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Sunday, April 8, 2018

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.

1 comment:

Comments and suggestions are welcome!