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Beogram 4002: Restoration of DC Motor Video Published - Check It Out!

By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...

Saturday, March 24, 2018

Beogram 4000: Cosmetic Items, Final Adjustments, and a Test Drive with George Duke on MPS!

The restoration of the Beogram 4000 from Germany is nearing its end. I adjusted the sub-chassis and arm height and then the arm lowering limit:
The correct adjustment of the arm lowering limit is an important safeguard against stylus damage should the record detection mechanism ever fail. The next step was the calibration of the tracking weight. I always install a M3 nut and washer to bolt the counterweight position safely into place, which is much more stable than the original setup with the locking washer. This shows the original setup:
and the M3 nut in place:
Once that was done I calibrated the arm weight with a digital scale:
The next step was to adjust the tracking feedback:
The blue pot that allows the adjustment of the tracking sensor LED comes in handy for fine tuning of the feedback. 
The final adjustment was the motor voltage. This shows the oscilloscope traces for 33
and 45 RPM:
Nice smooth sine curves as they should be!

The final steps were to replace the scratched and dented main aluminum platter and the arm cover. Conveniently, someone recently parted out a 4002 5501 Beogram and sold the arm cover on ebay, which I snapped up for this project. The early 4002s have the same arm cover like the 4000, i.e. it states "tracking weight adjusting" (instead of "tracking force adjustment" like one the later 4002s) on it. The aluminum platter was exchanged with a pristine one from a donor 4004 that I recently acquired.

And finally, it was time for a test spin. I selected a recently bought record by George Duke: "Liberated Fantasies" on the awesome German MPS label (MPS stands for Music Produziert im Schwarzwald - music produced in the Black Forest...;-). I really like the bunch of records that George Duke recorded in Villingen...in my opinion the most creative part of his career. Here is an impression:
Considering the initial state of this shipping-damaged Beogram 4000, it was an especially happy moment to see and hear it executing flawlessly. Mr. Duke is smiling, too!..;-)

Thursday, March 22, 2018

Beomaster 8000: Starting with the output amplifier boards and power suppy

I like bench testing the Beomaster 8000 output amplifier boards by themselves on the test workbench before using them back in the Beomaster. It is easiest for me to do that by removing the output amplifier assemblies to both work on and test. After doing that a few times I realized that it is also easier to work on the ±55 VDC reservoir capacitor replacement with the output amplifier assemblies out of the way. Sometimes it takes a while for the obvious to sink in :-).

So on this Beomaster 8000 project I am starting with removing the left and right channel amplifier boards plus the Beomaster power supply components.

Turning the Beomaster around I removed the cover for the output amplifier heat sinks.

Looking at the Beomaster insides again I noticed a couple of problems I didn't catch the first time I looked.

The small transformer has quite a bit of corrosion on its mounting hardware. That would suggest this unit spent some storage time in a damp location. I can also see some deteriorating damping grease leaking around the lid damping assembly (a very common problem with these units).

I will make a note to clean up the corroded mounting hardware and I will disassemble the lid damping assembly to clean it out and install new damping grease. 

Removing the output amplifier assemblies opens up the way to the large ±55 VDC reservoir capacitors.

On the output amplifier boards themselves I can see they were worked on before. In addition, at least one of these boards is not an original board for this Beomaster 8000. Maybe both came from another Beomaster. That doesn't really matter too much but it does make you a little curious about what this receiver has gone through.

In the above pictures of the two output amplifier assemblies you can see that the flat metal springs that hold the TIP 141 and TIP 146 Darlington transistors to the heat sink are different. There are other subtle differences you can observe between the boards. Remember that these two boards are the same B&O part. They are interchangeable between the left and right channels.

Here is a closer look at the boards.

The right channel has some different transistor packages for 5TR201 through 5TR203 (BC546B) and 5TR204 (BC556B). Some resistors have also been replaced on the right channel board. Both boards show some deterioration on their emitter resistors (5R236 & 5R237).

Both boards show some heat darkened places on the boards. The right channel board has a really bad spot. That board also has the replaced resistors.

The trace side of the boards show both boards have had repair work performed on them.

I have a number of Beomaster 8000 spare output amplifiers to compare to. Most of them are still in their original state so lets compare these two boards with some originals.

All of the boards have darkened boards underneath the resistors. A lot of heat over time. It seems a lot of Beomaster 8000 units had a rather hard life. That wasn't the case for the previous Beomaster 8000 project though. The output amplifier boards on the unit were very clean and untraumatized. 

Perhaps it is in the working environment these various Beomaster components lived in. I know that the previous project Beomaster unit was from an excellent environment and when the receiver was in use it was part of a complete Beosystem 8000. 

On this current Beomaster 8000 unit I noticed that the TIP 141 & 146 Darlington transistors that mount to the heat sinks had very little thermal grease actually on the transistor case. 

Whatever the cause is I will get these current amplifier boards fixed up with capacitors rated to at least 105°C and reworked transistor thermal protection.

Tuesday, March 20, 2018

Beogram 4000: Glueing a Delaminated Platter Back Together and Installation of New Cabined Guidance Washers

The Beogram 4000 that I am restoring right now unfortunately had suffered some transport damage due to the inept packaging job of the ebay seller. Since he did not take out the platter, it bounced around and in the process delaminated into two parts. This shows the bottom and top parts of the platter:
It is interesting to have a peek inside the platter assembly. It appears that the lower part of each platter, which provides the main weight of the assembly, was individually balanced, similar to what is done with car wheels after putting on tires. They did that by drilling out small portions of the materials on the too heavy side of the platter (you can see the red pen mark from the balancing process next to the drill pattern):
It is also clear from the markings on each of the ribs of the lower platter that both parts were glued together. Inspired by this finding I used a slow curing epoxy glue, which I applied next to the original glue spots:
Then I placed the top part off the platter and aligned it as good as I could. A 4002 platter served as weight to secure both halves in place while the epoxy cursed for 24 hrs:
In the meantime, I replaced the old cracked cabinet guidance washers with new 3D printed ones. This shows the original center washer at the front of the enclosure:
This one is a strange design aspect of the 4000...it seems to be a flat Teflon washer that has no guiding feature. This one is best replaced with a 3D guidance washer printed in black, since white ones are visible from the front through the crack between the cabinet and the aluminum panels:
This shows the complete set of five 3D printed guidance washers:
They are available through the Beolover Shapeways store (see link under 'pages' on the right).

Sunday, March 18, 2018

Beogram 4002: DC Motor Restoration and Dependence of RPM Performance on the Value of C10

I recently received two DC motors for restoration that were extracted from Beogram 4002 units that Sonavor is currently rebuilding. This shows one of the motors:
The first step for motor rejuvenation is performing an oil infusion of the brass sleeve bearings which are usually running try after 35+ years of operation. This requires a complete disassembly of the motor:
The bearings are on the black pad in the front. Their oil infusion requires placing them under vacuum while immersed in oil:
The air bubbles emerging from the pores of the bearings signal that the infusion process is progressing. As the vacuum draws the air out, oil replaces the air in the pores and the bearings regain their ability to form an oil film around the motor shaft. After 24-48 hours this bubbling typically stops and the bearings can be extracted for reinsertion into the motor. This shows the bearings in place:
Once the motor is back together it is time to give it a 24 hrs stability test with the BeoloverRPM device which allows the logging of the RPM over extended periods of time. This shows it in action:
The BeoloverRPM is available to other enthusiasts. Just send an email or use the contact form on the right if you are interested.

For a while now I am trying to figure out how to get these DC motors running with the highest stability. Since their control system is fully analog, factors like friction and temperature can significantly alter the stability of the system. Especially capacitor C10 which governs the feedback response of the system has a strong influence on the RPM stability. Over time B&O changed the C10 value between three values: 0.33uF, 0.47uF and 10uF. Our research has shown that most motors have a 'preferred' value for best performance. Which one can only be determined at this point by testing them with different values and then selecting the best. I did this for these two motors. This are the graphs that were measured:
The top two graphs show the RPM performance of both motors with 0.47uF installed for C10. Especially the red curve (motor #1) has some pretty big spikes to higher RPM, indicating that the feedback tends to over compensate the system and under certain conditions the RPM spikes. The same motor run with 10uF in place showed a perfect curve as shown below (light red). Motor #2 also benefitted from 10uF, but not very significantly. It already performed fairly well with the 0.47 uF value soldered in.
At this point it is a mystery why some motors react different from others. They show the same coil resistance and draw the same current etc...i.e. they are virtually identical in all measurable values. My speculation is that these performance variations may have to do with mechanical friction in the motors, which by itself is a damping factor that acts on the control system's transfer function. Until we understand this a bit better, I guess the best approach is to simply determine the best value empirically and enjoy some nice vinyl records!

Saturday, March 17, 2018

Beogram 4000: Upgrade of Signal Path with New DIN5 Plug and a Grounding Switch

As usual the original DIN5 plug on the Beogram 4000 that I am currently restoring was badly oxidized. This shows the original plug:
I replaced it with a new all-metal plug that has gold coated terminals:
Another useful upgrade of the signal output of any Beogram is the installation of a switch that allows connecting signal and system grounds. This is a useful feature if the Beogram is ever to be used with a DIN5 to RCA adapter. Since RCA cables do not carry the system ground, connecting it to the signal ground on the Beogram often eliminates humming. This shows the output terminal assembly of the Beogram 4000 in its original condition:
This open layout makes it very convenient to add a switch:
The shown switch position connects the two ground. Flipping the switch would open the connection.

Beogram 4000: Replacement of Sensor Arm Light Bulb with an LED

The final incandescent bulb to be upgraded with an LED in the Beogram 4000 that I am restoring right now was the one in the sensor arm. This LED is the most difficult to replace due to the compact dimensions of the compartment in which it is located. This required the design of a flex PCB based solution since this LED not only needs a current limiting resistor, but also a current bypass to simulate the presence of a light bulb drawing a much more significant current than a modern LED. This current is used by the control system to detect if the light bulb is working, i.e. the LED setup needs to draw a similar current. This shows the bulb compartment pulled out of the sensor arm:
I removed the bulb. This shows the bulb together with the already folded LED assembly:
Then I installed the flex PCB into the bulb compartment. This shows it powered up:
The warm-white LED emits enough red photons to properly light up the B&O logo like the original bulb.
Whenever the sensor arm light source is replaced the record detection circuit needs to be tested to make sure the absence of a record is properly detected. This shows the signal measured at the collector of TR14, which amplifies the sensor response for the control system:
It is as specified in the manual, having an amplitude of about 2V spanning from 0.2 to 2.2V. All good in the sensor department!

Thursday, March 15, 2018

Beomaster 8000: From One Beomaster 8000 Project To Another

Having just shipped back the most recent Beomaster 8000 and Beogram 8000 restorations to their owner I find myself continuing on with another Beomaster 8000 restoration project. Maybe 2018 will be the year of the Beomaster 8000. At least on the sonavor workbench :-).

As a Beosystem 8000 owner myself I understand these fellow owners wanting to have a fully working system again.

This works out good for me because my workbench is already in the Beomaster 8000 mode. I have the parts ready, the work bench ready and the test bench ready so it is time to begin.

First though, this Beomaster 8000 arrived in the best shipping box I have ever seen. It is not a Bang & Olufsen original box. It is better. I haven't ever seen a k├╝box shipping box before this one. It is very well designed and for the Beomaster 8000 well worth the cost I think. The Beomaster is a really heavy receiver. It benefits from a really sturdy shipping box to ensure safe transit. 

Another cool packing product the owner used on this shipment was Sealed Air Instapak cushioning. These are various size bags with foam cushioning that is activated during the packing process so the expanding foam molds itself to the item being protected. The shipping box limits the foam bag expansion so the bag forms itself to the space between the Beomaster and the shipping box.

Inside the shipping box was another nice Beomaster 8000. 

The cosmetic condition of this unit is pretty good. I don't see anything broken so far. There are some surface scratches though and as usual the flat, metal spring clips for the center bar are not a complete set.

This Beomaster was manufactured around the same time as the previous Beomaster 8000 project. Like that one this Beomaster has the later, improved microcomputer box that is easy to open up.

The restoration on this Beomaster will also be pretty much the same scope as the previous Beomaster 8000 project.  Electrolytic capacitors will be replaced with new high quality capacitors that are rated up to 105°C. Most capacitors 4.7uF and lower will be replaced with WIMA MKS polyester capacitors. The old LF353N audio OpAmps will be checked and likely replaced with new ones. The old, dry heat sink compound will be cleaned off and and replaced with SIL-Pads. The Beomaster display panel will be reworked with new light sources installed. The volume and tuning wheel sensors will be checked and probably replaced with new emitter and detector devices as I have been running into a number of these with failing sensors.

Enough talking about the project. It is time to get started.