Beomaster 6000 (2253): Rebuilding Output Amplifier and Power Supply, and Adjustment of Quiescent Current and Amplifier Offset

After replacing the volume belt in the Beomaster 6000 (2253) that I am repairing right now, the basic functionality was re-established. However, I noticed the the output got hot quickly without playing the unit very loudly. This was a certain sign that the quiescent current (no load current) was too high and that adjustments needed to be made. Once the board is up, it is a great idea to replace the electrolytic capacitors and upgrade the output trimmers to modern 25x turn encapsulated units. The next step is to also replace the reservoir capacitors in the power supply. They see elevated temperatures and have a tendency to go bad at this point in time. So, lets do it:

Here is a picture of the output (left) and power supply (right) boards:

The picture looks a bit 'jagged' since I used the 'pano' function of my iPhone camera app to capture the entire unit in one picture from a close-up distance.

The output amplifier board can be easily accessed via loosening two screws from the bottom of the unit that hold the heatsink down. Once they are unscrewed the entire heat sink with board can be pulled up and put into service position (use the foam piece that sits on the reservoir caps of the power supply if it is still in there to prop the board up. After removing the various plugs the unit can be extracted:

Removal of the five screws that hold the board to the sink allows to flip the board over to access the solder points:

This makes it straight forward to replace the electrolytic capacitors and the two output trimmers:

Note that the output trimmers should not have more than 50 Ohms across the relevant pins when you put them in. If they exceed 60-70 Ohms or so the current will be pretty high in the output transistors and damage may occur when the unit is switched on. At 50 Ohm you will see about 3 mV across the emitter resistors.

Then it is time to put the assembly back in and adjust the quiescent current by increasing the trimmer resistance a bit. I usually put them in in an orientation that CW adjustment increases the quiescent current:

The voltage across the emitter resistors of the output transistors needs to be 22 mV. 

After doing this also for the right channel, I then adjusted the amplifier offset to make sure that there is no DC on the speaker outputs. This is done by hooking up the probes across the output connectors

and adjusting the offset trimmer to until 0 mV is measured. This is usually achieved with a close-to-center trimmer position:

After finishing up the output board, I turned my attention towards the power supply. It can also be lifted up via unscrewing two screws that hold the heat sink from underneath the unit, but there are two more screws that hold the PCB on top. This shows the power supply in its original condition:

The reservoir capacitors for the smaller voltages are on the board. They are of the 'multiple negative contact pads type' that was used in many B&O types of this vintage:

While these caps can be exchanged with standard model types (the multi connector types do not seem to be made anymore) but care needs to be taken since some designs utilize the multi connectors to elegantly bridge traces on the PCB, thereby saving on the use of jumpers. This is the case with C1 (3300uF) on the power supply board. Usually the negative lead of the capacitor can be used to make the bridge. This is shown here (vertical wire bridge on the left side of the positive C1 solder point):

On the right you see the solder points of the C10 (2200uF) replacement. There no bridge is necessary since two of the contacts ended up in dummy pads that are only there to give the can more stability.

On to the main reservoir caps of the ±50V rails. This shows them in their original condition:

The right one shows signs of corrosion (dark bulge about 10 O'clock). I replaced them with modern 105C grade Panasonic units that are 5 mm 'thinner' than the original 40mm cans. this made it necessary to roll them in some cardboard to maintain mechanical stability in their compartment:

This shows the completed power supply:

and a detail shot of the two on-board reservoir caps:

all good now in the heat sink compartment! This Beomaster should be good for another number of years. Usually there are no issues with the capacitors on the other boards since they run much cooler than the boards in the heat sink area. The LED displays of this unit are miraculously still working on all segments (another major trouble spot, but luckily this is only a cosmetic issue and not essential for the operation of the unit)...so let's hope that they will do that for some longer!