Beomaster 2400 Type 2902 - Canada Project Indicator Lamps Replaced

 The indicator lamps are now all replaced on this Beomaster 2400.  The old lamps (except for one) were still working but the age of the lamps is unknown so with this restoration it will be documented when they were changed.

The only lamps I converted to LEDs are the three lamps for the Bass, Treble and Balance position indicators.  Those lamps have no function in the circuit except to illuminate the slider positions when the Beomaster is on.  From past Beomaster 1900/2400 projects I like using two LED devices per lamp position here to give the light coverage on the indicator masks.  The reason to change these three incandescent lamps is because the give off the most heat which I have seen cause damage to the indicator masks. You can also see in the photos that the center of the red indicators lenses on these three lamps is faded white due to the heat. The replacement LEDs will run much, much cooler.

The rest of the Beomaster 2400 (and 1900) lamps do directly affect the circuits they are indicators for.
So I will replace those using replacement lamps the Beomaster owner sourced from Martin Olsen.

Here are the Volume Control and Source Select lamp boards with their new lamps.

Now with the lamps all replaced I rechecked the volume muting circuit when a source is selected.

The resulting voltage levels and volume muting duration are pretty much the same as with the old lamps. This is something I can note and use when comparing the next Beomaster 2400 (or 1900) receiver.

I am now ready to recheck the service manual adjustments again before playing some music through this receiver.

Beomaster 2400 Type 2902 - Canada Project Volume Muting Circuit When Source Selection Is Made

Every now and then there are some discussions on Beoworld regarding the Beomaster 1900 and 2400 receivers' volume muting during selection of a music source. It is something that I never have really paid much attention to so the discussion caught my interest and I decided to measure the volume muting activity to see it in action.

Here is the Beomaster 1900 and 2400 source selection muting circuit.
One thing to note...I have a few service manuals for both the BM1900 and 2400 but neither show this circuitry for later model PCB3 Volume Control boards. The circuit didn't change but other things on the Volume Control board did. As a result, the later model boards have different reference designators and the location of the components changed some. I found that the new reference designator for the old 3TR6 transistor to be 3TR29. Since the documentation didn't change I will continue referring to the transistor as 3TR6 though.

Here is the trace side of the newer BM1900/2400 Volume Control Board (PCB3).  I have attached three wires (black, yellow and blue) for my oscilloscope probes to connect to.

Here is the source switching volume muting description from the Bang & Olufsen BM1900 service manual.

Now for a look at the signals on the oscilloscope.  For these measurements I am still using the Beomaster 2400 with its original lamps.

The first photo is of the signals being monitored (3TR6-collector, 3TR6-base, 3TR6-emitter and 5R3)...when one source has been selected (Beomaster 2400 "On").

The voltage measurements are pretty much like the service manual states...except the value of 5R3 was expected to be higher than the voltage at 3TR6-base. I will measure these again when I replace the lamps in the PCB 5 lamp board.

Here are the same signals when a source switching event occurs.

As the service manual described, the 3TR6 collector goes to about -12 VDC. That should mute the Beomaster 2400 volume. The duration of the volume muting event I measured is 324msec. The service manual says it should last about 500msec. So I will check the duration after I replace the lamps.

I performed another series of tests using a different PCB 5 lamp board. This board I replaced the source indicator lamps with LEDs and 1KΩ load resistors (to limit the current through the LED to its rated value).  I have used this type of lamp replacement before and it didn't cause any problem with source selection and the display of the selected source. However, I suspect that it does affect the short, ~500msec volume muting circuit.

Sure enough, the LED lamps negate the volume mute circuit during source switching.  The LED assemblies don't use much current compared to the incandescent lamps and the act of switching the selected source doesn't excite the base of 3TR6 to engage the volume muting.

I could experiment with changing the 5R3 resistor and adding some bypass resistors across the LED assemblies to come up with a combination that excites the muting circuit. I would have to be careful because too much current could result in the volume muting always being on.

For now I believe I will stick with incandescent lamps for these source indicator lamps. That way the volume muting during source switching will be functional. This time I will spend a little more time seeing if I can actually hear a difference during the Beomaster 2400 source switching events.

Beogram 4000 (type 5215): replacing sensor arm photo cell with modern one

I recently received 2 Beogram's (a BG4000 and a BG4002) on my workbench with broken photocell's in the detector arm. Normally these photocells do not wear out but the light bulbs may have their wires broken off. Beolover has already a LED replacement for these bulbs (see sensor arm light bulb replacement). However, this time the photocell had broken wires and even a broken cell.  Impossible to repair and, to the best of my knowledge, also impossible to find new ones.

Here is how the broken photocell looked like:

The photocell used in the Beogram 4000 series is in fact a photovoltaic cell (BP100). It produces energy (DC voltage) when photons from a light source hit the surface. So, it's sort of a micro solar panel! I could not find any photovoltaic cell of this size. Another sort of replacement needed to be found. 

After experimenting with photodiodes, photoresistors, ...I ended up using a phototransistor. They came out as the most sensitive and reliable components for this application. Placement inside the detector was however an issue, but luckily an "Osram opto sensor" was the perfect match. There was no need to adapt, file, cut anything. If perfectly fits snugly into the detector arm. 

                                                Dimensions are: 4,6 mm x 5,8 mm and 1,7mm thick

                                                                      type: OSRAM LPT 80A

The best way to fit the sensor into the detector unit is to first remove the light bulb, bend and isolate the wires of the new sensor and then snap it into the black detector housing. The sensor should touch the brown PCB so that the lens of the sensor is in line with the bigger lens on the detector unit.

isolate the wires with some heat shrink tubing

bend the wires to have a "drop in" into the housing and into the PCB holes

new sensor in place and soldered

light bulb back in place

Below is the new schematic with the added resistors. 

Some explanation:

The OSRAM phototransistor is an NPN type with open base. In other words, the transistor has only 2 legs (collector and emitter), the base is "open" and the incoming light triggers the transistor to start conducting. You need a power supply source off course to make the circuit working. After some experimenting, I ended up with a 22K resistor (R1) from the 6V DC available supply rail to the collector.

Since the phototransistor acts as an on/off switch, the out coming pulse is about 5V p-t-p. This is way higher than the 20 - 30mV that the original BP100 photocell was delivering. That is why a voltage divider (R2 & R3) was added so that the pulses after transistor 1TR14 where around 2,5 VDC as described in the service manual.

Note: since the new sensor is a NPN transistor, make sure the 2 wires from this sensor leading to the PCB are correct: the long leg on the phototransistor is the collector. Short one is the emittor.

Using a oscilloscoop, you can see the nice pulses generated by the phototransistor at the collector:

.....and the ones measured at the collector of 1TR14. Very close to what the service manual is indicating.

Since only 3 resistors needed to be added, I soldered them directly onto the PCB and on the connectors where the incoming wires from the detector arm are connected. To my surprise, a "spare" unused connector was installed. No idea why it is there, because it is not connected to anything. Anyhow, very convenient now....It's like the B&O engineers expected that somebody, someday, would need this 😊

These pictures are taken from the Beogram 4000. A Beogram 4002/4004/6000 will need different wiring, but the concept is exactly the same.

Beomaster 2400 Type 2902 - Canada Project Power Supply & No-Load Current Adjustment

 The hard parts of the Beomaster 2400 are completed. It is time to put the receiver back together so I can do some basic voltage checks.  

First though I need to re-attach the four output amplifier transistors that mount to the Beomaster 2400 heat sinks. I like using the Sil-Pad thermal interface product rather than the white thermal paste in these low profile receivers. The Sil-Pads do a good job and don't have the mess that paste does.

At this point I decided I wanted to check the Beomaster 2400 restoration I had done so far by powering up the Beomaster and performing some basic service manual tests.  In order to do that I re-attached the Filter & Tone Control board along with the Beomaster 2400 lamp boards.

If you think your eyes are playing tricks on you in the above two photos they are not. I didn't really like the new replacement 2C92 2200uF, 25V capacitor for +15VDC in the Beomaster.  The markings on that capacitor were kind of strange. It wasn't as clear as it should be about which end of the axial capacitor was positive and negative. Normally the (-) marking is a distinct arrow pointing at the negative end. This capacitor looks like it points in both directors and the data sheet on it wasn't any help...so I switched it out with a Vishay 2200uF, 25V, axial capacitor that has clear polarity markings.

Plugging in the Beomaster 2400 and turning the master power switch on resulted in the red Standby LED illuminating.

The first test I performed was for the +15VDC supply. I adjusted it to the required +15V per the service manual.

Next were the left and right channel no-load current adjustments.
The space for attaching measurement wires and adjusting the trimmers is very cramped in the Beomaster 2400. My method of performing the adjustment is to mount the measurement wires across the emitter resistors from the bottom of the Beomaster 2400 main board.

That provides the leads I need to attach my DMM for the no-load current measurements.
I adjusted the left channel for the target 12mV across the emitter resistor test points.

Then I adjusted the right channel.

So far everything looks good.

I want to measure the Beomaster 2400 muting circuit while I have the cover off and the original lamps still attached. That is the volume muting that occurs when the music source is switched.  Pressing one of the Beomaster music source switches should send a volume mute signal to the volume control until the source switching is completed.

I soldered on some test wires so I can watch the muting circuit in action with an oscilloscope.

Tomorrow I will measure what goes on with the volume muting circuit.

Beomaster 2400 Type 2902 - Canada Project Capacitor Replacement Part 2

Wrapping up the Beomaster 2400 capacitor replacement is just recapping the tone, filter and FM tuning board. Just eight capacitors on this board. This board also has the three slider controls that I cleaned with Deoxit fader lube. 

Here is the board before replacing the capacitors.

Here is the board after the recap. As with the other boards, the seven electrolytic capacitors were way out of tolerance (50% to 100% off their value). The lone 0.22uF tantalum was fine but in being consistent with my other replacements I replaced it with a WIMA MKS capacitor.

All total, there were 58 electrolytic capacitors and 19 tantalum capacitors replaced in this Beomaster.

Beomaster 2400 Type 2902 - Canada Project Capacitor Replacement Part 1

 I got through all of the work replacing old capacitors in the Beomaster 2400 main board (over 60 capacitors) and volume control board (2 capacitors).

Here is the Beomaster 2400 again prior to replacing the electrolytic capacitors.

This restoration was different than my usual Beomaster 2400 capacitor restorations.
The owner of this Beomaster had already purchased a Beomaster 2400 recapping kit from Martin Olsen.  It has been a long time since I used someone's capacitor kit but why not? The capacitors are good capacitors so I wouldn't want to waste them.

Martin's kit also included a new bridge diode for the incoming AC power so I will replace that.
This kit has all of the replacement capacitors for the Beomaster 2400 electrolytic capacitors except the two large 5000uF, 50V reservoir capacitors so I will add those two.

The kit also does not contain replacements for the Beomaster 2400 tantalum capacitors. The tantalum capacitors are typically still okay but I normally replace them as the restored audio component often has a long trip back to its owner. Replacing them is a bit of additional insurance to avoid having to send a problem component back and forth.

Of course you will find a lot of discussion on various audio forums about whether to replace tantalum capacitors with tantalums or a different type like electrolytic or polyester.  For capacitance values of 4.7uF and smaller I prefer using WIMA MKS polyester capacitors so that is what I will be doing here.

Here is the Beomaster 2400 after the recapping task of the main board and volume control board have been completed.

I also replaced the left and right channel no-load current adjustment trimmers with new multi-turn, sealed type trimmers.  There were three capacitors, the black ones in the photo, that were capacitors that had been replaced by someone else previously.  They measured good but I changed them so that all of the capacitors starting with this restoration are the same age. One thing to note...one of the previously changed capacitors was 33uF instead of the original 22uF capacitance value.  Martin's kit calls for the original 22uF capacitor which is also what I have always used. I have no idea why the 33uF was put in here.

For my own knowledge base I always take a little extra time and measure each capacitor I remove. I just want to know what state it is actually in.  On this Beomaster 2400 all of the electrolytic capacitors except the large ones were out of tolerance. The 1uF capacitors almost all measured over 2uF. The 2.2uF capacitors were either at 3.5uF or just over 4uF.  The 4.7uf were around 6uF. 

The tantalums were all still within their 20% tolerances but a lot of them were close to the upper limit.

The reservoir capacitors measured good but I still always replace them to insure against future problems.

Here are the two reservoir capacitors before replacing them.

Here is the after photo.

 

The reservoir capacitors are a little tricky to change. B&O connected a number of ground wires to the common point of the reservoir capacitors. The positive and negative DC connection points each have three wires connected as well. New power supply capacitors I use have small posts so connecting the number of wired the Beomaster requires means creating some good terminal solder points.

I joined two terminal posts together for the ground wires and soldered all of the ground wires to that ground terminal. I soldered the positive DC voltage wires to another terminal then soldered that to the positive post of the reservoir capacitor assembly. I also connected the negative DC voltage wires to a terminal on the negative post.

That makes for a really solid reservoir capacitor assembly and the wires are neatly connected.

Two of the black ground wires go to the speaker jacks.  The original wires are short and don't have much room to operate with.  One usually always breaks off when raising and lowering the main board during the recapping process. I always replace those with some longer wires that can handle the main board raising and lowering.

Here are the before and after photos of the volume control board.

Here is a photo of the two new multi-turn, sealed trimmers that will be used to adjust the no-load current of the output amplifiers.

The adjustment knobs for the trimmers are easier to access than the old trimmers.

The next restorations task is to do the capacitor replacement on the removable board with the tone controls, filters and FM tuning.  That will be followed by cleaning the slider controls and the replacement of the lamps.

Beomaster 2400 Type 2902 - Canada Project First Opened Up

I plugged this new Beomaster 2400 unit in and checked out the lights on it to see if any were burned out.  It turns out that all of the lamps are functioning.

 

As promised I opened up this new Beomaster 2400 receiver to check the components inside.

The Bass, Treble and Balance slider controls look to be in good shape.

The slider controls on this Beomaster 2400 are a different style than I usually see.  I have seen these before but they have a round bar for the slide control to travel on instead of the usual flat bar.

 

The black plastic bridge that holds the slider control contacts is different due to the round bar that it attaches to. As a result of that difference these controls don't have the problem of the broken contact mounts that typically require a repair.

Now to take a look at the Beomaster 2400 indicator lamps. 

First, the volume control lamps. I can see that one of these lamps has been replaced in the past.
So this Beomaster 2400 has been worked on before.

Here are the lamps for the Bass, Treble and Balance indicators.

The indicator masks for these controls were is good shape but you can see on the underside of the red lens caps that there is a lot of fading due to heat from the incandescent lamps.  I usually replace the original lamps here with cooler operating LED lamps.  These three lamps are always on when the Beomaster is not in standby and are not vital to any circuitry in the Beomaster.  They simply are powered on and off when the Beomaster comes out of standby and goes into standby.

The last group of lamps are for the FM tuning indicator, FM stereo indicator and the selected audio source indicators (Phono, Tape, FM1, FM2, FM3, FM4 and FM5). There is also a red LED lamp for the standby mode indication.

These lamps do wire into Beomaster 2400 circuitry and factor into the operation of the circuits they are part of.

Now the restoration can be planned and implemented.
I start on that tomorrow.