Beomaster 8000 - Output Amplifier Bench Testing Review

I felt like the left and right channel output amplifier boards were in good shape after the restoration and I was successful in presetting the no-load current adjustment during the bench testing.

However, I was left disappointed that I wasn't able to preset the DC offset adjustment of the boards during my bench test. I tried doing the DC offset adjustment but when I connected a dummy 8Ω speaker load for the adjustment procedure the current draw on the ±55V supplies was not balanced. There was quite a bit more current on the -55V supply than on the +55V supply. To go along with that I could only adjust the DC offset to around -380mV across the speaker load.

Here is a picture of the bench power supply load when attempting the DC offset adjustment.

The current draw imbalance occurs as soon as the 8Ω speaker load is applied. Even when the output amplifier is in the off state (+15V supply at zero volts).  Turning the output amplifier on (+15V supply above +14V) raises the current draw on the ±55V supplies but there is still a 50mA difference.

Why was that?  The DC offset adjustment procedure calls for the speaker connection to be on and the volume level to be at zero.

I have the 8Ω speaker load connection and I have nothing applied to the audio input connector on the output amplifier board.

Here is the schematic marked up with my test connections.

You can see that I have the audio input pins shown as having the option of being open or shorted together. I first thought that shorting them together would solve the problem because that should be the same as zero volts per the service manual instructions.

That was not correct. My next thoughts were that this board must have some fault preventing the DC offset adjustment. When I tried three more output amplifier assemblies I got the exact same result on all three. No way that I have four faulty boards. Especially with the exact same fault.

Here is a picture of my fourth output amplifier assembly DC offset adjustment test.

This meant that my stand alone bench test is not correctly representing what the board sees when it is properly installed in a Beomaster.

At this point I turned to Martin Olsen for advice. I get a lot of parts from Martin and he is an experienced Bang & Olufsen tech who has provided me with some great information on repairs over the years.

After thinking about the problem of attempting this stand alone bench test procedure Martin suggested that to be accurate in my audio input setup the two input pins should be grounded.
That was exactly the problem!

From the schematic picture above you can see how I thought shorting the two audio input pins would ground the signal so I would have zero volts. That turns out to not be sufficient for this stand alone board bench test. Looking at the output amplifier trace side and the service manual schematic again I can see that the audio input low signal is shown to be going to signal ground. Not board ground. There is no real signal with the output amplifier sitting on the bench by itself. Signal ground is really just undefined in this test environment.

Following the board traces that P39-1 connects directly to I can verify all of the signal ground connections indicated on the schematic. I have marked them up on my schematic below as the green, dotted lines to clarify those connections.

Here is the clarified schematic and my new test setup.

So connecting the audio inputs (P39-1 and P39-2) together is pretty much the same as leaving them open on this stand alone bench test. That is what I observed in attempting to perform the DC offset with the 8Ω speaker load. With the board being stand alone on the bench it requires P39-1 and 2 connected to each other and to board ground so there is a true reference to the return in this environment. That sets the input at a defined, 0 VDC level and the amplifier stable for the DC offset adjustment.

Marking up the Beomaster relevant schematic parts that show the output amplifier board connected to the rest of the Beomaster I can see the required ground connection I missed. The audio signal ground between the Filter & Tone Controls board gets routed to chassis ground. Chassis ground connects to the output amplifier board ground post, P37, which connects to the main ground.

Connecting up output board P37 ground post to the P39 input pins, I immediately noticed the result on the bench power supplies.

The current draw on the ±55V supplies is now close to the same. This is what I see with good output amplifier boards in a Beomaster 8000 receiver. The DC offset adjustment is now able to dial in to the expected 0 ± 0.5mV range.

Here are some pictures of the output amplifier board and the test setup.

Here are a few summary notes regarding the stand alone bench testing of the Beomaster 8000 output amplifier board.

1. Start with the audio input connector pins shorted to the ground pin on the output amplifier board.

2. Have the speaker load disconnected

3. You need two DC power supplies capable of +55 VDC and have current limiting protection. I set mine to stop at 150mA. This will prevent a bad situation should the output amplifier have a short somewhere.

4. You need a third DC power supply capable of +15 VDC to enable the output amplifier to turn on.

5. Dial up the ±55 VDC supplies first. Increase their voltage slowly from 0V to 55V. Observe their current draw as the voltage is increased. When new parts (including trimmer resistors) have been installed on the output amplifier board it is very common for the supply current to go above 130mA way before 55 volts is reached. When this happens you must dial down the supply current by adjusting the no-load current trimmer on the output amplifier board. Use that trimmer to keep the supply current around 30mA until ±55 volts are reached by the supplies.

6. Turn the output amplifier on by slowly increasing the +15 VDC supply from 0V to +15V. Observe the current draw on the ±55V supplies while doing this. The ±55V current should not go above 120mA. The typical current values I have seen are 100mA to 110mA.

7. With the output amplifier powered up and on, adjust the no-load current to the desired 18mV value across the emitter resistors.

8. Power down the +15V supply to turn the output amplifier off.

9. Connect the speaker load

10. Reapply the +15V to turn the output amplifier back on and observe that the ±55V supply current is in the 100mA to 110mA range.

11. Measure and adjust the DC offset trimmer to get 0±0.5mV across the speaker load.

That should do it. The output amplifier assembly is nicely checked out prior to installation. If there is a failed component on the board it should reveal itself during these tests and should be easier to trouble-shoot with this test setup.

Beomaster 8000: Right Channel Output Board Re-checked

I spent the last few days checking the right channel output board components as well as setting up my test bench to do an isolated test of the board.

With the right channel output board assembly completely removed from the Beomaster chassis I de-soldered the connections from the board to the power transistors. This allowed me to measure them with a transistor tester.

I won't show all of the measurements but I ended up checking all of the transistors and diodes on the board. Every component measured good in their out of circuit state. That being the case, I went ahead while the board was out to recheck capacitors. Again, everything measured fine.

Satisfied that none of the components had a hard failure I decided it was time to reinstall them.

Here are the heatsink components ready to be reinstalled. I cleaned off all of the old compound.

After studying the assembly to determine the best way to re-install them I decided I must do it while everything is outside the chassis. It would be really difficult to try and mount the components on the mica insulators and try to get the metal clips in place when there is heatsink compound involved.

The order I decided to go with was to install the middle transistor (IC205) first.

That wasn't too bad. The center transistor is tightly installed and now anchors the wiring harness so the remaining transistors will be easier.
The next transistor I installed was the top transistor (IC204).

That was followed by the small TR208 (BD165).

When I got to the bottom TIP146 (IC206), I realized I would end up having to install it upside down from its original installation in order to easily slide the mounting clip in place. I didn't want to do that
so the bottom transistor (IC204) was a little more difficult to get in than it should have been. The easier way to install these would have been to begin with the bottom transistor (IC204) and work my way up to IC206.

The left side installation (TIP141 transistors) is similar to the right. Here is everything reinstalled and cleaned up.

Here is a better picture of the reworked heatsink mounted components.

I want to retest this board outside of the Beomaster before re-installing it. By using bench power supplies to power the output board I don't have to worry about the rest of the Beomaster and should be able to find out for sure if the problem is on this board.

I had to wait for the arrival of two new 60 VDC adjustable bench power supplies to do this test as the output board rail voltages are ±55 V.  My existing 30 VDC bench power supply can provide the +15 VDC needed. Fortunately the new supplies arrived this week.

Here is the test setup. I created special test wires with connector plugs to attach the power and grounds to the board.

The power and grounds are marked on the picture and those lines go to the bench power supplies. The two mini-grabber probes are from the DVM to check the no-load idle current.
Here are the bench supplies after slowly ramping up their ±55 VDC rail voltages. That was followed by ramping up the +15 VDC voltage.

When I got the ±55 V supplies ramped up to their levels and the +15 ramped up to its level, I adjusted the no-load idle current to an initial value of 18mV. That resulted in about 0.11 amp current on each 60V supply which is what the expected value is.  Leaving the output board powered up I left it on for a while and the no-load idle current measurement (TP200 to TP201) crept up to about 22mV and settled there.

This is good news as it appears this output amplifier assembly is good. I will reinstall it in the Beomaster and re-run the burn-in test to see if the earlier fault appears again. That still concerns me as I haven't found a problem related to that yet. In performing my re-check of the output board components I pretty much re-flowed solder on all of the board.

Here is the right channel output board re-install. It can fit in through the back without having to remove the heatsink harness from the board. That is a relief.

Wired back up.

The power test reveals there is still a problem but it has more clarity now.

The Beomaster powers up into the Standby mode as it should. When I press a source function (TP2 in this case) the display shows the selected source at the saved volume level.

A missing key event is happening now. There is no delayed, second relay click for the ±55 V rail supplies to kick in. So the Beomaster is showing On but the output amplifier is not powered.

A few days ago, before I removed and checked the right channel output board, I ran tests where I had removed the rail voltage connections from the Beomaster output boards. When I ran the same power on tests the ±55V rail voltages would usually come on (the second relay would engage). However, occasionally the relay would not engage.

Tomorrow I will have to trouble-shoot the start up circuit in the back and see if there is a problem with the power relay.