A Pair of Beomaster 1900 Receivers for Restoration: Finishing up Unit #1

The first of the two Beomaster 1900 receivers is now complete.  

I wrapped up the testing tonight by checking the Total Harmonic Distortion (THD) at 1KHz with 20 Watts of power out of the Beomaster 1900 into my dummy 8Ω speaker load as well as the Frequency Response from 20Hz to 20KHz at an output level of 20 Watts.

Here is a photo again of the dummy speaker load test setup.

The Total Harmonic Distortion (THD) with a 1Khz sine wave into the Beomaster 1900 Tape source input and maximum rated power (20 Watts) across the dummy speaker load measured 0.068% which is well below the service manual specified maximum THD level of 0.2%.

Next I performed a frequency response measurement from 20Hz to 20KHz frequencies with and input level of 260mV into the Tape input and the power level at 20 Watts again.

The result was a ±1 dB swing from the 20Hz to 20KHz frequency range. That is pretty much the same as I measured for the Beomaster 2400 receiver I restored earlier.

 

Those are very satisfactory measurements for this Bang & Olufsen series amplifier.

This Beomaster will now go into a listening room while I get the second Beomaster 1900 unit restored.

Here are a couple of final photos from the workbench.
This Beomaster 1900 receiver shouldn't be seeing the workbench again for fifty years.

A Pair of Beomaster 1900 Receivers for Restoration: Unit #1 Reassembly for Testing

 After a bit of weather related delay it is time to get back on track with this Beomaster 1900 restoration.

The Beomaster has been playing music quite nicely in the workshop for over a week. It is safe to reassemble the cabinet parts and prepare the Beomaster for some performance testing.

Before I can put the touch button control and the tone control/FM tuning panels back in place the three display masks for Bass, Treble and Balance have to be reconnected.

The touch button control panel is a little tricky to reinstall.
Each touch button post on the main PCB has a corresponding copper activator with a small notch that fits against the touch button post.  Sometimes getting those notches lined up with the posts is difficult.
This one was quite easy and the notches fit right on the posts without me having to reach a tool inside the Beomaster to help the alignment.

This photo shows two of the copper activators.

For the reinstallation of the tone control and FM tuning panel the slider control buttons must line up with the sliders as the panel is installed. I slide all three slider controls to the bottom before attaching the panel.

A little bit of Nyogel damping grease is applied to the tone control and FM tuning panel hinges before installing the panel.

You can see in the above photo that the Beomaster 1900 wood trim is looking fresh and good.
I did some very light sanding followed by cleaning and application of some boiled linseed oil.

Tomorrow I will be able to connect the Beomaster 1900 to some dummy speaker loads and make some audio measurements.

Beogram 4002 (5513): Restoration of DC Platter Motor and Main PCB

I recently received another main PCB and DC platter motor of a Beogram 4002. As usual I started with the motor restoration since it can take a few days until the bearings are fully infused with oil. This shows the motor:

I took it apart to get to the bearings (the two small donuts on the black pad up front):

This motor was quite oily on the inside, an indication of 'previous human interaction'. This was confirmed by sorry state of one of the commutator brushes, which was bent out of shape:

I immersed the bearings in motor oil and pulled a vacuum. Immediately vigorous bubbling started:

This is an indication that the vacuum draws the air from the bearing pores, making room for the inter diffusion of oil.

While the bearings were infusing, I focused on the main PCB. This PCB also had already seen some work, as was obvious from the two trimmers that were soldered to the copper side of the board (upper right corner):

This shows the component side in original condition:

Here a detail shot of the sensor arm amplifier (TR3):

I replaced all the electrolytic capacitors and the RPM relay and RPM trimmers with modern encapsulated parts, and I installed a new transistor for TR3 since these have a tendency to go out of spec, and then the record detection mechanism stops working or becomes unreliable:

This shows the rebuilt RPM section in detail:

After the board was completed, I installed it in one of my Beogram 4002s and I adjusted the bias of the new TR3 to the specified 4V at the collector:

Then I moved the adjusted bias trimmer to the component side:

Now it was time for a measurement of the sensor response. This shows the trace that I measured with the empty platter spinning beneath the sensor:

Each dip corresponds to a passing by black 'rib' on the platter. The amplitude is larger than 5V, which is a good signal.

After about 72 hrs the bubbling of the bearings had stopped and I removed them from the oil:

Then I straightened the brush as good as I could 

and cleaned the motor components in an ultrasonic bath to get rid of the excess oil from the previous repair attempt. These motors run very unstable when the commutator or the brushes are oily. Then I put the motor back together for a RPM stability test with my BeoloverRPM device, which allows logging the RPM in 10s intervals over extended periods of time:

This is the curve I measured:

This is as good as it gets with the 4002 DC platter motor. This motor and PCB are ready for duty again!

A Pair of Beomaster 1900 Receivers for Restoration: Unit #1 Power Supply Checks and Adjustments

Today I adjusted the Beomaster 1900 +15 VDC power supply along with some other service manual adjustments. Everything adjusted and tested perfectly.

I will start with the +15 VDC check and adjustment.

There isn't an adjustment for the ±31VDC rail voltages but I always measure them to see verify they are good. If there is a problem I want to catch it early.

I attached measurement probe wires underneath the Beomaster on the collectors of output transistors IC200 and IC201. It is easier to measure the rail voltages that way.

In this case both measured good.

I do a similar thing for the No-Load Current check and adjustment of the output amplifier section. I attach some probe wires underneath the Beomaster 1900. Two wires for each channel across one of the channel's emitter resistors.

Per the service manual I adjusted the No-Load Current trimmers to 12mA for each channel. That is with the Beomaster 1900 volume level at zero and no speakers connected.

Here is the right channel.

Here is the left channel.

Before continuing with anything else I decided it was time to see what this Beomaster 1900 restoration work sounded like.

I connected up my usual workshop pair of Beovox S-55 speakers for the output and an iPod Nano to the Tape source for the input.

...and the result is some really great sounds coming through the Beomaster. 

The first play of music through a restored amplifier is always a great milestone to reach.

I took a rest and listened to music for about an hour.

The next step in the restoration is to pull off the wood trim pieces while they are so easily accessible. I would rather work with them removed than while they are attached to the frame.

A Pair of Beomaster 1900 Receivers for Restoration: Unit #1 Power Restored

The first of two Beomaster 1900 units sent for restoration is completed and ready for some testing.
To begin testing a restored Beomaster the work always begins with plugging it in to line power, turning it on and checking the power supply voltages.

With this Beomaster I was not getting any power.

In the previous post I suspected the +15 VDC power supply fuse (a 2A fuse) and the fuse holders that had burned spots on them. I also looked at the Beomaster 1900 mains power switch and found a wire exposed through the insulation.

I fixed the wiring on the main power switch.

The Beomaster 1900 has a nice way of securing the wire to the line voltage. It holds securely but it is easy to open up and work on.

There is a plastic wire holding clip that just slides out to loosen the wire.

I fed more wire into the Beomaster so I could cut off the bad section that had the exposed copper wire.

Then I wired the main power switch back up and put everything back in place.

That looks much better.

Next I changed the fuse holder for the +15VDC supply.

However, there was still no power. I could see now that I was getting proper voltage from the transformer to the bridge rectifier but no DC voltage out of the device. 

The reason I changed the bridge rectifier was because the Beomaster arrived with a pieced together diode bridge and I could see the charred remains of some past event involving power on this receiver.
There is nothing wrong with making a bridge rectifier that way but I wanted to return this Beomaster back to the way it originally was.

Here is the original state of the Beomaster 1900 +15 VDC power supply as I received it.

Here is the Beomaster 1900 with new parts: fuse holder, bridge rectifier and 2200uf smoothing filter capacitor.

There is no way the new bridge rectifier could be bad so I investigated my installation of it again.
Checking continuity of the signal paths on the trace side of the main circuit board I discovered that the solder pads for the bridge rectifier leads had two damaged pads. They didn't reveal their damage when I installed the new bridge rectifier but underneath they were not making contact with the trace on the board.

Here I show what happened when I pulled one of the AC voltage connections to the bridge rectifier.
The +VDC pad was also not making connection even though you could not visually see it.

I reworked the bridge rectifier connections and now it looks like this.

You will note that I also had to place a jumper on one side of the new fuse holder leads on the trace side.
This was required because the original two-pin holder clamp previously makes that jumper.
At first I thought that was the source to my power supply problem but after making that addition I still had no +15 VDC. 
There were still the two bridge rectifier solder pads that required repair.
As often happens in these repairs there can be multiple issues affecting one function. :-)

...and now there is +15VDC power on the Beomaster.
The transformer sends around 18 VRMS to the bridge rectifier whose DC output is around 23 VDC as shown on the meter below as measured across the smoothing capacitor.
That voltage then goes through a +15 VDC regulator.

Now I can finally move on to testing the Beomaster 1900 power supplies and make some service manual adjustments.

Beogram 400x DC Platter Motor Restoration Marathon

I recently received four DC platter motors from Sonavor for restoration. He needed them for a bunch of Beogram 400x restorations. Let the motor marathon begin! This shows the four motors:

In the following I only show the process for one of them. The first step is to take the motor apart to get the bearings out:

The bearings are the two small donuts on the black pad. The next step is to immerse them in motor oil and pull a vacuum:

If the bearings are in need of oil replenishment, the vacuum extracts the air from the pores of the bearing material. This manifests itself as bubbling from the bearings. After 1-3 days the bubbling stops and then it is time to extract the bearings: 

Then the motor is put back together and it is time for a RPM stability test with the BeoloverRPM device:

The BeoloverRPM allows logging the RPM over extended periods of time. I usually measure for about 24 hrs. These are the curves I measured for the four motors:

They are all pretty similar and show some small RPM drifts over time, which are most likely related to temperature changes in the circuit and its surroundings. The fourth (blue curve) is a bit more unruly. I get this sometimes. I have seen in the past that such motors 'quiet down' after they run for a while. Not sure why some motors behave like this. At any rate, the variations are much too small that they would be audible. The joys of analog audio! These motors are ready for duty again!