Beomaster 2400 (Type 2902): Full Functional Restoration

This post describes the bulk of the work done during the restoration of a Beomaster 2400 (Type 2902) that I received from a customer in Virginia. I already made two posts about this unit, which discuss specific FM tuner faults I fixed: (1) The tuner had very weak reception. This was traced to a dead transistor in the IF section, and (2) the stereo indicator was permanently on, which was fixed by replacing the stereo decoder IC.

This shows the final result of my efforts (I added an original antenna and "Controle Module" (spelling per Service Manual...;-) in these pictures):

What an amazing design! Beosexy! More and more I think the 2400 and the 1900 are B&O's masterpieces in the control unit department. No surprise that this design is featured in various modern art museums around the world!

Let's see what it took to get there:

This shows the unit as received. It arrived well-packaged, so there was no shipping damage:

It had the usual delaminating veneer panels on the sides. In this case, black electrical tape was used to fix the wood in place so it would not break off entirely:

The other side also already showed signs of delamination:

Otherwise, the unit was in pretty decent condition. There is a small ding up front in the edge of the aluminum panel:

The plexiglass section of the control panel also has a few damages:

When I opened up the enclosure, the side panels came off due to the usual broken off plastic tabs:

This shows the unit in 'service position' on my Lazy Susan:

Using a rotating work platform is a great idea for working on these Beomasters since it essentially eliminates the risk of breaking off some wiring connections while working on the main PCB. Here is a more detailed view of the main PCB in its original condition:

I replaced all electrolytic capacitors. It is also a good idea to replace the rectifier with a stronger unit. The original ones are a bit undersized and tend to fail. This shows old and new in comparison:

Note that the larger replacement needs its legs bent to fit into the footprint vacated by the original.

Another important item to replace are the quiescent current trimmers. The original ones are usually oxidized, which can result in a burnout of the output transistors. This shows the new 25-turn encapsulated trimmers that I usually implant:

I install them with extended legs so they can be oriented to show their adjustment screw through the opening in the PCB, like the original single-turn trimmers:

This shows the rebuilt PCB along with the replaced parts:

Next came the control panel PCB (#4). In the picture below, you can see the bass, treble, and balance sliders with attached indicator foils. In the center are the switches (loudness, mono/stereo, AFC, and start-up volume). On the right are the FM preset trimmers and the main FM dial.

First, I focused on the sliders:

The wide metal track in the center indicated that they were the older type, which is prone to failure. 

The first step was removing the precious indicator foils. They are clipped in with copper clamps that can be removed with a small screwdriver:

The next step is unsoldering the pins, and then they can be removed. This shows one of them:

Once they are on the bench, the slider part can be removed. This is done by straightening the two flattened 'side-pins' integrated into two of the pins that were soldered in.

Like so:

Then the fixture with the pins can be pried out with a screwdriver:

This shows the part with the two pins at the bottom from the side:

The smaller of the two pins on each side were the ones that needed to be straightened out for the removal of this part.

Now the slider with its track can be removed:

The part that needs to be removed is the plastic bridge that holds the contact terminals. The original part is still on the metal track with the terminals attached:

As the plastic degrades over time, the metal terminals come loose because the plastic tabs that hold them in place break off. The new parts designed by dksoundparts have an improved design, which hopefully will last longer. This shows the two terminal pieces:

It is interesting to note that these terminals each make a connection between the resistive carbon track and a parallel metal 'pickup' track. Therefore, the two ends are different. The left contacts are blank metal for connecting to the metal track, while the right ones have a small carbon plug implanted for making contact to the carbon tracks. 

Note that it is pretty easy to lose one of the little carbon plugs. They are only pressed into the metal parts. It is a good idea to do these repairs on a towel or similar to prevent parts from bouncing off the table.

This shows the three refurbished sliders. Nice new plastic bridges from dksoundparts!

The next step was replacing the electrolytic capacitors on this board. This shows the rebuilt board with the slide indicator foils reattached:

On to rebuilding the indicator boards with their many incandescent lightbulbs. I always replace all of the bulbs with the Beolover Set of LED Replacements for Beomaster 1900 and 2400 Indicator Light Bulbs. The reason is that LEDs last much longer than light bulbs. Light bulbs also degrade slowly over their lifetime, altering their light output. In addition, they emit heat, which leads to the bleaching of the red and green filter pieces. LEDs alleviate all these issues. All did not get worse in modern times!...;-).

First came the program selector board. This shows it in its original condition with the matching LED boards in front of it:

I removed the filter assembly from the PCB:

Underneath every red and green filter is a light bulb for each of the indicators.

I removed the light bulbs and implanted the LED boards:

Each board carries a small dedicated circuit mimicking the behavior of light bulbs in addition to the LEDs:

The reason for these added circuits is that in electronic circuits of the 1970s and 80s, the turn-on behavior of light bulbs (mainly the resistance increase once they heat up) actively participated in the circuit behavior. As an example, the bulb resistance change is used in the Beomaster 2400 circuit to temporarily activate the mute circuitry when a program button is pressed, preventing pops in the speakers. My LED replacements utilize capacitors and diodes to replicate the light bulb behavior, which keeps the circuit working as intended.

After the LED boards were installed, I clipped the PCB back into the lens assembly:

There is no interference between the LED boards and the plastic part. Everything looks like nothing ever happened behind the filter pieces!

The next step was replacing the three bulbs in the slider indicator illumination assembly:

After the removal of the plastic cover, the bulbs are revealed:

The LED replacement boards up front contain 9 LEDs each for a homogeneous background illumination. This shows the boards installed:

And after replacing the plastic cover:

The final board with bulbs to replace is the volume indicator:

It contains only two bulbs:

This indicator assembly is a bit special since it relies on the spatial emission of light from light bulbs. The plastic lens assembly also acts as a light conduit due to its funnel-like shape. This generates the impression of an increasing or shrinking illuminated area proportional to the selected volume. In order to keep this functionality, the LED boards need to be installed at a ~30-degree angle as shown here:

This shows the plastic assembly reinstalled:

After reinstalling the indicator boards, I fired the Beomaster up for the first time:

Beolovely! All of the LEDs worked as intended!

An important step of any Beomaster 2400 restoration is the proper adjustment of the quiescent current. The trimmers need to be adjusted for each channel to yield 12mV across the emitter resistors of the output transistors. Right channel:

and left channel:

This adjustment ensures that there are minimal distortions in the output signal and that the Beomaster runs as cool as possible. Be careful if you do this at home: if the voltage gets too high, it is possible to damage the output transistors!

After this adjustment, I hooked up speakers and tried listening to radio stations. Without attaching an antenna, I was not able to pull in stations in good quality. Also, I got a loud humming between the few stations I was able to receive. Something was wrong! Usually, the Beomaster 2400 is able to receive quite a few stations in decent quality in our house, even without attaching an antenna. It took me a while to figure this out, and I also had to order a few RF-capable transistors to fix this. This shows the new MPSH10 transistor installed that corrected the issue:

I wrote a separate post about this repair. See here.

After the tuner worked again properly, I realized that the stereo indicator light was always on, regardless of whether a station was tuned in or not. It was even on when playing vinyl records! Since this indicator is controlled directly by a pin of the monolithic MC1310 stereo decoder IC, the conclusion that the IC was at fault came easily. I ordered a few NOS ICs from a vendor in China who specializes in such obsolete ICs. After installing an IC socket and implanting one of the new chips,

 the stereo indicator was off! But it did not come on very easily anymore! It turns out that someone had de-tuned the 19kHz VCO that is used by the MC1310 for decoding the stereo signal. The VCO was running at 19.8kHz instead of 19kHz. I adjusted 2C24 until the frequency was back at 19kHz:

Here is an oscilloscope trace measured at 2TP4 after the adjustment:

Beolovely! This restored the proper functionality of the stereo decoder. I wrote a dedicated post about this repair, too! See here. At this point, everything seemed to work properly, so it was time to do some characterizing to make sure this Beomaster was ready for duty again. I set up my QA400 audio analyzer and plugged a 8 Ohm speaker sink into the right speaker output:

I fed a 1 kHz/0 dB (2.83Vpp amplitude) signal into the tape input from my waveform generator:

Then I turned the Beomaster on and increased the volume as high as I could, without seeing clipping in the output signal. This shows the measurement at the 8-ohm resistor: 

A nice clean sine wave! Usually, clipping starts at above 50Vpp output signal.

This shows the QA400 FFT spectrum of the signal at this point:

The Total Harmonic Distortion (THD) measurement came out to about 0.18%, which matches the "<0.2%" spec noted in the service manual. So far so good! After about 15 min the heatsink of the right output stabilized at about 55 °C, which is normal. This channel was operating at full capacity!

After this brute force measurement I measured the frequency response of the channel and received this curve:

Very linear as it should be for a good HiFi amplifier! In this measurement, the QA400 sends white noise into the amplifier input and measures the FFT spectrum of what comes out at the output. White noise is defined by a band-wide homogeneous mixture of frequencies of the same amplitude. Therefore, the FFT spectrum of such noise is a boring flat line.

I did the same measurement for the left channel and received very similar results. This Beomaster was ready!

Time to put this Beomaster back together!

Like most Beomasters of this vintage, this unit had delaminating veneer panels. I usually put some white wood glue under the delaminated fragments and then clamp them against a plastic foil-wrapped wood panel:

The plastic does not stick well to the glue. This allows 'rubbing off' the squeezed-out glue before it completely hardens. Not a perfect solution, but in the absence of replacement wood panels, a suitable workaround to get a decent cosmetic appearance:

After stabilizing the veneer panels, I put the enclosure back together. Sadly, a few of the touch buttons did not work. I traced this back to strongly oxidized contact pins and terminals. This shows the extracted oxidized pins:

I cleaned them with Bar Keepers Friend:

Beoshiny! The cleaning agent also works well on copper alloys:

After cleaning the contacts, everything was fine, and all the buttons worked!
Before I mounted the bottom plate, I realized the plastic feet had gone missing. I installed new ones:

Beautiful!:

I will now play this Beomaster for a bit longer, and then it will be time to send it back to its owner!