Featured Post

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...

Wednesday, January 17, 2018

Beomaster 8000: Power Supply Board Recapping

The Beomaster 8000 Power Supply board restoration consists of changing out several electrolytic capacitors along with a couple of tantalum capacitors with new 105°C capacitors. As I normally do I use WIMA MKS capacitors where the capacitor value is less than 10uF.

Here is the before picture of the power supply board.

The capacitor replacement is pretty straight forward with the exception of one capacitor located inside the metal shield box on the board and the three large, axial capacitors (2200uF and 4700uF) that require some glue to secure.

There is also a separate heat sink assembly for three voltage regulators. I started with that assembly and replaced the lone 10uF capacitor. While I did that I decided to clean off the old thermal grease and change out the thermal insulators.

On the three, large axial capacitors I scraped off the old mounting glue to prepare for the new capacitors.

For this type of application a hot glue gun works great as the glue works fast and can always be removed in the future if necessary.

The metal shield box for the Beomaster remote control receiver is not too bad to remove but does require de-soldering four mounting tabs. Here is the box removed and the power supply board recapped.

The shield box is soldered back in place, the board connector solder joints are reflowed and this board is done.

Monday, January 15, 2018

Beomaster 8000: Removing the Output Amplifier Boards and Power Supply for Recapping

It has been quite a bit colder in the workshop the last week. Another cold front is arriving tonight but work must continue on the Beomaster 8000. I had to plug in an additional heater in the workshop to keep things toasty.

The next step in the restoration is to replace the electrolytic and tantalum capacitors on the two output amplifier boards and the power supply board. The reason I am doing those as a group is because of their physical location in the Beomaster.

The output amplifier board is identical for the left and right channels. The only difference is the color coding on the wires that connect to the speaker switches and the headphone jack.

I prefer to do the recap restoration work with the boards removed. In the case of these output amplifier boards they can also be tested outside the Beomaster by applying the necessary ±55V and +15V supply voltages.

Removal of the output amplifier boards requires removing the output transistors from the big heat sinks. It would have been nice if the output amplifier board had enough room to slide out the back of the cabinet without removing the heatsink mounted components. There are just too many wires in way.

It is worse on the left channel and the cabinet ledge for the lid damper (damper is removed in the photo) blocks any chance of the board sliding out.

So heat sink component removal it is.

The thermal paste for the transistors has usually started to dry out by now so I don't mind the exercise as it should ensure long term protection for the parts.

Here are the left and right output amplifier boards removed and ready for recapping.

This is also a good time to pull out the power supply board and its heat sink with the voltage regulators.

While these boards are out I will clean out any dust and debris from the cabinet where those components live. I like the feeling of knowing the Beomaster is clean like new again after the restoration.

Beogram 4002 (5513): Replacement of Sensor Arm Bulb with an LED Assembly

After rebuilding the DC motor of the Beogram 4002 that I am restoring right now it was time to replace the light bulb in the sensor arm. This bulb is critical for the record detection circuit, and replacing it with an LED makes sense since LEDs last much longer than light bulbs. This shows the bulb compartment extracted from the arm together with the flexPCB based LED assembly:
The LED assembly 'mimics' the light bulb in terms of drawn current to be compatible with the light bulb detection circuit. This circuit monitors the current through the bulb, and disables the record detection circuit in case the bulb dies. LEDs draw much less current than the bulbs, and therefore an additional current path needs to be provided on the LED board in parallel to the LED itself.

The LED board folds into the compartment:
After soldering it in, I started the Beogram to see if everything works:
The warm-white LED has enough red emission to render the B&O logo properly red as if it were illuminated by the original incandescent bulb.

The final step of any light source replacement in the sensor arm is to check if the ribs on the platter are properly detected. TR3 amplifies the sensor signal and its collector needs to be at 4V DC when there is no signal (record present). Unfortunately, the circuit design of the 4002 biases the base of TR3 via a single resistor connected to the collector. This makes the working point of TR3 dependent on its current amplification factor, which can vary widely between transistors of the same type. Hence, many Beograms have a too small voltage at the collector of TR3, reducing the signal generated by the ribs on the platter as they go by underneath the sensor. This can be rectified by replacing R26 (1 MOhm) with a 2 MOhm multi-turn trimmer:
I usually install the trimmer first on the solder side to be able to adjust the collector voltage  with the board installed and the turntable on. Once the 4V are set, the trimmer can be installed from the component side:
The final step is to measure the sensor signal at the collector with the platter spinning underneath:
This Beogram passed the test with flying colors: The signal is very close to the prescribed 6Vpp. If the signal is too small, then TR3 is not biased correctly, and if the signal does not go to zero when a rib goes by, then the LED or the bulb is not in the correct location within the bulb compartment of the sensor arm.

Wednesday, January 10, 2018

Beogram 4002 (5513): Restoration of the DC Motor - Oil Infusion of the Oilite Bearings

After rebuilding all the components of the DC motor control system of the Beogram 4002 (5513) that I am restoring right now, it was time to look into the DC motor. Just spinning it by hand created scraping bearing noises, i.e. it was definitely time to re-infuse its Oilite sleeve bearings with oil. Most Beogram DC motors need this treatment at this point in time to be able delivering stable RPM performance again. This shows the motor as extracted from the Beogram:
I took it apart to extract the two bearings:
The bearings are the two 'donuts' up front on the black pad.

I submerged them in motor oil and pulled a vacuum. Immediately air bubbles emerged from both bearings:
After about 24 hrs the bubbling came to an end and I extracted the bearings from the oil and put the motor back together. This shows the 3D printed tool I use for installing the top bearing:
After the motor was together again, I installed it in the Beogram and ran a 24 hrs RPM stability test with the BeoloverRPM device. It allows logging the RPM over extended periods of time. A great feature to detect intermittent RPM instabilities. This tool is available to other B&O enthusiasts. Just send me an email or use the contact form on the right.

This shows the BeoloverRPM in action (it is also great for adjusting the RPM precisely):
This shows the resulting 24 hrs RPM graph:
This is as good as it gets with a DC motor Beogram. The slight variations of the RPM are probably caused by temperature variations of the ambient (or the moon phase...;-). They are within the specs of the Beogram and are not audible. The joys of analog technology of the 70s! This Beogram is getting close to a first test drive!

Tuesday, January 9, 2018

Beomaster 8000: Replacing the ±55V Reservoir Capacitors

The two most time consuming tasks on the Beomaster 8000 restoration are the changing out of the four, large reservoir capacitors and the recapping of the output amplifier boards. Both of those tasks require some cabinet disassembly to get to components.

Here are the four ±55V reservoir capacitor replacements on this Beomaster 8000 project.

The space to de-solder wires and install the new capacitors is tight plus the two reservoir capacitors against the back wall of the cabinet are secured by round, metal clips. To loosen the mounting clips on the rear capacitors more working space is needed. That requires loosening the rear panel screws so it can be moved enough to get some needle-nose pliers on the clips.

I removed the left channel capacitors first...

With the left channel reservoir capacitors removed you can see the metal mounting clip for the rear capacitor.

Now for the right channel...

Both sets are removed.

Here are the Beolover replacement capacitors. Modern 10,000uF replacement capacitors are quite a bit smaller in physical size. The Beolover replacement includes a 3D printed adapter so the replacement capacitors fit in the same space as the originals.

Note: Any DIY enthusiasts that are up for this challenge themselves can purchase these Beolover replacement reservoir capacitor kits through the contact form on the right.

And here are the left and right channel reservoir capacitors secured and connected up.

The right channel side of the reservoir replacement had more wires that needed to be moved out of the way during the recap. That included the gray cable for the smaller transformer. With the reservoir recap completed I routed the wires back through to the small transformer but left them disconnected for now as I will soon be recapping the power supply and output amplifier boards.

Friday, January 5, 2018

Beogram 4002 (5513): Replacement of RPM Trimmer Panel Bulbs with LED Assemblies

After rebuilding the electronics of the Beogram 4002 (5513) that I am currently restoring it was time to replace the light bulbs in the RPM trimmer panel with LED assemblies. LEDs not only last much longer than incandescent bulbs, they also generate far less heat, which is beneficial for the RPM stability of the DC motor that drives the platter. This shows the RPM control panel removed:
To get to the light bulbs their covers need to be removed:
After unsoldering their leads they can be removed together with their aluminum clips:
The replacement LED assemblies
are drop-in and can directly be soldered into the same place:
These parts are available to other B&O enthusiasts. Just send an email or use the contact form on the right if you are interested in getting them.

Once this was done I replaced the covers and tested the panel:
The authentic 'incandescent look' of these LED implants is a result of using carefully intensity tuned red/green LEDs that can mimic the emission spectrum of a low emission light bulb. Now everything is in place to test the DC platter motor after its restoration, which will be next.

Thursday, January 4, 2018

Beogram 4002 (5513): Restoration of PCBs

After rebuilding the arm lowering and tracking systems of a Beogram 4002, my next step is to rebuild the PCBs and replace the reservoir capacitor. This shows the main PCB in its original condition:
This is a detail picture of the RPM control section:
I replaced all electrolytic capacitors with new 105C grade Japanese types, and installed a new relay and 25-turn encapsulated RPM trimmers. This shows the relay and trimmers in old-new comparison:
These components are available to other enthusiasts (like all Beolover parts featured on this blog). Just send a email our use the contact form on the right if you are interested.
This shows the rebuilt board together with the extracted parts:
The trimmers are placed that their adjustment screws protrude through the PCB. This is important since one needs to adjust the RPM while the board is installed and the platter turning.
The next step was to rebuild the output PCB that accommodates the output grounding relay:
I replaced the delay capacitor and the relay, and also installed a switch that allows connecting the system and signal grounds. This is a convenient feature if one operates the deck with a RCA adapter, which often cause some degree of humming. Connecting the grounds often cures the issue.
This shows the rebuilt board:
The final step of the electronics rebuild was to implant a new reservoir capacitor. This Beogram 4002 featured the double capacitance type that is not available anymore.
It has a 4000uF and a 1000uF section. The 1000uF section is only used if a CD-4 board is installed (usually it is not). The 4000uF is essential for the Beogram to run. The black wire in the back is the ground connection for both capacitors. White connects to the 4000uF and orange to the 1000uF ends. I usually replace these caps with a two-capacitor assembly based on a 3D printed adapter that fits into the mounting bracket of the original capacitor:
The 20k resistor across the 1000uF cap serves as a bleed to ensure the capacitor discharges when the deck is unplugged for service or transport. The next step is rebuilding the RPM trimmer panel with LED assemblies to replace its scale illumination light bulbs.