Beomaster 6000 (Type 2253): Runs Too Hot, Output Stages Rebuild, Volume Belt Replacement, and Repair of an Intermittent Power Supply

A few months ago I met a couple while riding my bike around in my neighborhood in Albuquerque. It turned out that they lived close by. And that they have close ties to Denmark. The plot thickens!...;-). Long story short, I was invited to look at a Beomaster 6000 (Type 2253) that they bought in the 1980s from a local store. It was running hot and the volume could not be adjusted anymore.

Recently they went on a trip for a month and it was a good moment to give it a bit of TLC. So I picked the unit up and put it on the bench:

It is in pretty good cosmetic condition. Only very minor damage to the veneered side panels:

I put the unit into service position (which was easy since the plastic enclosure screw fittings had broken out - see below for my repair of this issue) and had a look at the volume servo:

As expected the rubber belt was gone. Luckily, this is an easy repair if one knows that the pulley on the string mechanism can be pulled off for installation of a new belt:

So with a bit of care the new belt can be installed without needing to mess with the string that moves the volume indicator foil. I put a Beolover Volume Servo Belt for Beomaster 6000 on the shaft:

and after pulling it onto the motor pulley:

I turned the unit on to test the volume drive. I noticed that one of the two light bulbs that illuminate the volume scale was dead:

I replaced it with a red LED and an appropriate resistor:

Once I had figured out what resistor to use for a good intensity I also replaced the still working bulb:

This is how it looks like with the scale bezel installed:

Beolovely! On to the other major issue, the hot heat sink. I removed the metal shroud from the heat sinks, which revealed the output amplifier board. It is bolted to the heat sink, which cools its output transistors:

Removal of two screws from below releases the entire assembly and it can be pulled up:

I unplugged it and put it on the bench:

After removal of the screws that hold the board to the heat sink, the board can be flipped up so one can access the solder side:

I usually replace all the electrolytic capacitors and the quiescent current trimmers. This Beomaster was one of the initially produced units, which have two non-polar electrolytic 47uF capacitors installed. They are evident by the absence of polarity markings:

In later production runs of this design they were replaced with back to back standard polar units and a slightly modified board to accommodate them. Of course I did not have any non-polar 47uF electrolytic capacitors since they are a special item. But I have a wide selection of surface mount components for my development work. So I made two small adapters for standard SMD multilayer ceramic capacitors in a 1210 package using small Dremeled pieces from a development board and male board headers:

This shows one of them installed:

Next came the quiescent current trimmers. They are a bit of an Achilles heel of this amplifier design since they can lead to overheating output transistors when they oxidize as they age. Oxidization can increase the resistance across them and higher resistance means more quiescent current through the transistors, even if no music is playing. In the extreme, these trimmers can go open circuit and then smoke will come from the heat sinks, followed by a shutdown of the receiver due to blown fuses. I removed the two trimmers for the left and right channels and measured them. Unsurprisingly, one of them yielded a fairly high resistance of around 80 Ohms:

Normal is maybe 60 when they are adjusted properly. I replaced them with modern encapsulated 25-turn trimmers, which allow a much more precise and long-term stable adjustment of the current. Before the installation I made sure they were set to a resistance lower than 50 Ohms (usually, trimmers come set to center from the factory, i.e. in this case of 100 Ohm trimmers they should be set to around 50 Ohms, but it is a good idea to check):

Then I soldered them in. This shows the restored board together with the evicted original parts:

I plugged everything back togehter and turned the unit on. Then I adjusted the trimmers to get 22 Ohm (cold receiver) between the emitter resistors of the output:

this concluded my work on the output stages. In this vintage B&O it is a good idea to also replace the electrolytic caps in the power supply. They also get pretty warm. They are often out of spec due to leakage or drying out. The power supply is to the right of the output stages using the remaining heat sink real estate in the 'chimney' in the back of the enclosure. Removal of another screw from below the chimney liberates this board with its attached heat sink. This shows it in original condition. Two of the reservoir capacitors are mounted directly on the board. They are the two big cans in the center:

I replaced them with a couple new replacements:

The next step was replacing the two main reservoir cans (that stabilize the output stages) sitting in their own separate compartment next to the power supply board:

I installed two Beolover Reservoir Capacitors for Beomaster 6000 and 8000, which use 3D printed adapters to give modern capacitors the same form factor like the original ones: 

Assembled:

This makes transferring the wiring a snap. I removed the original capacitors with their ground connection still intact:

Then I transferred the ground wire to the new assemblies keeping the orientation of the terminals about the same:

Then I put the connected cans into their compartment and soldered the power rail wiring back on:

This concluded my work on the power supply (or at least so I thought at this point!...;-) and I focused on repairing the broken out enclosure screws. This shows the 'as-received' situation on the right side

and on the left side of the enclosure:

(I put the screws and nuts where they need to be when in their unlocked position).

A typical issue of this generation of B&O designs from the early 1980s when plastic was king (remember the 1981 Plymouth Reliant? What breathtaking progress in the automotive sector!!...;-). Luckily, 3D printers were invented since, making repairs of such design sins a bit easier. I designed parts that fit on the remaining stumps. This shows the right side component including the original bolt and square nut:

This one needed to be epoxied to the remains of the original fixture for lack of space. I used two toothpicks against the circuit board to hold the part in place while the glue cured:

This shows the corresponding part for the left side:

On the left there is some space below the fixture that can be used for putting in a bolt. I drilled a hole through the bolt orifice in the part for precise fit:

Note that this photo shows an earlier version of the part, which had to be modified later since the left side has much less space available due to a metal bracket that is bolted to the mating area on the bottom part of the enclosure. This shows the final part in place. I also used epoxy to fuse the part as good as possible to the stump, while the bolt gives it additional structural stability:

A shot from a different angle:

At this point I thought I was done and I set the Beomaster up with a Beogram 4002 that I just restored to listen to a record.

Sadly, I was not able to turn the Beomaster on anymore. Something had happened between my bench and the sideboard where the Beogram was! Frustrating! The symptoms were these: I plugged the unit in and the lights under the volume indicator came on, and I also heard the click from the relays. But otherwise it was completely dead: Nothing on the displays, no power dot, no response to the keypad. Nice! Exactly what one wants to happen when fixing something!...;-).

I opened it up and had a look around. My earlier experience with units from this B&O vintage suggested a cracked circuit board, a cold solder joint or a broken board wire (they often disconnect right at the soldering tab within the insulation, and therefore one cannot see it when it happens). I started 'wiggling things' and after a while the power dot in the display came back on and I was able to turn the unit on again by selecting an input source button on the keypad.

Some more wiggling things and I was pretty sure that the power supply board (16) was the culprit. Not surprising, since its voltage regulating transistors are bolted to the heatsink, while also soldered to the PCB. This is generally a solid recipe for getting faults like this since the solder points can easily get damaged due to thermal expansion strain or just vibrations of the heat sink against the board when moving the unit, like I did.

Indeed it turned out that the fault could be reliably provoked by bending the board a bit against the heatsink. So my first response was to re-solder all the sink mounted transistors. But this did not fix it. So I thought it is probably a cracked trace close to the transistor solder points. I soldered short magnet wire (which has a heat removable polyurethane coating) pieces to the transistor solder points and made direct connections to the next circuit node, thereby circumventing the traces in close proximity to the points. Sadly, this also did not affect a change. At this point I started bending the board a bit more 'selectively', i.e. just on one end and then the other trying to get some insight into which area of the board might be affected. It seemed to me after a while that the left side of the board was a bit more sensitive. So I soldered some more wire bits between connected nodes and this finally cured the problem! I was now able to bend the board in any direction I wanted and the unit stayed on! This shows the final result of my efforts:

It was finally time to enjoy a record through this Beomaster! While I listened to it, I noted that I was not able to lower the volume below maybe 10% on the volume indicator. Even the '0' button of the volume presets did not mute the sound. In theory one should be able to lower the volume to zero with the volume up and down buttons. So I opened it up again and had a look. The problem became clear quickly: the volume indicator foil had an issue to go all the way to the quiet end, and due to this blockage, the motor was not able to drive the volume potentiometer all the way to its end point. 

This shows the volume indicator foil as it appeared to me at this point:

If you look closely at the right end of the foil mechanism, where the white string connects, it is apparent that the white 'plate' that connects to the string hangs down a bit before the plastic bracket that keeps the ribbon on track. And that prevented the foil from being pulled all the way to the end. I figured the white piece could use something like a ramp to guide it up a bit, so it could move all the way up to the plastic bracket. I 3D printed a few shapes that did not obstruct access to the solder points underneath the ribbon but were able to act as a ramp. This is the shape I ended up using:

It seems to work well and now the white pice makes it to the end of its travel every time zero volume is dialed in:

Here a photo from a different perspective:

Ok! Back to some more testing of this Beomaster 6000! I will play it for a while to make sure there are no more intermittent issues!