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Sunday, August 26, 2018

Canada Beomaster 8000: Rebuilding the output amplifier boards

The output amplifier assemblies on the Beomaster 8000 are two identical boards (same Bang & Olufsen part number). One for the left channel and one for the right channel. Each board seems small enough where the restoration task should be quick and easy.  At least it always starts out that way in my mind.

Actually doing the restoration work takes a lot more time than that one would think.  Most of the time is spent cleaning off the old, white thermal paste  where the transistors are mounted to the big heat sinks.  In more recent restoration projects I have started to use Sil-Pads for the thermal insulation of the output amplifier transistors. It works well and eliminates the messy white paste.

During this step I replace the electrolytic capacitors on the output amplifier board as well as the no-load current trimmer resistor and DC offset trimmer resistor.  I also check the output transistors while I have them disconnected during the thermal insulator replacement.

Here are the left and right channel output amplifier boards for this Beomaster 8000 project.

It is a good idea to work on them one at a time so you always have a complete output amplifier board available as a reference.

There are only six capacitors and two trimmer resistors (both 100Ω max.).  Here is the output amplifier board prepared for its new parts. The two axial electrolytic capacitors measured half the capacitance value they were supposed to be.

Here is a test of one of the PNP Darlington transistors.

All of the transistors on this Beomaster's heat sinks measured good. Note that the hFE value of the Darlington looks low.  While this transistor tester is very good it cannot accurately determine the hFE of a Darlington transistor.

Now for the removal of the old mica insulators, the white thermal paste and the transistor components

The first set of output amplifier transistors are installed with new thermal insulation.

The PTC resistor on the second set of transistor components still gets a dab of white thermal paste for its insulation.

The newly mounted output transistors and heatsink are now ready to connect back to the output amplifier board.

Here are the completed left and right channel output amplifier assemblies ready for bench testing.

On to the bench test of these reworked output amplifier assemblies.

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