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Sunday, August 12, 2018

Canada Beomaster 8000: Removing the output amplifier boards

The Beomaster 8000 has two identical output amplifier assemblies (a circuit board plus a heatsink with transistors). When I replace the electrolytic capacitors and the trimmer resistors on these boards I like to remove the output amplifier assemblies from the Beomaster completely. It is possible to recap the board by leaving it in the Beomaster and tilting the board up but I feel like I do a better job when I can deal with the board by itself. Plus I can test the output amplifier on my workbench outside of the Beomaster.

Removing the output amplifier boards doesn't sound too difficult. The Beomaster 8000 service manual says that you just removed the heatsink mounting screws and the little speaker panel. After that you rotate the circuit board ninety degrees and pull it out through the rectangular opening in the back of the Beomaster cabinet frame.

So let's try that.

Here is the back of the Beomaster 8000.

The rear cover plate has eight screws connecting it to the two amplifier heat sinks. Removing those eight screws allow the metal cabinet cover to be lifted off and expose the heatsink assemblies.

The left and right sides of the cabinet each have a screw holding the speaker terminal plate to the cabinet frame.

The back of the cabinet center plate then has four remaining screws that are attached to the two heatsink assemblies. Both heatsink assemblies have two screws that connect them to their cabinet mounting brackets. All of those screws must be removed. Here is the left channel heatsink with the screws removed.

The heatsink assembly can now be pulled away from its mounting brackets exposing the rectangular opening in the cabinet frame that the service manual refers to.  This left channel is particularly tight due to the small ledge inside the cabinet for the lid damping assembly.

You can see in the above photo that there is a nice size cable bundle under the lid damper shelf that makes the board removal tricky. The photo also shows six connectors at the rear of the board that must be removed.  Those are the ±55 VDC and ground inputs to the board along with the output amplifier outputs.

Here are those connectors disconnected.

It is starting to look like there is more room now to possibly get the board out through the back of the Beomaster. The next step is to rotate the board ninety degrees. There should be just enough slack in the cables from the board to the heatsink mounted components to perform the maneuver.

The left channel output amplifier is oriented properly for exit from cabinet now. There is still an obstruction from the large cable that goes underneath the damper lid ledge.

To provide some slack in the obstructing cable the left channel reservoir capacitors need to be lifted.
Doing that allows that cable to be moved towards the front of the damper lid ledge and providing just enough space for the output amplifier board to finally slide out.

There...it is out.

Not quite as easy as it sounds. In fact this is only the second time I have done it this way. In previous Beomaster 8000 project posts I remove the retaining clips for the heatsink mounted components and pull them through the opening to remove the output amplifier board from the inside of the Beomaster cabinet.

That technique works well for removing the boards especially since the heatsink mounted components are going to be checked anyway and new thermal protection installed.

Re-installing the boards after the restoration is a different story. Once the boards are recapped and tested out on the bench I will want to get them back into the Beomaster cabinet. It would be nice to be able to install them with the heatsink mounted components installed. That is why learning tricks to using the service manual method of removal is a valuable technique to master.

The right channel output amplifier is a little bit easier to manage but it is still pretty tight and there isn't much operating room. However, there isn't an obstructive ledge and it is easier to move the other cables out of the way.

Definitely more room here.

The right channel output board easily rotated ninety degrees and out it came.

Here are the Beomaster 8000 output amplifier boards ready for restoration and then bench testing.

While these boards are out I will start the recap work on the Beomaster power supply board and reservoir capacitors.

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