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.

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.

A Pair of Beomaster 1900 Receivers for Restoration: Unit #1 Reassembly for Testing and a Power Problem

 The next step before testing the Beomaster 1900 was to reinstall the four output transistors that mount on the rear heatsinks. I had removed the heatsinks and the original thermal insulating paste. On the reassembly I like to use modern SIL-Pad thermal insulators. They do a great job and don't have the mess that thermal paste does.

Here are the parts laid out for reassembly.

I like the way Bang & Olufsen chose to mount the transistors to the heatsinks. They use curved, metal spring clips that press the plastic face of the transistor against the heatsink...sandwiching the thermal insulating material in between.

Here are the output transistors reinstalled.

Everything is installed that is necessary to start testing the Beomaster 1900.

It only took the amount of time for me to connect up the variac and dim bulb tester to try powering up the Beomaster to discover a problem.

The Beomaster 1900's +15 VDC power supply was not turning on. No Standby lamp illuminated.
I exercised the main power switch again to make sure it wasn't dirty but nothing.

I unplugged the power cord and started to check around the +15 VDC power supply.  I could not measure anything across its fuse.  I had noticed the fuse holders were badly tarnished but this unit had powered up prior to starting the restoration. I pulled the fuse and it was okay. I just couldn't get anything through it from the fuse holders.

Here are the fuse holder clips and where I removed them from on the board.

Age has not been kind to these fuse holders. I remember now that the original bridge rectifier had been replaced on this Beomaster and there was a large, charred mark under where the bridge rectifier had been mounted. It is likely some big power event had occurred sometime in the past.

In any case I need to order some of these fuse holders on the +15 VDC power supply line.  If I order them today Mouser will deliver them on Tuesday.

The power supply incident got me thinking about all of the input power to the Beomaster 1900. I had seen a bad power switch on a Beomaster 2400 in the past so I should look at this one more closely.

I am glad I did. The switch actually measures good and I did some cleaning on it with Deoxit.  However, I found a cut in the insulation on one of the main power wires delivering line power to the Beomaster.  It probably wouldn't hurt anything but I don't like bare wires where they are not supposed to be.

I will rework the wiring on the power switch and fix that problem while I am waiting for the fuse holder clips to arrive.

Here is the cleaned up and prepared power switch.

A Pair of Beomaster 1900 Receivers for Restoration: Installing New Lamps (Unit #1)

I replaced all of the indicator lamps in the Beomaster 1900 today.

For the Bass, Treble and Balance indicator lamps I change from the original incandescent type lamps to LED/resistor type lamp assemblies. The reason for that is the incandescent type lamps generate more heat and I can see from white spots on the red lamp lenses that the heat has had an effect.  The LED lamp assemblies will operate much cooler.

Here is the board with the original Bass, Treble and Balance lamps.  I was out of my LED replacement lamp assemblies so I built more than the three I needed for this project as I know I will be needing the extra assemblies soon.

Here are the new Bass, Treble and Balance LED lamp assemblies installed.
Polarity matters with LEDs so I marked where the positive lead has to go on the board.

For the remaining twelve indicator lamps I replaced the original incandescent lamps with new incandescent lamps that I source from Beoparts. They are the correct type and necessary in these indicators so that the underlying Beomaster 1900 circuits work as intended.

There are two lamps on the volume control board and ten lamps on the selected source and FM tuning indicator board.

Here is the before photo of the lamps. I can see that these are replacements that were probably installed when this Beomaster had its bridge rectifier for the 15 volt power supply replaced.

Here are the new lamps.  All uniform...all the same type.

Now this Beomaster is ready for reassembly then on to the power supply testing.

A Pair of Beomaster 1900 Receivers for Restoration: Recapping the Tone Control & FM Tuning Board (Unit #1)

The most time consuming tasks of the Beomaster 1900 restoration were completed in the last post.
This post shows the final board that needs capacitors replaced....The Tone Control & FM Tuning Board.
The capacitor replacement is easy. There are just ten capacitors and they are easily accessible. The difficult task on this board is cleaning and repairing the three slider controls (Bass, Treble and Balance).

Here is the Tone Control & FM Tuning Board before photos.

I started with the the cleaning and repairing of the slide controls.

After some disassembly of the Bass and Treble sliders I could see that an early repair attempt had been made in the past. Probably at the same time the bridge rectifier on the main board was replaced (though the two items are not related).

These slide controls have a weak part of their design and that is where the control contacts mount to the sliding control bridge. The small, black, plastic piece of the bridge the contacts fit into tends to break.
You can see where there was an attempt to repair that problem with some glue on the bridge of the bass control.

Here is the slider control for treble.

A good slider control bridge repair kit was probably not available when the glue repair attempt was made. That repair probably worked for a while. Eventually that type of repair will fail again so the fix made today is to use a replacement bridge that is supplied by Martin Olsen of Beoparts.

This photo shows the new Beoparts bridge next to the original bridge for the bass control slider.

Here are the bass slider control contacts mounted on the new bridge

Another task of the slider control repair is cleaning and lubricating of the rails the contacts mate with.

I use some Deoxit and some conductive carbon grease. This bass control slider is ready for reassembly and installation back on the Tone Control & FM Tuner Board.

Here is the treble slider control ready for reassembly.

I am starting to get anxious to listen to this Beomaster receiver.  Still a little ways to go though.

The third and last slider control, the balance control, is a different style of slider. It looks similar to the others but its construction is such that it does not require the repair that the bass and treble controls did. I prefer this style of slider that the balance control has. It seems to be more durable.

It does require cleaning and lubrication though.

Here are the before and after photos of the cleaning of the balance slider control.

Now for the capacitor replacements.
Like the other electrolytic capacitors in the Beomaster these were out of tolerance.

Here is the worst one, a 2.2uF capacitor.

Wow... 6.27uF.

Here is a photo of the restored Tone Control & FM Tuning board.

The final restoration tasks before testing the results is to replace the Beomaster 1900 indicator lamps.
I will do that tomorrow.