Beomaster 2400 (2902): Functional Restoration and Test

This post discusses the functional restoration of a Beomaster 2400 (Type 2902) that I received from a customer in Connecticut. My first assessment of the unit is posted here.

The elegant flat design of this Beomaster type required putting most circuitry onto a single board, which is connected via wiring harnesses that are directly soldered to the board. Some of the auxiliary boards already received wire to board connectors, which helps a bit with the disassembly. 

After unsoldering the FM receiver front end the main PCB swings out into a 'service position' of sorts. In order to protect the many fragile wires that connect to it I had the idea putting the entire setup on a large Lazy Susan rotary platform that I fashioned from a pizza cutting board and a 20" turntable bearing. This allows accessing the PCB from both sides without continuously bending the wires:

The biggest task is the replacement of the close to 100 electrolytic capacitors that this design carries. after more than 40 years these capacitors are often out of spec and need to be replaced. Some of them are inside the EMI shields that are soldered to the board. This shows the stereo decoder can:

Its 'lid' is on the backside of the board:

I unsoldered both shields to get to the single electrolytic cap inside:

I replaced it

and put the shields back into place. Next came the remote control receiver:

Removal of the can revealed the electrlytic cap inside:

I replaced it

And put the can back into place. Next I focused on the small piggybacked board above the volume control assembly. It has four Ta capacitors on it:

This board is a circuit modification that became necessary after they realized that there were issues when using non-B&O tape decks with the Beomaster 1900/2400. This board changes the tape output signal levels compared to the original setup. Since the main board was already designed they elected to add this small PCB. I guess in those days it was a lot of work to change the design of a board due to the absence of digital design tools. The added board is soldered to the original component solder points and adds a few more components to the circuit. I unsoldered it from the main board to be able accessing the solder points of the capacitors:

This shows it with new capacitors:

I soldered the small board back in:

Then began the arduous task of replacing all the other electrolytic caps on the board. This simply involves unsoldering a couple then pulling them out and putting new ones in minding the correct polarity. After this was done, I replaced the two main reservoir capacitors:

I replaced them with modern 10000uF units that have the same form factor. The original units are 4700uF, but in this case 'more is better' applies since they act as current reservoirs for the amplifier. This shows the new ones soldered in place:

Here a shot of the rebuilt board:

All the replaced components:

While the main PCB is still in service position it is the perfect moment to get to the electrolytic capacitor in the FM front end:

Its covers come off easily:

And from the other side:

The single blue Ta electrolytic capacitor needs to be replaced:

This shows the new one implanted:

Then it was time to address the other PCBs. The board that sits behind the controls under the aluminum lid (PCB#4) has a few electrolytic capacitors on it:

I replaced them:

On to PCB#3, the volume control and oscillator board. This Beomaster 2400 was fitted with the upgraded 128 step volume control, which only has one Ta electrolytic capacitor on it (blue):

I replaced it:

This concluded the capacitor replacement process. 

An important item to address in these Beomasters are the quiescent current trimmers in the output amplifier design. With these trimmers the working point of the output transistors is adjusted. The problem is that when the trimmers resistance goes up the quiescent current increases. This means when the trimmers oxidize the output transistors can overheat and literally go up in smoke. So it is a great idea to replace them with modern encapsulated multi-turn trimmers. This shows the original single turn trimmers after I removed them:

I implanted new 220 Ohm 25-turn trimmers

in a way that their adjustment screws poke through the original adjustment orifices in the PCB:

I also installed small wire 'eyelets' on the emitter resistors, which allowed making a solid connection with the multimeter for adjusting the quiescent current:

The service manual advises an adjustment to 12 mV after the unit has warmed up. 

In the meantime I finally received a batch of 10 amp bridge rectifiers as replacement for the original 3 Amp unit that this Beomaster still had implemented. It is the black square to the left of the reservoir capacitor stabilizing the 15V rail:

The 10 amp replacement is a bit bigger than the original unit, i.e. the legs need to be bent together to fit into the original footprint:

This shows it installed:

After this I decided to try if I could get any sound out of the Beomaster and connected it to my bench speakers. But no luck. Only at fully turned up volume I was able to hear a faint radio station. It turned out that this was caused by a faulty balance slider, which had lost its contacts. This is a frequent issue with the old style sliders that can be recognized by the broad metal spine on which the plastic bridge with the contacts slides. I unsoldered the slider:

The next step was removal of the attached plastic foil with the display apertures. This is done by removing the small copper clamp that holds it to the top of the plastic bridge:

The next step was removal of the metal spine:

Once the spine is off, the old slider can be slid off. This shows the original slider still in place. One of the contacts has broken off. The replacement part from the dksoundparts store is shown next to it:

I replaced the original part with the new one:

After soldering it back into place the Beomaster played radio through the speakers. Time for some measurements! This shows my setup:

I usually use a homemade 8 Ohm speaker dump fitted with a -40dBV (1/100) attenuator that allows me to directly connect my QuantAsylum QA400 for analyzing the output signal. The QA400 can only take a maximum voltage of 4V at its inputs, while the speaker outputs of the Beomaster can easily exceed 30V amplitude at high volumes. Therefore a suitable voltage divider is necessary for such measurements.

My first measurement focused on characterizing the Total Harmonic Distortion (THD) of the unit. For this measurement I used a RIGOL wave generator for putting a 1 kHz 0 dBV (2.83Vpp) signal into the tape input. Then I set the Beomaster to the highest volume level that still yielded a non-clipped clean sine wave on the oscilloscope. This is the spectrum that was measured by the QA400 for the right channel (the left was very similar):

The QA400 calculated 0.19% THD from this graph. This makes sense: The voltage difference between the fundamental 1kHz frequency and its first harmonic at 2 kHz is about 55 dBV, which corresponds to a 562x smaller signal, or 0.17% distortion. 0.19% is close to the THD < 0.2% spec stated in the service manual. So far so good!

Another concern with any amplifier is its frequency response. It should typically be as linear as possible that the amplifier does not alter the sound of the signal that is amplified. For this measurement I set the balance, bass and treble sliders to their center positions and the loudness switch to 'off'. Using the FR setting of the QA400 and connecting its outputs to the tape input of the Beogram I measured these frequency response curves for the 20-20kHz range:

This seems pretty linear, and it can be expected that the amplifier will not superimpose any 'color' onto the amplified sound.

While I had this measurement setup in place I also set the bass and treble sliders to their minimum and maximum positions and these are the curves I measured:

In absence of a calibrated voltage dial I started the Beomaster with the volume preset set to high to have a reproducible volume setting. The adjustment ranges seem to be around ±12dB at the low end and maybe ±15dB at the top end. This is a bit less for the bass range than the value stated in the service manual: "Bass at 40Hz: ±18dB", while the treble range matches the stated value: "Treble at 12500Hz: ±15dB" fairly well. Not sure why the bass range is a bit off. Maybe this has to do with tolerances in the sliders or filter components in the tone control circuit.

Since I like listening to vinyl records, I also measured the frequency response of the phono input. It is always interesting to see how well the frequency response matches the RIAA de-emphasis curve. This is the graph I measured:

For this measurement I actually had to make a special cable with a 40 dBV (1/100) attenuator for connecting the QA400 outputs to the phono input of the Beomaster. Even at the lowest signal setting the QA400 output still produces a too large signal for phono inputs. Phono signals are around 100x weaker than standard high level input signals, i.e. amplitudes in excess of 10mV can already drive a RIAA pre-amp into saturation. 

The measured curves show the expected gain reduction between low and high frequencies. The RIAA curve suggests ±20dBV variations for both bass and treble ranges. The Beomaster RIAA seems to match this fairly well for the high frequencies where the gain drops by about 18dBV between 1 kHz and 20kHz. In the bass range the change is only about 10dBV, suggesting that bass frequencies are slightly under-amplified making the sound a bit brighter than it should be. Of course this could be compensated by setting the bass tone control slider a bit higher.

In practical terms, I already played a few records on my bench 4002 through this phono input and I thought they sounded pretty good. So maybe this is not a big issue.

Stay tuned for my next post about this Beomaster, which will address replacing the front panel light bulbs with LEDs.

Beogram 4004 (5526): A New Arrival from New York - First Look

I recently received a Beogram 4004 (Type 5526) from a customer in New York for a full restoration. The unit came safely packaged in a Beolover shipping container (watch the packaging process in the video posted on my 4002/4004 page, it reliably prevents shipping damages). I extracted the unit and put it on the bench:

The hood is quite scratched as if a miniature ice dancer had spun her pirouettes across it..;-):

In absence of new reproduction hoods from the dksoundparts store this hood could be polished back to a decent look, while we wait for them to get their act together (all eyes are on you!...;-). The hood hinges seem in good condition without any discernible cracking. Unfortunately there are a few damages on the inside, which cannot be polished out. But this hood could certainly look more presentable than now after polishing it.

The keypad has the usual use-smudges and also a few scratches:

These smudges come from touching the keys which over time thins and damages the coating. Luckily, we now have a restoration process in place that could make this pad look like new again. Original keypads that still lookg decent can be protected from further decay by installing the Beolover Commander Remote Control for Beogram 4002 and 4004, which allows full control of all functions without needing touching the keys ever again.

The platter is in pretty good condition except one fairly long scratch in the center:

The plinth is also still fairly decent. It only has one small chip on the left corner:

If perfection is desired a beautiful new CNC machined plinth could be installed. They are of much better quality than the original veneer on particle board frames.

The right corner is still pristine:

I removed the aluminum plates and the platters:

The deck looks fairly original under the hood. I found the usual orange fragments from degraded transport lock bushings:

These will be replaced with a Beolover Transport Lock Bushing Set for Beogram 4000, 4002, and 4004.

After this visual inspection I plugged the Beogram in and pressed START. Nothing happened at all. I had a look at the fuses, but both were still o.k.. This suggested an issue with the carriage drive circuit. I manually drove the carriage off the SO ('switch off') switch by turning the carriage pulley a few rotations. After the switch was cleared, the light at the end of the sensor arm came on, indicating that the unit powered up. Pressing the << >> < > buttons yielded no response. Also the platter motor did not rotate. But I was able to raise and lower the arm. So clearly some mayor issues are present in this unit. The platter motor seemed completely seized and I was not able to turn it by hand. A non-standard too-long screw on the motor mounting plate indicated that there some previous interventions already happened:

Overall, I think this unit is a pretty decent candidate for a nice restoration, especially if the cosmetic issues would be alleviated. I am confident that the functional issues can be cured and like-new functionality will be achieved once I am done with my process. Stay tuned!