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.

Beogram 8000: Finishing Up Some Odds and Ends

I am ready to put this Beogram 8000 back together and start playing records in my office but there are a few odds and ends to take care of first.

The muting relay is a component that has acted up on the last few Beogram 8000 projects. Because of that I now always replace it. The original muting relay on the Beogram 8000 is a National type relay.

Beolover makes an adapter board for several of the old relays Bang & Olufsen uses in their turntables. National is one of them.

This Beolover adapter board was originally created for the Beogram 400x turntables. I am able to use the board here and mount an Omron 12 VDC relay replacement for the National.  Due to the polarity of the Omron relay and the Beogram 8000 relay board layout I have to mount the board upside down.

Another quick task to do is to close up the microcomputer PCB box. I have left it open up to now in case there was some problem (i.e. the failed display segment I had to fix earlier).

I applied fresh thermal compound to the heatsink bar and reattached the lid to the shielding box.

From the end of the box where the microcomputer IC is you can peak inside and check that the heatsink and thermal compound made good contact with the device.

Last is to repair the tonearm compartment lid so the damping lever slowing closes. Like a lot of these turntables the original damping grease has been removed. It likely deteriorated over time. This unit has some sort of grease as a substitute but that really doesn't provide the correct damping.

I use Nyogel 767A. It is specifically designed for damping applications and it has worked great on these Bang & Olufsen components that have lids/doors that are dampened.

The damping on the Beogram 8000 tonearm compartment lid is performed by a plastic lever that is mounted on a pivot. The lid assembly presses on to this lever. The damping grease slows down the movement of the lever.

Here is the damping mechanism re-installed and ready for the lid assembly.

Now I can finally put all of the Beogram components back together.