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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 31, 2018

Beogram 4000: Hood Restoration

The last step of my current Beogram 4000 project was to exchange the badly shipping damaged hood with one from a donor 4000 and restore it. The hood that I had available had the usual scratches from careless storage but otherwise was in good condition:
I sanded the surface down below the scratch depth with 320 grit sand paper. Then it was time to polish it back to shiny and translucent. This shows the hood after the 800 grit step:
almost there. After several more polishing steps it finally looked pretty good again:
Good news for this Beogram 4000! Now it looks almost like new again! A few more days of testing and then it should be ready for its big trip to Hong Kong to its new owner!

Thursday, March 29, 2018

Beogram 4004 (5526): Restoration of the 'Drive Train'

I recently received a Beogram 4004 exhibiting RPM stability trouble. I rebuilt the 'drive train' components of the unit to restore RPM consistency. Most Beogram 400x have this issue at this point in time, and the most likely trouble spots are dry brass bearings in the DC motor, corroded contacts in the RPM relay and/or thermal influence of the light bulbs in the RPM trimmer panel on the trimmers themselves. I started with the motor. This shows the motor removed from the Beogram:
This shows it disassembled to extract the bearings (they are the two small 'donuts' on the black pad):
I submerged the bearings in motor oil and pulled a vacuum:
The typical bubbling started immediately, indicating air was leaving the bearing, making room for oil to diffuse into the bearing to refill its pores. This was a particularly thirsty set of bearings and the bubbling took about 72 hrs to subside (which indicates that the bearings are full of oil again).

While the bearings were infusing, I updated the main board with a new RPM relay and new 25 turn 
RPM trimmers. This shows the RPM section on the main board in its original condition:
and with new components installed:
Then I focused on replacing the incandescent bulbs in the RPM trimmer panel. This shows the panel extracted from the Beogram:
I removed the bulb covers
and unsoldered the bulbs:
Then it was time to implant the LED assemblies replacing the bulbs. This shows the two LED boards before installation:
Like all parts featured on this blog they are available to other enthusiasts. Just send an email or use the contact form on the right. This shows the boards implanted:
They are 'drop-in' replacements, i.e. perform in the circuit identically to the bulbs. The covers fit back on like nothing happened:

Once the bubbling stopped I extracted the bearings from the oil:
And installed them back in their places in the motor housing. This shows the tools I am using for placing the upper bearing:
After reassembly of the motor it was time to test the RPM stability with the BeoloverRPM device, which allows logging the RPM over long periods of time. A very valuable tool to identify intermittent (most are) RPM stability issues:
It is of course also great for quickly checking the RPM and adjust it if necessary. The BeoloverRPM is also available to other B&O enthusiasts.
I ran the Beogram for ~24 hrs and the blue curve in the graph below is what I measured:
Unfortunately, that was not a good result indicating that the motor also needed new spark snubbers. In some motors they seem to develop some significant intermittent conductivity at voltages they should not, which partially short circuits one or more coils on the rotor. This typically results in RPM drops. I opened the motor up again and extracted the rotor:
The two (visible, there are three, one is behind the commutator) yellowish devices around the commutator are the snubbers. They can be removed by unsoldering the three points where they connect to the coils. This shows the original snubbers with their modern replacements ready for implantation:
This shows the TVS diodes soldered in place:
If you try this at home, make sure that the TVS diodes are located as close as possible to the coils that they not interfere with he brushes when the rotor spins.
After putting the motor together again, I measured the red curve in the graph above, and we can conclude that the patient was finally cured! I listened to a few albums on they Beogram, and it seems to be working very nicely now. Time to send it back home!

Monday, March 26, 2018

Beomaster 8000: Power Supply PCB Recap

The rust I found in the previous post focused my attention to that as the first order of business. 

Besides the +5 V, ±15V power transformer there were some rust spots on the bottom plate of the Beomaster cabinet.

I used a rust neutralizer on the bad areas of the transformer first. After that dried I used flat black rust preventative paint. Here is the cleaned up cabinet compartment where the +5V, ±15V transformer and power supply board go.

On the power supply board recap I cleaned dust and debris from the board and replaced the electrolytic and tantalum capacitors. This power supply board must have been worked on in the past. A few of the capacitors do not look like original capacitors what would have come from the Bang & Olufsen factory.  The large +5V, ±15V reservoir capacitors are not fixed to the board with double-sided tape like most of these boards.

I always like to measure the capacitors I remove just to see what condition they are in. Most of the capacitors are within tolerance but there are always quite a few that are borderline to way out of tolerance. 

Here is a good example of failed capacitors from the power supply board. One of the 10uF, 63V electrolytic capacitors measures 39uF now! A little off I would say ;-).

Failing the opposite direction this 22uF capacitor is only 11uF now.

The ESR values are also quite high on those two capacitors.
On capacitors that are still in tolerance we always change those to new capacitors as well. They have lived longer than expected but time is not on their side. With the Beomaster opened up it is best to replace them now and be assured they won't fail for another forty (or more) years.

When replacing the +5V, ±15V reservoir capacitors I run a bead of hot glue along the bottom to secure the capacitors to the PCB so that won't move. The hot glue dries right away and can easily be removed if necessary.

Here is the power supply board with all of the capacitors replaced and new thermal insulators on the +5V, ±15V regulators. The metal shield around the remote control circuit is kind of a pain to remove and reinstall but that housed the failed 22uF capacitor so it is a necessary step.

With the cabinet and transformer cleaned up the parts are reinstalled.

It's starting to look like new again.

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.