Beomaster 2400: Indicator Lamp Checks

Now the project gets to the fun part...reassembly and testing.

First thing is to put all of the parts I removed back and turn the Beomaster 2400 on.

I want to see if the replaced lamps all work and I will want to measure the Beomaster power supplies.

This post will show the testing of the new lamps.  The volume level indicator lamp module and the Bass/Treble/Balance module have to be re-soldered in place first. 

Here is the volume level lamp module reinstalled.

...and here is the Bass/Treble/Balance lamp module.

With the Beomaster powered on these two lamp modules illuminate.  

The source and FM tuning indicator module has two connectors so no soldering is required to reinstall it.

The lamps on this module all worked except one, the FM3 source selection lamp.
It turned out that I had the LED replacement lamp installed with reverse polarity.  I switched it around and then it too worked.

I was curious as to how I got the polarity wrong on the FM3 source indicator board. I used the previous Beomaster 2400 restoration project as a reference so the polarity should have been correct.
Looking at the previous project I now see that this indicator lamp panel has differences more than just the two board connectors.

Here is the Beomaster 2400 Type 2902 indicator board from my previous restoration project. The FM3 lamp (between the two board connectors) does not have the jumper that the current Beomaster 2400 does.  The same is true for the Beomaster 1900 Type 2903. The wiring path for that lamp is indeed different...as I found out.

On to the power supply and no-load current adjustments.

Beomaster 2400: Lamp Replacements

This post shows my Beomaster 1900/2400 lamp replacement method.

I replace the two volume level indicator lamps with new incandescent type lamps. These lamps are part of the the volume level circuit and need the lamps to be the incandescent type of lamp.

This is the board with the original lamps.

Here is the board with two new incandescent lamps installed.  I source them from Martin Olsen.

Next is the update to the Beomaster 2400 source selection and FM indicator lamps.
Again, here is the original lamp board for the source selection and FM indicator lamps.
I marked the positive side on the lamps I will be replacing with LED type bulbs.

First is the replacement of the three incandescent bulbs for (FM Stereo indication and FM tuning).

Next are the replacement of the source selection indicator lamps with LED modules.
These are a bunch of my homemade LED assemblies. The single ones are for the Beomaster 1900/2400 source indicator lamps. The dual LED assemblies are for the Bass, Treble and Balance position indicators.

Last are the replacement of the incandescent lamps for the Bass, Treble and Balance indicators.
Here is a picture of the board with the original incandescent lamps.

I always change these lamps to LED type lamps.  I often find that the plastic position indicator masks that fit over these lamps are damaged due to heat from the lamps. The LED replacements operate at a much lower temperature so should be better for the long term and are not damaging to the plastic indicator masks.

In the next post I will reassemble the Beomaster 2400 next and check the results of the restoration work so far.

Beogram 8000: A Fun Evening With the Record Detection Circuit (the Conclusion)

This is a follow up to this post. After I made the post about the failed transistors in the record detection circuit of a Beogram 8000, I received this forwarded input from the owner of the 8000 who has a friend who understands circuits:

“I think he [the Beolover] replaced every transistor he came across until he got to tr16.  Then he measured it, it was OK, so he didn’t replace it.  I think he should replace TR16.  The reason is TR16 operates with 6 ua of base current, maybe his curve tracer is a different current.  Clearly he has the right waveform going into TR16, and noise is still coming out.  With the beta he measured this should not be happening, so something is wrong with the measurement.  This thing where all the transistors wear out is disturbing.  Of course this circuit calls for very high beta in every stage, but it shows us how transient our technology is.  A transistor is only a transistor because of the sandwich of 3 different types of impurities, which we know move around. After 40 years, it seems they move around a lot.  Clearly these parts worked at one time, but they all changed enough to stop working.  It’s rare to see something this old with good enough documentation and people willing to keep it going.”

Point taken. I also wondered why this circuit was still not working properly even with TR16 measuring o.k., and I have no idea what exactly my little transistor tester does in terms of applied voltages etc...when it does its magic.

Only one way to find out: I restored R72 to 1M and put R70 back into the circuit, and then replaced TR16 with a 2N2222 that yielded an hfE of 225 on my transistor tester. It still did not detect the absence of a record. So I rummaged through my parts collection and finally found a new BC547B that yielded an hfE of 430, and put it into the circuit, and that fixed it:

The yellow trace is the measurement point between R72 and R71, and the green trace is HH (collector of TR16). This qualifies as a good logic LOW, and the micro controller apparently thought so, too. It swept the arms across the platter in search of smaller records, and then returned home and turned off, as it should!

So the conclusion is, yes, the circuit can work as designed (indeed, a satisfying result!...;-)

One question remains: Why does TR16 need such a high hfE? Sure, the base current is pretty low due to the 1M resistor. In this particular case, the minimum voltage in the sawtooth at R71/72 is about 6V, i.e. the minimum base current into TR16 should be about 5.3uA. The current across CE should be limited to 5V/33kOhm=151uA if the transistor is fully on. That is only about a factor 28, i.e. a hfE of 225 should be plenty to pull the collector of TR16 down with 5.3uA.

This got me motivated to look a bit deeper into measuring hfE values. It turns out they are strongly collector current dependent. This figure is taken from the BC547B datasheet provided by ONsemi:

So we see that at the low 151uA current that TR16 sees, the current gain is only about 60% of the maximum value at about 20mA. The 2N2222 has a similar curve (see here), and that (together with the strong variation of hfE values in general between individual transistors of the same series) may explain why the 2N2222 was not able to perform properly in this circuit. 

Beogram 8000: A Fun Evening With the Record Detection Circuit

*************************This post has a follow-up. Please, see here*****************************

I recently started working on a Beogram 8000 from California. As the usual, as a first step, I replaced the electrolytic capacitors and the motor AC cap in the power block, then re-flowed the solder points of the headers, after which it was time to give the unit a first try. I pressed start and the carriage started moving, and without a record on the platter the unit activated the tonearm lift at the LP setdown point. Not what it was supposed to do. I concluded there was an issue with the record detection circuit. 

This is the relevant part of the Beogram 8000 circuit diagram:

I should point out that the Beogram 8000 featured here did not have the trimmer R64 at the sensor input, i.e. is one of the earlier models.

The first spot to check is the collector of TR14, which amplifies the signal of the sensor 5PH1 as the platter ribs pass through under the sensor arm. I drove the arm manually further onto the platter to get a stable signal, and then put the oscilloscope probe to the collector. This is the signal I measured:

The dips correspond to the ribs passing under the sensor and blocking the light reflection. Unfortunately, there is no prescribed wave form in the manual, but judging from the very similar circuit in the 4002/4 models this signal was way too low.

the next step was to measure the sensor signal directly at C15, which yielded about the waveform shown in the manual:

Therefore, I suspected that TR14 had a too low gain (hfE), which is often encountered in the Beogram 4002/4 models. I extracted and measured it:

It only showed with a gain of 94, which is well below the minimum gain of 200 specified for the BC547B. I replaced it with a 2N2222 that demonstrated a gain of 215, and measured again:

Much better. The dips go to zero and the amplitude is almost 10V. Unfortunately, there is no spec in the manual for this measurement, but judging from the 4002s the above qualifies as a decent signal. I hoped at this point this would fix the issue. But no cigar. The Beogram again dropped the arm at the LP setdown point in absence of a record. 

If the sensor signal is good, then the next measurement spot is at the junction between R71 and R72. I got the yellow curve in the screen below:

The triangles peak at about 7V and dip down to about 3V, in contrast to the specified 10-to-7V drop shown in the manual. This means there is a too small voltage at the base of TR16, which may mean trouble pulling the 'Disc Detector' signal (HH) properly to a logic LOW state below 0.2V.

I measured HH and that is the green curve in the graph above. At this point I though, bingo, there must also be an issue with TR15 that does the tugging at the base of TR16. I extracted it for measurement:

It came up with an hfE of 158, which is also a bit low. I replaced it with a new BC557B with an hfE of 275, above the minimum 200 spec. So far so good. I measured again at R71/72 and now the curve was according to specification:

Unfortunately, record detection still did not work, and the green curve was a bit better, but the spikes, while a bit weaker, were still present. Ok, I thought, TR16 must also be out of spec. I measured:

...and got a happy hfE of 434 (it is normal that the hfE varies a lot, and all is good as long as it is above its minimum spec, at least for most circuits.

To make sure that the chain of command was intact between TR16 and the micro controller input I grounded the collector of TR16, which should permanently disable record detection, and it should always behave as if there were no record. I pressed Play, and indeed, the arms ran across the platter in search of a smaller record, and then turned around and went back home. So that was working.

At this point I was a bit at a loss. For some reason, the pull at the base of TR16 was not strong enough, and it did not fully turn on. A mystery!

I decided to play a bit with the circuit and I replaced R72 with a 100k resistor, increasing the current by a factor 10 into the base of TR16 (or so I thought). This did not fix the issue. It turns out that the higher current was depleting C19 too quickly, reducing its voltage too fast between the passing of the platter ribs, again causing spikes in HH. This meant I needed to increase the current into C19 also, which was easily done by removing R70, which partially drains the collector of TR15 away from C19. After eliminating R70 from the circuit, it started working properly with HH going fully to zero when the ribs were visible to the sensor:

Luckily, HH also quickly restored itself to 5V when covering the sensor (simulating a record), i.e. the record detection mechanism of this Beogram 8000 is doing what it is supposed to be doing.

At this point, I am not fully satisfied with this result since I do not fully understand what is going on. I guess another 8000 where this circuit works 'as is' will need to show me the light some other day. The B&O excitement just never ends!...;-)

Beomaster 2400: Bass, Treble and Balance Slide Controls

The tone controls and balance sliders of this Beomaster 2400 felt good and appeared to be okay.  Closer inspection of the Bass control slider revealed some suspicious looking contact attachments.
That being the case I opened up the Bass control slider and sure enough the plastic bridge that holds the contacts had broken tabs.

The failing contact mounts on these slider control bridges are present on almost every Beomaster 1900/2400 unit I have encountered so far.

Fortunately Martin Olsen makes replacement bridges that will fix these sliders. I will use another set to restore this Beomaster.

Here are the original and replacement bridges side by side.

Here are the slider contacts installed in the new bridge and the slider all cleaned up.

The Bass slider control put back together.

The Treble and Balance slider controls still had bridges with the mounts for the slider contacts intact.
I went ahead and replaced those bridges with new ones however so the controls would feel the same.

Here is the Tone Control & FM Tuner board with the repaired slider controls re-installed.

The lamp replacements are the next step.  After that this Beomaster should be ready for a bench test.

Beomaster 2400: Finished the capacitor replacement

The last two boards to replace capacitors on are the Volume Control and Tone Control/Tuning boards.
Before starting those however I finished up work on the main board by replacing the left and right channel trimmer resistors for the no-load current adjustment (which will be performed later).

Here are the left and right trimmer resistors for no-load current adjustment on the Beomaster 2400 output amplifier.

I like using multi-turn trimmers for the no-load current (or idle current) adjustment. They make fine adjustments much easier.  Here are the new, Bourns 250Ω trimmers next to the original trimmers.

Here are the new trimmers installed on the main board.

The small adjustment screw is also easier to get to for the adjustment procedure once the Beomaster is put back together.

Moving on...here is the Beomaster 2400 Volume Control board recap before and after pictures. There were six tantalum capacitors on that board that I replaced with WIMA MKS capacitors.

Last is the Tone Control and FM Tuning board.

As I saw on the main board, all of the electrolytic capacitors were either way out of tolerance or close to the tolerance limit. Here are some samples of the measurements I made of the old capacitors from this Beomaster.

The next tasks of this Beomaster restoration will be to check and repair the Bass, Treble and Balance slider controls.

Beomaster 2400: Recapping the Main Board

Whew! The main board of this Beomaster 2400 is fully recapped now. Seventy capacitors were replaced and there are still a couple of boards left that also need recapping.

Here are a couple of photos of the main board after the recap

The bulk of the restoration work was on this main board recap.

Now let's go back to the start of the recap and look at some of the details.
Here is the board before any of the capacitors were replaced. There are three metal shield boxes on the main board. Each shield box houses one capacitor that I needed to replace. The three metal boxes are highlighted in the picture below. The box in the top right is for the FM Front End, Tuner. The box below it is for the FM Detector. The third box just to the right of the Beomaster 2400 reservoir capacitors is for the remote control receiver.

For me the best way to do the recapping is to just systematically go through and replace each capacitor one by one. I tried removing a bunch at one time then spent a bunch of time rechecking the work to make sure polarities and capacitance values were correct. Doing them one at a time and keeping track of the polarity as each capacitor is replaced is a much safer way to go.

Looking inside each of the metal boxes we find the following.

I found that almost every capacitor, except for the reservoir capacitors and tantalum capacitors, was way out of tolerance. A lot of the capacitors were almost double the capacitance they were supposed to be.

The 10uF, 63V electrolytic capacitor inside the detector box measured over 20uF...and so did all of the other 10uF, 63V capacitors on the board.

The 10uF, 10V tantalum inside the Front End, Tuner box measured okay but I still replace it as I already planned to do in this full capacitor restoration.

The remote control receiver box also had a 10uF, 63V capacitor that measured over 20uF.

The Beomaster 2400 has a small "mod" board installed by Bang & Olufsen at some point in going from the Beomaster 2400 Type 2901 to the Beomaster 2400 Type 2902. The board changed components used for the tape record audio signal and has four capacitors that this recapping exercise replaced. The small board had to be de-soldered from the main board so the trace side could be accessed for the capacitor replacement.

There are three power supply reservoir capacitors that I replaced.
One reservoir is a 2200uF, 25V electrolytic capacitor that is used in the +15VDC supply.

The other two are 5000uF, 35V electrolytic capacitors for the ±31VDC rails used by the output amplifier section.

I replaced the originals with 5600uF, 50V capacitors. They each measured around 5200uF which is nicely in the 20% tolerance of the original value.

As I mentioned earlier, most of the capacitors on the main board were badly out of tolerance.
Here are some examples of what the old, original capacitors measured as I removed them.

The main board is where most of the restoration work takes place on these Beomaster 2400 (and 1900) receivers. I still need to replace the two trimmer resistors on this main board for the Left and Right channel no-load current adjustment. I will finish recapping the last two boards then do the trimmers.