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Thursday, April 5, 2018

Beomaster 8000: Output Amplifier Restoration - Right Channel

I observed in a previous post that one or both of the output amplifier boards in this Beomaster 8000 are not an original board to this unit. That is okay as long as both boards work but it raises interest in this Beomaster's history.

Here are the right and left channel output amplifier boards again.

Most of these boards use BC546 and BC556 transistors for 5TR201 through 5TR204. The right channel board here has BCY65 and BCY77 type transistors instead. I am guessing they were replacements for original BC546 and BC556 transistors that failed (or were suspected of failing). The right channel board also has a really bad burn spot on the PCB underneath 5R244, 5R247 and 5R246. Those resistors have been replaced. The electrolytic capacitors on both boards are all replacements (from the originals) and are all the same brand so these boards were together for a previous recap.

Here is a closer look at the right channel output amplifier PCB condition.

The damage on this board by the original resistor is significant. I decided to fill the hole with epoxy to help protect the integrity of the board.

I installed new carbon film resistors for 5R244, 5R247, 5R246 and 5R245. I raised the resistors off the board a bit when I soldered them in.

The burn damage on the PCB had also created some damage to the trace side of the board.

The previous fix looked like it probably is okay but due to the observable damage I decided to check all of the components on the board as well as reworking the traces on the board.

I also recapped the board with 105°C rated capacitors.

I cleaned off the old mica transistor insulators from the big heat sinks and installed sil-pad strips for the Darlington transistors.

Here is the reworked right channel output board ready to be bench tested.

As I have done on other Beomaster 8000 output amplifier bench tests, I apply ±55V rail voltages, ground and a +15V control voltage that turns the output amplifier on. This will allow me to check that the output amplifier components are working and adjust the no-load (idle) current of the amplifier board.

On my two bench power supplies that provide the ±55V I setup the supply current limiting to 150mA. This will protect the board and supplies if something is wrong.

The ±55V power is applied first with the +15V control power supply at 0V.

Once the ±55V voltages are set I apply 15V with the third power supply to turn the output amplifier on. From other Beomaster 8000 output amplifier bench tests I expect the "on" state to result in 100mA to 110mA of current on the ±55V supplies.

Now I can check and adjust the no-load (idle) current setting for the output amplifier board. The service manual calls for 18mV as soon as the output amplifier is enabled.

The initial idle current setting has been made for the right channel. I will do the same thing for the left channel output amplifier board.

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