Beomaster 8000: Opening Up The Beomaster 8000 To Service Position - Part 1

When working on a Beomaster 8000 for repair or diagnostics we open it into its service position. Occasionally we have to provide instructions to an owner to do this. Bang & Olufsen never intended this to be something an owner would do but in cases where it is too far and expensive to ship us the Beomaster we have to get creative in the trouble-shooting.

This blog post demonstrates step-by-step the opening up of a Beomaster 8000. The photos are from various units we have worked on (including my own Beomaster).

First a caution...
Always have the Beomaster unplugged from the AC power.  It is very easy for a metal screw to fall into the unit and short something out or a screwdriver to slip and short out a component. 

Starting with the Beomaster 8000 service manual instructions on opening the unit up...

Here is what it looks like on the real thing.

There are three flat springs under the release rail that apply pressure to keep it in place and provide the movement necessary to trip the catch on the Filter & Tone Control panel.

The springs just set in place so they easily fall out of the Beomaster when the release rail is removed. So far most Beomaster 8000 units I have encountered are missing at least one of the springs. 

Tip...
Place the small loose parts that you remove in a plastic container (small box or bag). You may think they will be safe sitting somewhere on your work area but somehow parts get lost.

Once the release rail is removed there are four screws that are now exposed and need to be removed so the bottom Beomaster panel can be opened.

There are three catches extending from the bottom panel that were holding the front of the panel to the Beomaster cabinet frame. Sliding the panel forward releases those from the cabinet frame and allows the panel to be tilted upright into the service position. The same three catches fit into slots on the top of the cabinet frame front edge to hold the panel in the service position. A nice design that had a technician in mind.

Next is the removal of the glass panel that covers the display board.

A plastic trim panel over the speaker switches is fastened by two screws. The trim panel must be removed to slide out the glass panel over the displays.

I find that it also a good idea to loosen (not remove) three more screws for the cooling tower in order for the metal tabs of the glass display panel to slide out.

With the glass display panel safely stored away there are two more screws to remove so the display board and microcomputer board assemblies can be lifted into service position.

The display board and microcomputer boards are stacked together to form one assembly. The microcomputer board is the one on the bottom and is attached to a metal hinge that has a slot. The two board assembly can carefully be lifted up, pivoting on the hinge until the hinge catches in the slot. That will leave the microcomputer board and display board upright in the service position.

The right channel output amplifier board and power supply board are now visible.

That leaves the Filter & Tone Control panel to lift into service position so the left channel output amplifier, preamplifier, FM and FM Interface boards are accessible.

Those are the first steps in putting the Beomaster 8000 into a service position that allows the Beomaster to still function fully while a number of measurements and adjustments can be made.

In Part 2 of opening up the Beomaster components will be removed where the Beomaster will not remain intact and the receiver will no longer function.

Beomaster 8000: Starting with the output amplifier boards and power suppy

I like bench testing the Beomaster 8000 output amplifier boards by themselves on the test workbench before using them back in the Beomaster. It is easiest for me to do that by removing the output amplifier assemblies to both work on and test. After doing that a few times I realized that it is also easier to work on the ±55 VDC reservoir capacitor replacement with the output amplifier assemblies out of the way. Sometimes it takes a while for the obvious to sink in :-).

So on this Beomaster 8000 project I am starting with removing the left and right channel amplifier boards plus the Beomaster power supply components.

Turning the Beomaster around I removed the cover for the output amplifier heat sinks.

Looking at the Beomaster insides again I noticed a couple of problems I didn't catch the first time I looked.

The small transformer has quite a bit of corrosion on its mounting hardware. That would suggest this unit spent some storage time in a damp location. I can also see some deteriorating damping grease leaking around the lid damping assembly (a very common problem with these units).

I will make a note to clean up the corroded mounting hardware and I will disassemble the lid damping assembly to clean it out and install new damping grease. 

Removing the output amplifier assemblies opens up the way to the large ±55 VDC reservoir capacitors.

On the output amplifier boards themselves I can see they were worked on before. In addition, at least one of these boards is not an original board for this Beomaster 8000. Maybe both came from another Beomaster. That doesn't really matter too much but it does make you a little curious about what this receiver has gone through.

In the above pictures of the two output amplifier assemblies you can see that the flat metal springs that hold the TIP 141 and TIP 146 Darlington transistors to the heat sink are different. There are other subtle differences you can observe between the boards. Remember that these two boards are the same B&O part. They are interchangeable between the left and right channels.

Here is a closer look at the boards.

The right channel has some different transistor packages for 5TR201 through 5TR203 (BC546B) and 5TR204 (BC556B). Some resistors have also been replaced on the right channel board. Both boards show some deterioration on their emitter resistors (5R236 & 5R237).

Both boards show some heat darkened places on the boards. The right channel board has a really bad spot. That board also has the replaced resistors.

The trace side of the boards show both boards have had repair work performed on them.

I have a number of Beomaster 8000 spare output amplifiers to compare to. Most of them are still in their original state so lets compare these two boards with some originals.

All of the boards have darkened boards underneath the resistors. A lot of heat over time. It seems a lot of Beomaster 8000 units had a rather hard life. That wasn't the case for the previous Beomaster 8000 project though. The output amplifier boards on the unit were very clean and untraumatized. 

Perhaps it is in the working environment these various Beomaster components lived in. I know that the previous project Beomaster unit was from an excellent environment and when the receiver was in use it was part of a complete Beosystem 8000. 

On this current Beomaster 8000 unit I noticed that the TIP 141 & 146 Darlington transistors that mount to the heat sinks had very little thermal grease actually on the transistor case. 

Whatever the cause is I will get these current amplifier boards fixed up with capacitors rated to at least 105°C and reworked transistor thermal protection.

Beomaster 8000: Project Start - Initial Check

The Beomaster 8000 is now on the workbench and work begins with an initial check of what works and what doesn't. This Beomaster is in very good shape and when I connected up a couple of music sources (iPod Nano into TP2 and the BM8000 P1 FM station) I was a surprised the Beomaster played beautifully. I started to wonder if any work should be done on this receiver.

The happiness was short lived as the Beomaster stopped producing sound after ten minutes. Here is the Beomaster playing the local classical music FM station.

The first time the Beomaster stopped producing sound the display still showed 2.9 on the volume level, P1 as the tuner selection and 101.1 as the station. I put the Beomaster back into Standby mode then reselected P1. The music started playing again but after a few minutes, no sound. The second time the Beomaster display changed to tuner P and channel 87.5.

The Beomaster is definitely malfunctioning. It is time to open it up into service position and assess the work. Like the other Beosystem 8000 (and 6000) components from this early eighties period, Bang & Olufsen provided nice service access to work on these units.

Here is the Beomaster with the rear cover off, the front panel in service position and the display cover removed.

Here is the rear panel of the Beomaster.

When I unlatched the Beomaster locking bar for accessing the service position I noticed that one of the leaf springs that keeps the locking bar in place is missing. I will have to see if I have a spare in a parts unit.

This picture shows a better look at the Beomaster 8000 display board. The restoration plan is to convert the original LEDs with the SMD LEDs like those that were used in the Beocord 9000 display repair. So quite a bit of soldering is ahead.

Diving further into the insides of the Beomaster I discovered another missing piece. Like the Beocord 9000 unit, this Beomaster has a missing lid dampener. I am guessing this one was also leaking damping fluid. I also noticed that the Beomaster is missing the far left leaf spring for the locking bar. I will look for a spare dampener as well as the two missing leaf springs.

The previous picture was the left side of the Beomaster. Here is the right side (as viewed from the end looking in).

Most of the electrolytic capacitors look like they are original so that puts them in the mid-thirty year old range. The big black axial on the Power Supply PCB looks like it might be a replacement. This unit has been serviced in its past apparently.
Here is a better look at the left and right channel output amplifier boards.

It is time to take inventory of the capacitors and trimmers I need to replace in this Beomaster.