Beogram 4002 (5501): Random Stop Issue Fixed - Defective Photo Resistor in the Spindle RPM Sensor

A Beogram 4002 (5501) that I restored last year developed a strange new problem: When playing a record the OFF (>>) function  would be triggered randomly during play as if someone pressed the >> key on the keypad.

Before I will discuss the fix, let's have a look at the circuit diagram (click on the diagram to get it in full resolution):

TR17 in the 'Electronic Switch' is responsible for triggering the >> function of the turntable. Whenever its base is pulled to GND its collector goes high to ~18V and then the arm is lifted and the carriage is driven home. There are three ways this can occur: Via two mechanical switches, one being the "End Switch" (ES) under the carriage and the other the ">>" key on the keypad. The End Switch is the one that sends the carriage back whenever it is driven all the way to the left (i.e. manually via the << key or automatically if there is no record on the platter and the arms go all the way searching for one).

The third way to trigger >> is via the end groove detection mechanism ("RUN-OFF STOP" on the diagram). This works via TR20 whose base is pulled up when the end groove is detected. That connects its collector to ground, and with that the >> function is activated.

The end groove detection mechanism works via an 'analog rotary encoder' that detects light flashes impinging on a photo resistor (OR2) from a bulb (OIL1) that shines its light though four holes in the carriage pulley as they pass by (creating light fluctuations on OR2). 

These light pulses short OR2 to GND causing TR21 to shut down, which increases the voltage at the collector of TR21 to about 19V. These voltage pulses (see oscilloscope trace schematic on the circuit diagram) charge C33, which, via the voltage divider formed by R88/89, pulls up the base of TR20, triggering the >> function.

It is interesting to to note that the bulb OIL0 only comes on when its switch to GND is closed. This switch is activated by the carriage when the tone arm gets close to the record label. This switch is activated by the same tab on the carriage assembly that activates the 17 cm (singles) set-down point switch ("B" in the service manual). This is the reason that the tab that activates B has a long flat shape. Anyway, this switch aims to prevent triggering the the RUN-OFF STOP mechanism when the carriage briefly moves faster between two tracks of a record when the carriage is still far away from the label.

Why does the mechanism not trigger RUN-OFF STOP when the carriage advances normally while playing the last track when OIL1 is already on? C34, R91 and D30 form a network that discharges C33 when there is no light on the sensor. This means there is a competition between charging during illumination episodes, and discharging when there is no light on the sensor. So if the pulley rotates only slowly and occasionally during playback of a track, discharging 'wins' and the base of TR20 is not pulled up high enough. But when the light flashes happen quickly like when the carriage moves fast pulled along by the end groove, then the charging mechanism wins and TR20 is turned on. The beauty of analog control systems!!

Ok, back to the 'Random Stop Issue': After verifying that the ES and OFF switches were working properly, I traced the signals from the base of TR17 into the RUN-OFF STOP circuit. This shows the oscilloscope traces that I measured:

The traces are assigned as follows:

• Yellow: TR17 collector (when this signal goes high the measurement was triggered - the graph shows ±5 sec around the >> event)
• Blue: TR20 base
• Green: TR21 collector
• Red: TR21 base

These traces show that before the >> event happens there is a random statistical fluctuation of the voltage at the collector of TR21 (instead of being close to GND if there is no end groove event). These voltage flashes obviously correlate with the signal at the base of TR21 (suggesting that TR21 is working properly). This leaves only one conclusion: The sensor OR2 randomly short circuits to GND causing these voltage spikes. These spikes can be enough to charge C33 and when the voltage one C33 gets high enough due to these random events, the >> function is triggered. This is seen in the blue trace, which shows that the voltage exceeds the ~0.6-7V threshold needed to turn on a standard silicon based transistor when TR17 is activated.

All this told me that OR2 was faulty. This meant it needed to be replaced. Since this is a special B&O part, which is not available anymore, I decided to design a 3D printed assembly to replace the entire bulb/sensor assembly on the pulley. This shows the setup after I implanted an LED to replace the light bulb last year:

The orange part contains the LED (Newark 78R6602) and its 2k current limiting resistor (Newark 26R3983). It was designed to stick onto the OR2 sensor housing. The current design replaces the entire encoder setup. These are the structural parts of the assembly:

Assembled they look like this:

And with LED, resistor and photo resistor (type "5516", ~10k resistance when dark, ~500 Ohm when illuminated) installed:

And after installation and in action:

Once the set-up was installed I measured the signal at the collector of TR21 when the LED came on:

This signal looks pretty much like what the manual demands. I tested the Beogram by playing some records, and it seems everything is working again! So I am hopeful that this fix took care of the issue.

Beogram 4002 (5513): Second Unit From Houston

Several weeks ago I finished the first of two Beogram 4002 (Type 5513) turntable projects from an owner in Houston. Now I am starting on the second one. When I first received these two turntables I pulled their platter motors and sent them off to Beolover for the motor rebuilds. He finished those and posted a blog entry about them.

I don't see any reason not to restore the platter motor if you are restoring a Beogram 400x turntable. It would be like never changing the oil in your car. It is important to have the platter motor rebuilt before doing the electrical restoration on the Beogram main board. Beolover's motor restoration includes full testing of the speed stability which includes determining the correct value of the main board 1C10 capacitor.

Here is the second Beogram 4002 (5513) unit opened up. You can see (lower left corner of the cabinet) where the platter motor is removed.

Like the first unit, there was quite a bit of oil throughout the turntable chassis. I will start with a dismantling of the Beogram so I can inspect and clean. Restoration of parts will come after that.

This Beogram was worked on before. A new phono cable was installed for this Beogram 4002 to be used with preamplifiers that take an RCA jack instead of the B&O DIN plug. The cable looks like it is in good shape and has nice RCA jacks. I am not sure I like the way the new cable transitions to the plug on the Beogram output board though. I might have to rework that to take the cable closer to the board plug before separating out the shields.

The previous Beogram 4002 unit I restored for this owner had transport lock bushings that were still in great shape. They were pliable and healthy so I did not replace them. I only added a little plastic/rubber conditioner on them.

This unit has the usual crumbling of the bushings. These bushings have become brittle and just touching them causes them to break apart.

To clean up the bushing debris that already started to crumble I removed the tonearm assembly from the cabinet.

There is quite a lot of oily film from too much lubrication of the tangential drive screw. When the motor turns the oil flies off the spinning drive screw and lands all over the inside of the Beogram cabinet.

The tonearm assembly is affected by the oil as well. 

There is still some more clean up on the tonearm assembly but the Beogram cabinet is all clean and ready to go again.

The next step is to finish cleaning up the tonearm assembly and performing the initial mechanical checks and adjustments.

Beomaster 8000: Shipping Back Home

After a couple of weeks of functional testing it is time to pack up this Beomaster 8000 receiver and send it home. I hooked up an FM antenna, a Beocord 9000 and a Beogram 8002 as music sources to the Beomaster. It has been a joy playing the system in my workshop while I clean up to begin work on some new projects.

This Beomaster 8000 unit came to me packed better than any I had received before. The owner had it packed in a custom kübox and filled with special foam padding that was molded to the shape of the beomaster.

Packing it back up was just a matter of putting the Beomaster in a plastic bag then onto the bottom foam mold.

The rest of the side and top mold pieces were added and this box is ready to take to the shipping office.

The shipping place will add some banding straps to the kübox and off the package will go.

I am sure the owner will love this Beomaster 8000 again...as I have for the past few weeks :-).

Beogram 4004 (5526): Final Adjustments and Test Drive with Volker Kriegel on MPS

My current Beogram 4004 (5526) restoration project is coming to a close with making a few essential adjustments and tying up some loose ends. First, I replaced the original black plastic carriage pulley with a new one machined from aluminum:

Then I replaced the metal sheet screw that holds the tracking aperture in place

with a nice stainless steel 2 mm bolt that can be tightened with a ball hex key...much better than the original Philips head to get it fully tightened to survive the rigors of shipping in the calibrated position. This shows the new bolt in place:

Then it was time to adjust the tracking feedback sensitivity:

The intensity adjust trimmer (small blue box) on the Beolover tracking sensor light source comes in handy for fine-tuning the mechanism after coarse adjustment of the aperture and sensor positions.

Then I adjusted the floating sub-chassis to be horizontal, did the arm to platter distance adjustment while making sure that the platter is flush with the surrounding aluminum panel. Then it was time to adjust the arm lowering limit that the needle would miss the ribs if it got accidentally lowered onto the platter without a record present. An important fail safe in case the record detection mechanism fails at some point in the future:

Before playing any turntable for the first time it is important to calibrate the tracking weight. I usually start out by replacing the flimsy locking clip that holds the counter weight adjustment screw in place with a nut to be able securing it permanently once the calibration is done. This shows the original condition:

and with the M3 nut in place:

Then I calibrated the weight adjustment wheel to be accurate around 1.2g, the tracking weight for most B&O cartridges:

This is preferably done with a small digital gauge, which are very precise these days due to advances in integrated sensor chip design (even if they are cheap).

After giving the aluminum surfaces a deep clean it was finally time to present this Beogram in its full glory and play it for the first time. I selected a recently acquired vinyl by the awesome German MPS label: "October Variations" by Volker Kriegel and the Mild Maniac Orchestra, which they recorded in 1977.

In my opinion this is among Kriegel's best albums for its consistency and wealth of great melodies and jazzy fusion sound. The second track on the first side "Ballad Garden & Palm Dreams" is my favorite...perfect for taking a Beogram DC motor apart or soldering SMD components...;-). I will now play this Beogram for a bit longer, and then it will be time for the trip home to its owner.

Beogram 4002: Restoration of a Very Oily DC Platter Motor

I recently received a Beogram 4002 DC platter motor from Berlin, Germany for restoration. I immediately noticed that the motor was quite oily on the outside, probably from an attempt to lubricate it to enhance RPM stability. This shows the motor as received:

I disassembled it to extract the bearings for oil infusion:

The bearings are on the black pad up front. The inside of the motor was also very oily, but of course this does not help the bearings, since they need to be infused with oil under vacuum. This is necessary to pull out the air in the pores of the Oilite brass material that the new oil can penetrate into the bearing. This shows the bearings submerged in motor oil after pulling a vacuum:

The bubbles rising from them is the air exiting the bearing. This process usually takes 24-72 hrs depending on the state of the bearings.

After the bubbling stopped I extracted the bearings from the oil and put them on a paper towel to wick the excess oil away:

Then I re-assembled the motor again and implanted it into one of my 4002s. Then I ran a 24 hrs RPM stability test with the BeoloverRPM device, which allows the logging of the RPM for extended periods of time:

The BeoloverRPM is available to other enthusiasts. Just send me an email or use the contact form on the right. The very unstable blue curve on the graph below is what I measured:

This indicated that I was not done yet with this motor. First I thought the motor probably needed new spark suppressors since that usually causes large downward spikes, but the overall pattern looked somewhat different. Finally, I remembered that I had a similar issue when I just started developing the restoration procedure for these motors a few years back. Initially, I also thought that simply putting a lot of oil in everything would probably help. But when I squirted some of the oil on the commutator of the motor (to 'further reduce friction'...;-), the motor performance dramatically worsened, like in this case. It seems that the oil creates an insulating layer on the rotor impeding the current flow though the rotor, which causes RPM reductions.

So I took the motor apart again and cleaned rotor and brush carrier (together with the pulley - it was also oily and I worried that I would not be able to glue it to the shaft once done) in my ultrasonic cleaner:

I put it in warm water with some dish washing detergent. Immediately after starting the cleaner the water became very cloudy.

This indicated that an emulsion was forming between the oil on the parts and the water. I replaced the water three times and ran the cleaner a couple minutes in each batch. Then the water finally stayed clear and I gave it a fifth run in pure ethanol to get the water out of the rotor windings etc...The I put everything out for drying:

After re-assembling, I tested the motor for another 24 hrs and I was able to measure the nice red curve in the above graph. So everything is fine now with this motor, and it is ready for return shipping to Berlin!

Beogram 4004 (5526): Installation of A New Output Cable

After replacing the light bulb in the sensor arm with an LED assembly it was time to look into the RCA plug 'conversion' of this Beogram 4004 (5526). This shows the output cable in the condition when I received this Beogram:

Not a very inspiring sight. The blue and yellow leads are the Beolink signal lines that allow controlling the 4004 via a Beomaster 2400. We decided to return this 4004 to its original DIN7 output scheme. Originally the 4004 came with a convertible male DIN7 plug, which can be turned into a standard DIN5 by unscrewing the two pins that are extra in the DIN7 format. This shows a plug as seen on a 4004 I restored a few years ago:

The two outer pins were removed so that the plug essentially acted like a standard DIN5 phono output. Unfortunately, such DIN7 plugs are not available anymore. A solution I came up with is to install female DIN7 plug that has all the signals on it. If extended with a DIN7 cable it turns into a DIN7 male output that will properly work with a Beomaster 2400. Extend it with a DIN5 or a DIN5-to-RCA adapter and a standard non-B&O phono output is achieved.

This shows the female DIN7 plug during the installation:

Note that the leads need to be soldered in a mirrored pattern compared to that applied for a male DIN7 plug, since male-male DIN5 and DIN5-RCA jumper cables mirror the leads for symmetry reasons.

This shows the DIN7 connected to a DIN5-to-RCA jumper cable:

The system ground connection is on a separate wire (not visible on this photo) that it can be connected to the GND terminal of the RCA input amplifier.

Beomaster 8000: Wrapping Up The Performance Tests

The current Beomaster 8000 restoration project has been going through quite a bit of testing in the last few weeks. Its owner mentioned that this Beomaster would occasionally come on by itself. Sometimes in the middle of the night it would come out of Standby mode and start playing music. It's always nice to listen to a Beomaster 8000 but you do want it to obey your commands.

I was confident that the restoration work I performed would address that issue but to test it I left the Beomaster plugged in on Standby mode for three full days. No hiccups and it never went into play mode by itself.

It was back to more listening tests while I set up the bench for some performance tests.

As Beolover recently posted, we don't really have the specific test information and test equipment that Bang & Olufsen used when they tested these Beomaster 8000 amplifiers with respect to what is in their technical specifications. On amplifiers there are usually only two service manual adjustments. No-load current (idle current) and DC Offset. That is the case with the Beomaster 8000. Other than those two adjustments a restored amplifier is expected to perform at the manufacturer listed specs for their design. We can use their published specifications as a guide as we collect our own measurements using test equipment available to us today. With these new measurements we will have measurable and repeatable values to compare our amplifier restorations.

Both Beolover and I use the QuantAsylum QA400 Audio Analyzer to make measurements. We also both use 8Ω fixed loads (power resistors mounted to large heatsinks) as dummy speaker loads during the tests. A small 0.08Ω to 0.1Ω resistor is added in series on each speaker load to allow direct measurements to the QA400 analyzer.  That is because the full voltage across the 8Ω load would damage the analyzer inputs. I have a pair of  QuantAsylum QA190 low noise, differential probes that allow measuring directly across the 8Ω (actually 8.08Ω in my case) dummy load. It should be noted that the updated QuantAsylum QA401 analyzer has built in differential inputs so the external probes are no longer necessary.

Here are some pictures of my test setup.

To run a direct measurement to the QA400 analyzer inputs from the 0.08Ω series resistor I use a coax cable with BNC connections for the analyzer and mini-grabbers for the 0.08Ω resistor.

Here is the Beomaster under test with the QA400 analyzer in the background and its measurement screen on my lab computer.

The QA400 analyzer, as described in Beolover's post, outputs a tone burst test signal for single frequency stimulus tests and a square wave impulse for frequency response test stimulus. I can run both left and right channel measurements at the same time as the analyzer provides the input to the Beomaster Tape 1 or Tape 2 left & right channel source inputs.

This test setup is easy to use and I can connect a bench DMM to the speaker loads to measure the actual voltage put out by the Beomaster output amplifier. One problem I run into is occasional noise that mucks with the measurement device sometimes. Usually down at the line 60Hz frequency. In my test setup the direct measurements across my 0.08Ω series resistor just don't provide as clean a measurement as the QA190 differential probes. I also have to monitor the QA190 probes though as they are battery powered (so the batteries need to be fresh).

For these Beomaster 8000 performance tests I decided to first grab a THD measurement for a 1KHz input signal right before it goes to the Beomaster Output Amplifier board. The audio signal goes from the input source (Tape 1 or 2) through the Preamplifier then to the Filter & Tone Control board. The path from the Filter & Tone Control board to the Output Amplifier is via connector 4P24. For this low voltage test I did use some direct probes with alligator clips to the QA400 inputs and not the QA190 probes. That is because of the space available to connect probes to.

Here is the THD measurement for a 1KHz input signal at 4P24.

The left and right channel THD is very low after going through all of the preamplifier and tone control circuitry.  Though it is very low (nearly -100dB) you can see the effect of some 60Hz noise picked up by the measurement cable.

Next was the 1KHz signal THD measurement after the Output Amplifier. This is measured across the 8.08Ω fixed speaker load at different output levels. It should be noted that during these tests I took the opportunity to calibrate the Tape 1 and Tape 2 input level trimmers again so I could make sure they are equal. I also tried to set them where I could set the Beomaster volume control on a level that produces 100W across the 8.08Ω load.  The result of that is a Beomaster volume level of 5.4 producing my 100W test output. The next volume level up (5.5) with this setup causes the clipping light to illuminate.

Here are left and right channel THD and SNR measurements at volume levels of 5.4 (~100W) and 5.3 (~67W).  I also turned the analyzer A Weighting mode on as the B&O specs use that.

The next volume level up (5.5) on the Beomaster puts the output amplifier into clipping so THD of 0.01% at the rated maximum power into 8.08Ω is very good. The next volume setting down (5.3) drops the THD to 0.008%.

Next is a check of the Beomaster frequency response. Beolover also describes this measurement with the QA400. I am showing the measurement picture produced by the QA400 measurement software (with a few of my own text comments added). It has frequency markers for three points I wanted to focus on (1KHz, 10KHz and 20KHz).

My results are pretty similar to what Beolover got although it appears the QA190 differential probes helped my measurements out on the low frequency noise. The drop at 20KHz was right around 1dB which is more than what the B&O published specs state but then our measurement methods are not exactly the same. Between these Beomaster performance results and the last few we have done it compares with very similar results. The results are definitely a pass for this Beomaster receiver.