Beomaster 1900 Type 2904: New Project From the East Coast

It is time to switch my workbench from Beomaster 8000 and Beogram turntable mode to Beomaster 1900 and 2400 restoration mode.

First is a Beomaster 1900 Type 2904 receiver. The 1900 was a very popular receiver for Bang & Olufsen and was produced from 1976 to 1982. I remember seeing them in the local audio shop (I'm dating myself) and was impressed by their modern touch buttons. That was quite something back then. Of course that went along with the super low profile, sleak Jacob Jensen design. A really nice design that held up with time. It is not surprising that so many owners still want them working today. However, at over forty years old some restoration is required.

That is what the task is with this one. This Beomaster 1900 is in decent physical shape. It needs a little cleaning and polishing. The rosewood veneer trim has some places where it is coming loose. I will have to be careful with those and see if I can save it.

Looking at the bottom side of the Beomaster I see that all four rubber feet are missing.  It looks like Martin Olsen is carrying remanufactured replacements for those so I will be placing an order for those.

I was a little leary about what I would find when I opened up the cabinet. In carrying the Beomaster to the workshop I could hear rattling inside.

Fortunately the rattle was just from pieces of the broken feet. The rubber material is quite deteriorated.

Peeling back the rest of the Beomaster 1900 first layer showed nothing to indicate there are any major problems. I was glad to see that the plastic masks for the bass, treble and balance indicators all look good on this unit. The previous Beomaster 1900 I restored was a Type 2903 and the indicator masks had started to deteriorate from the heat of the illuminating lamps. I have printed replacement masks but those won't be needed on this unit.

Quite a nice unit. Definitely worth getting back to working order.

I will continue to disassembly the Beomaster so I can clean and get to all of the electrolytic capacitors that will be replaced. I will change out the lamps. The volume indicators and FM tuning indicators will get new incandescent lamps as their properties play into the circuit function. The bass, treble, balance and source selection indicators will be switched to LED replacements.

The bass, treble and balance slide pots will be cleaned and lubricated. A common failure in these receivers are the plastic mounts for the slide control contacts so I will check that those are good. If not I have Martin's repair kit for them.

Beomaster 8000 From Canada: Taking care of some loose ends before shipping home

The Beomaster 8000 receiver from Canada that I blogged about here has been undergoing extensive testing in my listening room.  The input sources (FM, phono, tape 1 and tape 2) have been exercised for countless hours. This unit has performed great.

When it was time to unplug the Beomaster from my Beosystem 8000 listening room setup I got a surprise when I unplugged the Beogram 8000 cable. The Beomaster 8000 Phono DIN jack came out with. Part way anyways. The wires in the Beomaster source input connector box kept the DIN jack from getting too far.

I hadn't noticed any problem with the Phono DIN jack previously but now that is was out I could see that one tab (of two) that prevents the DIN jack from pressing into the connector box was broken and both side tabs, that keep the DIN jack from pulling out, were broken.

Here are the pictures.

Fortunately for this type of problem Beolover makes a 3D printed repair kit.

I had used the 3D printed parts for the top and bottom edges of the phono DIN jack on the Beomaster 8000 from Texas.  That DIN jack had intact side clips though so the top and bottom Beolover parts were all that were needed on that project.

This Beomaster 8000 phono DIN jack had its bottom tab intact but was missing the top one. In addition, both the left and right side clips were broken.

The next two pictures show the installation of the phono DIN jack left side stop piece. The original side clips were a flexible, spring like plastic tab. A replacement part that works as a spring clip isn't really an option. Beolover came up with side pieces that glue to the DIN jack housing in such a way that the DIN jack cannot be pulled out (the top and bottom replacement pieces are designed to not let the DIN jack press down into the connector box).

Note that just one side (the left side) was installed first. Since this DIN jack still had its original bottom tab I only needed one of the Beolover parts for that part of the assembly.

With the left side stop piece glued in place and the top Beolover replacement part installed I was able to push the phono DIN jack back into the source input box.

The final step was to glue in the right side stop piece.

The phono DIN jack was now securely mounted where it will no longer fall in or pull out of the source input box.

Here is the completed repair.

Now back to another round of listening tests before the packing up for shipment back to Canada.
That step cannot be skipped. Whenever working with the source input connectors it is really easy for one of the small wires to break.  I do not want to risk shipping this Beomaster back to Canada only to discover there is some problem I could have easily caught with a little more testing.

Beogram 4002 DC Motor Restoration: Repair of a Damaged Pickup Coil and Oil Infusion

I recently received a Beogram 4002 DC platter motor from Australia for repair and restoration. This shows the motor as received:

I was told that the motor would always run at a high speed. This immediately pointed towards a damaged RPM feedback coil. If there is no feedback signal back into the circuit that tells it how fast the motor is running, the circuit puts the pedal to the metal, so to speak, and the motor runs as fast as the circuit can accelerate it.

I took the motor apart, and it became immediately clear that 'human creativity' had occurred. This shows the bush carrier of the motor. The brushes were installed wrongly. Their contact terminals need to be underneath the contact past and not above like I found it in this motor. Also there is some solder burn next to one of the terminals.

The two small square objects are the pickup coils. They send an AC voltage back to the circuit via the white lead. This AC signal is generated by energized rotor coils passing across the coils, which causes the induction of a current due to the changing magnetic field. The frequency of the current is proportional to the RPM of the motor (see oscilloscope trace below).

One can measure the resistance through these coils, which are connected in series, by measuring between the white and blue jumpers. When everything is o.k., about 41 Ohm should be seen. This motor yielded open circuit, indicating that the connections of one or both coils were ripped out. This happens very easily while taking these motors apart, since the magnet wire used on these coils is of a very thin gauge.

I completely disassembled th motor to get to the bearings for infusion. This shows the parts spread out:

I immersed the bearings (two small donuts on black pad) in motor oil, and pulled a vacuum. Immediately vigorous bubbling started:

This indicated that the vacuum was drawing air from the porous bearing material, creating space for oil to diffuse into the bearing instead.

While the infusion process took its course, I looked into the pickup coils. One of them was still o.k. and I was able to measure 20.5 Ohm across it. The other, however was open circuit. I looked at ti under the microscope trying to identify the loose ends where the wire was broken. This is a  sisyphean task even if nothing happened but pulling a bit too hard on one of the leads on the coil. In this case, a repair effort seemed to have made things worse, and several windings had been cut with a tool, probably while trying to get some loose ends for splicing together again. I had to unwind several loose ends from the coil until I finally found an end that would connect to the other contact lead of the coil. I measured about 15 Ohms:

Luckily, one of the disconnected parts of the cut windings was a bit longer, yielding close to 3 Ohms:

I connected the main winding and the fragment and this resulted in a 18.5 Ohms coil:

I decided that this would be enough windings for getting a decent feedback signal and I fixated the mess in place with some soft double sided tape to insulate the solder spot from the motor housing and to keep things neatly together:

At this point I let things be until the bubbling of the bearings stopped after about 48 hours. I extracted the bearings from the oil:

Then I put the coils and the lower bearing back into the brush carrier:

The top bearing was installed using a 3D printed tool for pressing the bearing retainer tabs back into the original shape:

Then I put the motor back together and measured the feedback signal while the motor ran freely at 5V:

This looked like a healthy feedback signal at about 150mV amplitude, enough to satisfy the motor control chip. This indicated that the coil repair had been successful.

Encouraged by this result I put the motor in one of my Beogram 4002s and ran a 24 hrs RPM test with the BeoloverRPM device:

The BeoloverRPM allows the logic of the RPM for extended periods of time, a great feature for pinpointing RPM stability issues. After about 24 hrs I extracted this curve:

This is a pretty good result for a Beogram 4002 DC platter motor. The small peaks are a result of minute friction changes, while the bearings get used to their new alignment etc...This usually goes away after a playing the deck for a while. At any rate these fluctuations are much smaller than what human can identify, i.e. this motor is back in business.

Beogram 4004 (5526): Hood Restoration

While I am enjoying listening to the Beogram 4004 (5526) that I recently finished up, there was still its scratched hood that needed a good polishing. This hood showed the usual scratching, but was otherwise undamaged. No cracks in the hinges etc..., i.e. a perfect starting point for a restoration. This shows the original condition:

I was able to feel some of these scratches with a fingernail, i.e. this hood needed an equalizing  sanding before polishing it back to translucency. Since the scratches were not too bad, I started out with 320 grit sand paper and went at the surface until all the scratches were no longer visible:

Then it was time to polish it with ever finer grit paper. After 6 steps it looked pretty good again:

Most of these hoods had rubber bumpers installed at the front corners, but they are usually broken off. I usually drill them out with a 2mm bit:

And then I glue snippets of a 2mm O-ring into the cavities:

After the glue is hardened I cut them to 1 mm length with a razor blade over a 3D printed 1mm thick washer:

This hood is ready to be installed! Soon this Beogram 4004 will travel back to its owner!

Beogram 4004 (5526): A New DIN Plug, Final Adjustments and a Test Drive with Bob James!

After restoring the electronics and the motor of the Beogram 4004 (5526) that I am restoring right now, it was finally time to make some final adjustments, clean the aluminum surfaces, add a new DIN plug and then enjoy a first vinyl on it. Before making the adjustments, I added a M3 nut to the counterweight screw to enhance the stability of the weight calibration. This shows the original setup with a circle and some paint to hold things in place:

I added the M3 nut and a washer

and balanced the arm for zero weight.

Then I adjusted the tracking weight with a digital scale:

Then I adjusted the platter height and then the arm lowering limit that the needle misses the lower areas of the ribs (located at the set down points) by about a mm:

This is an important adjustment to ensure that the needle does not hit the ribs should it ever be lowered onto the empty platter due to a malfunction in the record detection circuit.

Then I adjusted the floating chassis to ensure a flush alignment between platter and surrounding aluminum panels. 

The final adjustment was the tracking feedback. The final fine tuning was done with the brightness adjustment trimmer on the LED light source for the sensor:

The last thing to do was to update the corroded DIN plug:

The Beogram 4004 came with a convertible DIN7 plug with the two 'additional' prongs removable that it could be turned into a DIN5 when used with non-B&O equipment. Pins 6 and 7 on the DIN7 carry the Beolink signal. This allowed using the remote control of a Beomaster 2400 to start and stop the connected 4004. 

Unfortunately, these convertible DIN7 male plugs are not available anymore. I usually install a female DIN7, which allows preserving the original DIN7 functionality, while allowing the easy conversion to DIN5 and RCA with a suitable adapter cable. 

This shows the DIN7 installed:

and connected to a DIN5 male-to-male jumper, which allowed me connecting the 4004 to my Beomaster 6000 4-Ch for testing.

I selected a just acquired vinyl record by Bob James: Lucky Seven: This album was recorded in 1979 and it has a few very nice smooth jazz funk tracks on it, most notably (IMO...;-) "Blue Lick" on the A side. Of course, I cleaned it ultrasonically with a CleanerVinyl Easy6 setup after I received the vinyl from a Discogs seller.

This shows the 4004 in action together with Bob:

Beautiful! And so far it is performing perfectly. The new 1985 motor runs very stable and it is very quiet.

Beogram 4004 (5526): Adaptation of a 1985 Platter DC Motor and an Exciting New Motor Failure Mode

The Beogram 4004 (5526) that I am currently restoring came with a strange platter motor installed with some hot-wiring:

It had a 1985 production date on it:

I wondered from what Beogram this motor may have been extracted. Dillen of BeoWorld.org helped out with his encyclopedic knowledge of B&O details. He stated:

"That motor is part# 8400098

It was used in Beogram 1700, 2200, 2202, 2400, 2402, 2404, 3400 (type 5726), 3404 (Type 5727), 6000 (Type 5751,5753,5754) and Beocenter 5000".

I tested the RPM and this motor indeed ran at 33 RPM, but not very stable. So far so normal (most DC platter motors have dry bearings at this point in time, i.e. they often do run not very smoothly). I had a closer look at the platter motor control circuit and it had been modified by replacing resistors R16 and R17 with wires and the two RPM trimmers (normally 5k) had been replaced with 2.2k trimmers.

This indicated that this motor needed a different Schmitt trigger timing in the motor control circuit. This pointed to a different RPM feedback mechanism in this "younger" motor.

I decided to take the motor apart and have a look, and restore the dry bearings while in there.

This shows the extracted motor:

The first difference to note is that it has four wires instead of the normal three. These wires were connected into the regular three prong plug. I removed the metal pulley and removed the outer housing:

The plot thickened as it became obvious that the red and blue leads went into one end of the motor, while the two yellow ones into the other.

I opened the motor up:

Immediately it became clear that this motor was different. The gear like wheel at the bottom end of the rotor obviously is an updated RPM feedback mechanism, which is not based anymore on induction of a current in two small coils under the rotor, as the powered rotor poles travel across them. This motor has a more modern type of mechanism, based on a magnet disc with 39 field shaping 'poles' around it. This disc runs next to a coil that is connected to the two yellow wires. This causes a feedback signal that has a much higher frequency per RPM than that of the original motors.

Before taking it apart, I ran the motor from a bench supply and measured the feedback signal:

This shows the earlier measured signal from an original motor at the same 5V (i.e. at a comparable RPM):

Note the different time and voltage scales. The signal of the 1985 motor has an approx. amplitude of 1V and the frequency is 268Hz. The original motor has a signal of only about 0.15V and its frequency is much slower at 36 Hz. This about 7x faster frequency of the 1985 motor is a consequence of the larger number of field shaping poles on this motor compared to the two coils and three rotor poles in the original motor. And the larger signal comes from the much closer proximity between pickup coil and magnetic field from the disc. So an all around improvement.

Back to the bearing restoration: This shows the motor completely apart with the bearings extracted (two small donuts on the black pad):

I immersed the bearings into motor oil and pulled a vacuum. Immediately the usual bubbling started, indicating air being extracted from the porous bearing material. This allows fresh oil to diffuse into the bearings:

After about two days the bubbling stopped and I extracted the bearings from the oil:

Then it was time to re-install them in the motor. I had to realize that my standard tool to press the bearing retainer back into shape did not fit due to the different feedback mechanism. I redesigned the part:

And this allowed me to put the bottom bearing back into place:

The I inserted the top bearing:

In this motor the brushes are located on the top end.

I put it back together and measured its RPM stability with the circuit modification in place to see how it would perform. This is the curve I measured after about 8 hrs:

This did not look great at all. If such such sudden RPM breakdowns are still there after re-infusing the bearings, it is usually a sign of one or more bad spark snubbers on the motor coils. So I took it apart again, and replaced the snubbers with modern TVS devices. This shows the rotor with the original snubbers (devices with red dots) in place and the new ones prepared for soldering on the black pad:

On these motors one cannot just remove the entire snubber assembly and put the replacements between the solder points (see here for a comparison with the original motor types). There is not enough space between rotor and brushes due to the new feedback mechanism. So I left the wiring in place and just cut the snubbers out with a wire cutter and soldered the new ones onto the wires..

Then I put the motor back together and ran another test:

This second surgery apparently did not help the patient at all! It even got worse. So I realized that there must be a yet another motor failure mode! Exciting!!

I took it apart again, and measured the resistance across the coils. Two of them measured the usual ~20 Ohm, but the third one showed only about 18 Ohm. This seemed a bit odd. While measuring I noticed a fluctuation of the value, so I wiggled the wires a bit and was able to reduce the resistance to about 5 Ohm with that. After a bit of more messing around, it became clear to me that one of the snubber wires apparently had chafed through the insulation of the magnet wire that is on the rotor poles and that way short circuited part of the coil whenever the contact intensified. My exchanging the snubbers must have made this issue worse as is visible from the increased RPM breakdowns.

I put a piece of double sided tape between the snubber wiring and the coil to insulate them from each other:

Now the coil measured the usual ~20 Ohms. So it was time for putting it back together and run another test:

This curve now looked pretty normal! So this motor is back in business. 

What is still did not like was the way the motor control circuit had been adapted for this motor. The person who did the modification realized that the RC time constant for the Schmitt trigger needed to be sped up to account for the much higher feedback frequency, and so he reduced the R component by removing the resistors and inserting lower resistance trimmers. This led to a faster response of the circuit, but with the unwanted side effect that the user accessible RPM trimmers above the keypad were now much more sensitive i.e. their resolution was much reduced, while their range was much larger than the ±3% prescribed by the scales. This was not Beolovely at all! When we make modifications or improvements, we try to make them in a way that preserves the original user experience.

It immediately occurred to me that the 'feel' of the RPM adjustment mechanism could be preserved if the C component were to be modified instead, while leaving all the resistances original. Thereby the ratio between the RPM trimmers to the rest of the resistance would stay the same after the modification. 

At this point I felt it was time to seek out the motor control circuit of a B&O product designed for this motor. I downloaded a few service manuals from the list Dillen gave me, but they all had monolithic motor control circuits in their diagrams, i.e. no information about the capacitances in these circuits.

But finally I found a manual with more information, the service manual for the Beocenter 2800/4600 series manufactured 1978 - 1980, concurrently with the Beogram 4004. Apparently the 4004 still carried the old motor for legacy reasons, while the Beocenters already seem to have had a motor with the updated feedback installed as is evident from the much changed capacitances in the control circuit. But it still used the old discrete circuit. This is the circuit shown in the Beocenter manual:

I marked the three capacitors that are different compared to the standard 4002/4 circuit, which is shown below:

The resistances in the Schmitt trigger RC legs of both circuits are similar, but the RC capacitor (C3 in the 4002/4 diagram and C2 in the marked up Beocenter circuit) are much different (220nF vs 68 nF). This makes the Beocenter Schmitt trigger much faster.

At this point it may be a good idea to briefly review how this circuit works. The AC feedback signal (see oscilloscope traces shown above) is fed into pin 1 and amplified with a large gain into a square wave. This square wave is fed into a non-symmetrical (two thresholds) Schmitt Trigger. When the feedback square turns the trigger on its output goes high (pin 6). This signal charges C8 via R19, and this voltage is fed into the motor amplifier via pin 5, and the motor receives power.

At the same time the trigger output signal on pin 6 is fed into the RPM controlling RC elements 2R2/R17/R15/C3 (33 RPM) and 2R1/R16/R14/C3 (45 RPM). Depending on the RPM setting C3 is fed via one of the two resistance chains, which raises its voltage. At a certain voltage the the upper threshold of the Schmitt trigger is triggered and its output voltage on pin 6 goes low. Then C3 discharges and during that time the Schmitt Trigger output remains low until it can be turned on again by the motor feedback square. This essentially creates a pulse width modulated (PWM) signal that allows controlling the energy flow into the motor.

This timing mechanism is the reason why the RC time constant needs to approximately match the motor feedback frequency, otherwise the motor gets too little power and runs only weakly. This is what probably prompted the original modification. The modifier turned the trimmer pods all the way down to zero, but was not able to get the motor up to the right speed. Then he had the idea to remove the fixed resistances and put in smaller trimmers to get up to 33 RPM.

I performed a number of tests with different capacitances for C3, C7, and C10 in place to find the best values. In order to do that with the table running, I broke the three capacitors out from the main PCB and ran wires to a breadboard:

This allowed me to exchange the caps without putting the Beogram on the bench for soldering. It turned out that 39nF for C3 gave the RPM trimmers a similar range like for the original motors. C7 seems to have little influence on the RPM stability, so I left it at 1nF like in the Beocenter circuit. C10 of course needed adaptation, too, since it controls the responsiveness of the feedback of the motor drive amplifier together with C9. The Beocenter value 47nF seems to work well, i.e. I kept it, too. This shows the relevant section of the circuit with the original capacitors in place:

And with the new ones:

I think it is a nice option to have these newer motors as a go-to for keeping the 4002/4s running. Sometimes the motors cannot be restored, and there is no obvious modern replacement motor available. This shows the motor wiring in more detail after I cleaned it up:

The yellow feedback wires need to connect to the white and the red leads of the plug. The original motor only has three leads since the yellow-red connection is made already inside the motor.

While working on this motor mod, I also replaced the electrolytic capacitors and relays on the main and output PCBs. This shows the main PCB after restoring it:

And the output board:

This shows the output relay area in detail:

As usual I installed a switch that allows connecting the signal and system grounds in case there is a humming issues when using RCA adapters. We are getting close to listening to some nice vinyls on this Beogram!