to restore the damping that was based on a 3D printed paddlewheel that I clamped on the back of the encoder shaft and then filled the cavity with a high viscosity damping grease. While this worked fairly well, I always had the feeling I was on the wrong track with this. So yesterday, I finally took the plunge and opened up the volume encoder to have a look under the hood with the goal to understand how the original damping had been done. After taking out the encoder sensor assembly I cut the retaining clips off with a wire cutter (unfortunately this seems to be the only way to get them off) and removed the encoder:

It is held by four adhesive strips and I needed to use a screwdriver to lift it up one corner at a time. Once I had it out I thought what a beautiful big design compared to today's encoders...Anyway, once the encoder is off, one can simply pull out the wheel from its precision sleeve bearing:

And after I saw this everything was clear: The original damping was simply done by putting some damping grease on the shaft and then inserting it back into the bearing. Due to the fairly tight fit this creates a nice damping effect. Unfortunately, over time this grease is entropically driven out of the bearing, and the damping effect wanes gradually. I put some Nyogel 767A on the shaft

And inserted the wheel into its bearing and the wheel was damped again. I put the encoder assembly back onto its posts and clamped it down with new 3mm retaining clips and nylon washers to protect the encoder housing in case it needs to be opened up again in another 20 years for a re-greasing:

Then I adjusted the plexiglass clamp in the back to ensure scraping free operation while being flush with the surface of the Beomaster keypad. I guess the evolution of my fix to this issue is another example of live-and-learn. I should have taken one apart much earlier...;-).

Beomaster 8000: Display Restoration with SMD LEDs

After I replaced the indicator light bulbs of the Beomaster 8000 that I am restoring right now I did the 7-segment displays. The 7-segment displays are essential for the signature looks of the 8000 and dead segments are at best annoying. I usually recommend to rebuild the displays even if they are still o.k. since the death of individual segments is virtually unavoidable considering the age of the units and the potential rigors of shipping. The original LEDs in these displays are not encapsulated, i.e. the bonded contact wires are not protected against vibration and thermal challenges etc.... Therefore, the usual failure mode is loss of contact at the bonding locations. Final failure is often preceded by intermittence as these wire bonds. This Beomaster had a few segments in this stage, which may or may not explain the claim by the seller that the displays were in good working condition. I posted many entries in the past about my restoration process, a summary can be found on my dedicated Beomaster 8000 page. The pictures shown here are posted to document the work done on this particular Beomaster 8000.

This shows the display board after I extracted it:

This shows the displays after unsoldering them. It is mandatory to use a desolder gun for removing them to prevent damage to the fragile pre-FR4 age PCB:

Then I liberated the displays from their plastic mounts,

and opened them up:

Then I removed the original LEDs and then soldered SMD LEDs into place. Then it was time for my 24hrs test where I power the boards from a test-jig that I set up with a couple solders breadboards:

Then I put the covers back on and tested them again to make sure all segments survived the procedure:

Then it was time to solder the displays back in. Here you can see them mounted back on their plastic mounts:

And back on the PCB: 

And fired up together with the newly rebuilt indicator lights (unfortunately I forgot to use a FM preset, so the source display is dark in this picture). The volume is a 6.0 to get the clipping indicator to light up:

On to the uProcessor board.

Beomaster 8000: Replacing the Incandescent Indicator Light Bulbs with SMD LEDs

I always enjoy rebuilding the display board of Beomaster 8000s. So much of the signature looks of these units depends on the gorgeous large 7-segment displays and the masked indicator lights for clipping, filters, mono and manual tuning. With this particular unit I started out by replacing the indicator bulbs with my recently designed SMD LED based replacement boards. There are two versions of this board accommodating the two left and right incandescent bulb cabinets. The contact pads for the bulbs are mirrored in these compartments, while the polarity remains the same. This mirror symmetry requires different boards to accommodate polar devices like LEDs. Here is a picture of the boards that I installed:

When I developed these boards I made a video how to install them:

It is important to keep in mind that the LED boards run on a much smaller current than the light bulbs, and this requires the removal of the resistors R34/36/38 and 40 on the display board. Otherwise the LEDs will always be on.

This shows the original light bulbs in their cabinets:

And after replacement with the SMD LED boards:

And in on condition:

On to rebuilding the 7-segment displays.

Beomaster 8000: Recapping the Tuner Boards

The final boards to be recapped in the Beomaster 8000 that I am currently restoring were the FM tuner boards. Here are some impressions of this effort. This shows the boards with the original capacitors:

And here after replacing all electrolytic capacitors with 105C grade Japanese units:

With these boards it is especially important to clean the contacts between the boards and coat them with DeoxIT. I had several cases in the past where one channel did not work on the tuner, which could directly be traced to a contact issue between these two boards. After I put everything back together I tested the Beomaster and everything works fine now!

This concludes the recapping of this Beomaster. On to the displays and the uProcessor boards. On into the heart of the dragon...;-)

Beomaster 8000: New Electrolytic Capacitors for the Control Panel and the Preamplifier

I made some progress with the recap of the Beomaster 8000 that I am working on right now. Today I did the control panel and the preamplifier boards. The control panel pcb is bolted to the control panel fixture, and one needs to unsolder the leads to the signal strength meter. After that is done the PCB can be extracted:

Here is a shot of the board after replacing all electrolytic capacitors with 105C grade Japanese units:

As usual, I also reflowed all solder points on the wire-to-board connectors and coated the pins with DeoxIT D100 for longterm stability.

After I put the control panel back together I moved on to the preamplifier board. It seems one 10uF capacitor had already been exchanged at some point. These red electrolytic cans are often problematic at this age.

I replaced all of them with new ones and also did all the connectors:

Here a detail shot:

On to the tuner boards, which will conclude the recap of this Beomaster 8000!

Beomaster 8000: Rebuilding the Power Supply Board

After replacing the original main reservoir capacitors of the Beomaster 8000 that I am restoring right now it was time to do the remaining power supply. The golden reservoir capacitors often fail at this age. Another often found dead capacitor is the 5V rail cap, since it is directly mounted on the 5V regulator (on the heat sink on the right side of the Beomaster enclosure), and therefore sees elevated temperatures. Here is a photo of the original power supply board:

and after replacing all the electrolytic capacitors with quality 105C grade Japanese units:

Before putting the assembly back into the Beomaster I also cleaned and reflowed all headers, and coated the pins with DeoxIT D100 for lasting oxidation protection. On to rebuilding the remaining boards...

Beomaster 8000: Replacing the Speaker Switches with Modern Encapsulated Units and Laser Cut Adapter Plates

After my second round of output PCB troubles in the Beomaster 8000 that I am restoring right now, I put in new speaker switches. The heatsink cover was still off, so this was a 'good moment'. I implemented modern encapsulated switches and laser cut adapter plates that make them fit into the original bays for the switches. A while ago I made a short video about this process, that also shows the laser cutter in action...what's not to like about digital manufacturing methods!:

These pictures document the procedure on the particular Beomaster 8000 that I am rebuilding. This shows the original switches in place:

Here they are 'folded' out and the original mounting plates are visible:

This shows the original switches in comparison with the new replacements and their adapter plates. Note that the here used most recent design iteration has laser etched tabs on the narrow sides of the plates since I was not able to find plexiglass sheets of the same thickness as the original base plates of the switches:

These switches and adapter plates are available to other enthusiasts, just send me a message.

Here they are soldered in place:

This picture shows them held in place in their bays. The laser cut plates exactly fit in the cutouts and the precisely laser etched thickness of the tabs ensures that the switches are at the right height when the heatsink cover is installed:

Beomaster 8000: More Output Trouble - Broken TR207 Collector Trace

After I fixed the broken PTC thermistor lead in the left output of the Beomaster 8000 that I am restoring right now, I thought I was finally done with the outputs, but no: When I tested it for a while it ran happily, and then out of a sudden the fault switch triggered again a shutdown of the output supply. This one was difficult to find since it turned out to be intermittent. After a while I finally figured out that the trace that connects the collector of TR207 (voltage gain stage) to the positive 55V rail was cracked. After removing the delaminated trace, I fixed it with magnet wire:

I like to use magnet wire for fixing traces, since the polyurethane insulation of the wire can conveniently be burned off with the soldering iron, resulting in short circuit proof connections that are insulated right up to the solder point.

Unfortunately, this did not fully fix the output issue. While the 8000 came on again reliably, I realized that my repair efforts must have caused some other problems. More trouble shooting finally yielded another broken off lead to the heat sink. It must have broken off while I lifted the board up to do fix the trace. The many alterations on this board and also the left channel suggest that the previous owner had a hard time to pinpoint the issue with the intermittent trace and the fault switch issue, and therefore the board was lifted a few times too many for replacing components finally compromising the integrity of the leads that go to the heatsink. This time the (green) lead to the base of IC205 broke at the solder terminal on the PCB. This caused this Darlington to be turned off. It was remarkable that the output still amplified under this condition. I only noticed the issue since after repairing the trace I ran the output with external power supplies while measuring the voltage drop across the R236/7 emitter resistors, which could not be raised anymore above 3 mV. This indicated that there was something wrong. After I reconnected the base of IC205 everything was finally good again. Live and learn. No Beolove without some degree of pain, I guess...;-)

Beomaster 8000: Replacing the Output Reservoir Capacitors

SinceI had the Beomaster 8000 heat sink cover off while working on the output stages it was a good moment to also replace the original 10000uF reservoir capacitors. I always enjoy replacing these with modern units and my custom designed 3D printed adapters to make them fit snugly into the original bays. Here are a few pictures:

Here is a shot of the replacement units with their 3D printed adapters (these adapters are available to other enthusiasts - just send me an email). I currently use quality Japanese 105C caps from United Chemi-Con (EKMH630VSN103MA50M):

This shows the original capacitors of the left channel:

And after installation of the replacements:

Here is the original right channel:

And the replacements installed. I had to put on shrink tubing (yellow in the photo) onto the original leads to make them short circuit proof. Some amateur overheated the insulation while messing with something back there and it was a bit frayed at the ends.

On to the speaker switches!

Beomaster 8000: Fault Switch Gets Triggered by Disconnected PTC Thermistor

Oh well...I guess I was a bit too enthusiastic when I declared victory in my last post about the output amplifiers of the Beomaster 8000 that I am restoring right now. It turned out that there was an intermittent issue that triggered the 'fault switch' and that in turn turned off the relays that enable power to the output stages. Here is the relevant circuit as shown in the Beomaster 8000 'Technical Product Information' booklet:

Here is how it works: TR16/17 form a latch that is triggered by a >~1.5V voltage coming from the "fault output" of the output stages between R254 and R253. When the latch is set it turns on TR15, and 15V apply to R68. That pulls up the base of TR11 and it turns on, which in turn turns on TR12. This pulls up the base of TR18 and the relay looses power. This cuts the power to the output boards, and the 8000 goes silent. All other circuits remain on, i.e. the display are functioning normally etc...

The two failure modes that can trigger the latch are:

1. A DC voltage at the 'AF OUTPUT' (this usually means that one or more TIP transistors burned out due to overload or failing quiescent current trimmers
2. The heatsinks get too hot. This increases the resistance of the positive temperature coefficient (PTC) thermistor that is connected to the junction between R353 and R252 and GND. When it is cool it has about 50 Ohms which pulls the fault output to GND. If the resistance increases, a DC voltage develops due to the pull-up to 55V and the latch is triggered.

In my particular case, the receiver came on, played for about 30 sec on the left channel and then it went silent. One could also hear three clicks in rapid succession right after starting it up, which is one too many. Normal are two clicks, which correspond to the start-up sequence during which the two relays that control the current into the toroidal transformer get switched in succession to limit the inrush current into the transformer. The third click meant that the fault mechanism turned the relays off again. In the beginning I was a bit confounded by the fact that the left output was continuing playing for half a minute. It turned out that this is possible due to the charge that is in the reservoir capacitors, i.e. it shut down when the voltage dropped close to zero in these capacitors. Why it did not do that on the right side, comes from the fact that the right supply also powers the preamplifier via an attached ±30V supply. This causes the charge on the reservoir caps on the right side to dissipate more quickly.

Since the output stages showed perfect current values during my test with external power supplies, I first thought that the fault switch itself was at fault. So I tested it by grounding the fault output from the output stages, and this caused the issue to disappear.

Note: This test needs to be performed from a cold start, since the latch in the fault switch only resets when the 15V rail turns off, i.e. the Beomaster needs to be turned off for that. However, the Beomaster runs a turn-off sequence after pressing the off bar on the keypad, where the 15V remain alive for another 60-90 sec. This means that one needs to wait a couple minutes before turning it on again to see if the grounded fault output disables the fault condition.

In my case this test yielded a functioning fault switch, and therefore the issue had to be in one of the outputs! It turned out that the left stage had a PTC that was intermittently disconnected from R253/R252, which caused it to loose its ability to pull the fault output to zero at normal temperatures. The picture shows the broken white-blue striped lead that connects the PTC (which is mounted on the heatsink) to the fault switch voltage divider. The tricky part here was that the lead looked perfectly o.k. since it was attached by the insulation to the solder terminal and the lead was only broken off in the inside.

After restoring this connection, the Beomaster started working normally.

Beomaster 8000: Rebuilding the Outputs, A New Interesting Output Failure Mode, and First Startup

I started working on the Beomaster 8000 that I recently acquired via eBay on behalf of an B&O enthusiast in the UK. The 8000 is definitely one of my most favorite restoration projects. An absolutely awesome design. The Beomaster 8000 was sold under the premise to be fully working but to have had some 'amplifier trouble', which supposedly had been 'professionally' repaired. Oh well, I heard such claims before, and especially on eBay the unsuspecting buyer needs to be aware that there are not many repair outfits left who can or want to tackle these units. Instead there is a lot of amateurish effort going on with the aim to make a fast buck and turn these sought-after units around for a ransom 'in restored condition'.

With that in mind I approached this Beomaster with caution and gave it my usual 'fix outputs first and then turn it on' treatment. So I opened it up and found a fairly clean situation. However, three screws are missing under the speaker switch cover:

And the cardboard shield that is supposed to keep the solder points on the display board from shorting against the toroidal transformer was installed incorrectly:

An interesting feature is the late-series microprocessor board that has soldered on EMI can lids and a sticker that says 'Sealed unit. To be returned to the distributor for service'...we'll see to that...;-)

After I lifted the display/uProcessor boards into service position and took the input socket panel tray out I looked at the output PCBs. And I made an interesting find: The quiescent current trimmers were replaced on both channels with 47 Ohm resistors. A very creative, but questionable fix to the corroding trimmer problem that commonly leads to frying the transistors in one or both outputs when the trimmers go open circuit. Here is a picture of the left channel board:

The resistor in question is in front of the third electrolytic cap from the left. Very crafty! I hooked the right channel board up with three external power supplies providing the ±55V rails and the 15V control bias:

The multimeter shows the voltage across the two emitter resistors R236/7 as 4.7mV. This is much lower than the prescribed 18mV. Well, at least the 47 Ohm resistor errs on the 'better side' by running the output Darlingtons IC203/204 at a too low operating point. Had the 'expert' who did this elected to use the next standard resistor value of 56 Ohms, most likely the unit would have run pretty hot. The problem with running at a too low operating point is that class B amplifier cross-over distortion is introduced in the output signal since the two half-waves are not 'connecting' smoothly at the cross over point. Since all TIP141/146 pairs are slightly different, trimmers are needed to find the perfect operating point for each unit independently. 

I installed as usual 25 turn encapsulated 100 Ohm precision trimmers

in both of the output boards and also exchanged all the electrolytic capacitors with 105C level quality Japanese units. This shows the left channel:

And here is the right:

Then I ran the right board with my external supplies and adjusted the trimmer to yield 18mV across the emitter resistors:

The power supplies showed the usual 0.11A (-55V), 0.1A (+55) and a close to zero current on the 15V control voltage pin

Very good! Then I did the same for the left channel, and I was surprised: No matter to what value I would set the trimmer, the multimeter would show zero voltage across the emitter resistors. Something was not working!

I poked around and measured the voltages across the TIP141 and TIP146 'columns' in the output. The voltages seemed reasonably close to the values shown in the service manual. Very strange! Then I measured the voltage at the base of the npn Darlington (IC203) while turning the potentiometer. Nothing happened. It stayed constant at about 1V. This was strange, since the potentiometer is supposed to adjust this voltage to about 1.2V to set the operation point of the Darlington. I did the same at its pnp match (IC204) and this base changed the voltage depending on the trimmer setting. A bit of head scratching lead me to the root cause of this phenomenon: The yellow lead between the emitter of IC203 and the PCB was open circuit. I unsoldered the lead and found that it was only hanging on by a shred of insulation, but that the wire had disconnected:

I took some insulation off and soldered the lead back in, and then everything was as it should be.

Time to plug this baby in and finally turn it on:

Allright! First light! This marks the start of another Beolover restoration!