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Thursday, March 22, 2018

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.

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