Beomaster 6000 (2702) restoration: restoring the back metal frame and wood plinth

The black aluminium anodised frame needed some TLC. A good cleaning of the metal and a new foam sealing did the job.

I used the foam that is typically used for speaker cabinet building. It's self adhesive and comes in different dimensions.

The wood plinth was already in a great shape with nice corners and no big damage. I light sanding was done followed by a good rubbing with proper rosewood oil.

After sanding and oiling..

The black felt that is glued on both top sides of the frame was also removed and replaced with new ones glued with contact glue.

New felt strips cut to the correct size

Next in line is the key panel.

Beomaster 8000 - 55V Reservoir Capacitors & Chassis Cleanup

I was expecting to quickly replace the four ±55V reservoir capacitors and reinstall the two output amplifier boards that I restored in the last post. However, once I removed the old reservoir capacitors and started to do some cleaning I found more rust spots like I had found during the power supply restoration. I decided a simple cleaning was not going to be good enough. I really needed to remove the dirty and rusty bottom of the cabinet to clean it up.

Here are some pictures of the old reservoir capacitors and their removal.

There was quite a bit of dust and grime as I had already seen in the cabinet. That is when I spotted more rust areas.

Here is a picture of a Beomaster 8000 I don't often take. The top and bottom cabinet pieces removed.

I gave the two removed, bottom chassis pieces a bath and repainting.

There are still a few small areas on the rear cabinet pieces that will require touching up using a small paint brush but the Beomaster feels very clean now.

Time to install the new reservoir capacitors. Modern capacitors of the same capacitance and voltage are smaller than the originals so I always use Beolover's replacement reservoir assemblies. These are new reservoir capacitors fitted into a 3D holder that matches the original reservoir capacitor size. This makes for a perfect replacement.

Removing the old capacitors and wiring the new ones up is kind of a pain with the way these components were connected up. I am trying something new on the new connections for this Beomaster where I installed some terminal lugs on the wires that solder to the reservoir capacitors. I am happier with the result.

It looks like I am ready to reinstall the left and right channel output amplifiers finally.

Beogram 4002 and 4004 DC Motor Restoration

I recently received two DC motors from a Beogram 4002 and a Beogram 4004, both located in Denmark. As usual, I started with the oil infusion process since it solves RPM stability issues in most cases. This shows the 4002 motor as received:

I took it apart:

The brass bearings are the two small donuts on the black pad up front. I immersed them in motor oil and pulled a vacuum:

Immediately, bubbles emerged from both bearings. This indicates that the oil infusion process has started. After about 48 hours the bubbling stopped and I extracted the bearings:

Then I reassembled the motor for testing. This shows the tool I use for pressing in the top bearing:

After assembling the motor I installed it into one of my Beogram 4002s and ran it for 24 hrs recording the RPM with my BeoloverRPM device:

I did the same process for the 4004 motor. This shows the RPM curves that I measured:

The red curve on top shows the 24 hr RPM performance of the 4002 motor. It performed fairly well with a 0.47uF capacitor in place for C10 in the motor control circuit. The dark blue curve shows the same measurement for the 4004 motor. some upward spikes are evident. I ran this motor a second time with a 10uF value in place for C10. This quieted the situation down and a near perfect curve was measured. So I recommend to replace the 0.47uF with a 10uF value if the 4004 did not come with a 10uF value factory installed (some do).

Time to send this motors back to their Beograms. They are ready to serve for another 35 years...;-).

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 6000 (2702) restoration: mounting the FM dial display and fixing a new dial cord

The FM dial display has been restored before (click here). One thing I forgot to mention is the importance to have the dial band under the tiny clips that guide the band in a straight line and avoid it rubbing against the red plexi panel that comes on top. These clips are so tiny that it's easy to not spot them.

On most units that I have restored the dial cord is broken (or about to be). The cords used on a Beomaster 6000 quad are very thin (about 0,4 -0,5 mm diameter)  compared to cords on many other vintage radio's. That's probably why it is extremely difficult to find vintage cord with that size. The best alternative that I have found is fishing line. I know that some hardcore vintage restorers will hate this, but it works, if you select the correct type ! The typical yellow/transparent nylon ones will NOT work. They are to stiff and slippery. I have good experience with high quality DAIWA 8EVO Tournament Braid type of 0,45mm.

There's a drawing in the service manual that describes exactly how to fit the dial cord. Important is the 3 turns on the main pulley, the 2 turns counterclockwise on the shaft coming in from the top and leaving at the bottom.

But before you break your head on this drawing and try to figure out the correct sequence of fitting the cord, watch this excellent videoclip by Beolover that shows you exactly how to do it in this post: click here.

Below is the DAIWA fishing line cord installed on this unit.

The dial cord is attached to the FM dial band with a plastic clip with 3 dents. Best is to remove some tension from the cord before attempting to fix it on the clip. A small dot of glue will help it stay in position once you have the correct setting. Tuning into a known FM frequency and then set the red line to the corresponding numbers on the red plexi display is the way to do it. It often requires a bit of trial & error to get it correct.

The next posts will mainly be on the restoration of the key panel, the red plexi display and the wood plinths. We want this Beomaster 6000 quad to look as good again as it sounds ! And that will require some different skill sets other than electronics.....