Beogram 4004 (5526): Restoration of the Keypad

The Beogram 4004 (5526) that I am restoring ring now had the usual wear pattern of the keypad. The coating gets worn down over time and one starts to see which keys were used most (you guessed it: START is among them...;-). This shows the keypad as I received it:

After recoating it looked like this:

Beolovely! On to final adjustments and then some testing!

Beogram 4004 (5526): Repair of a Cracked Plexiglass Hood

The Beogram 4004 (5526) that I am restoring right now came with a hood with a crack in the hinge region. This shows the crack after moving the metal hinge part:

Many Beogram hoods suffer this fate since the holes for the mounting bolts of the hood hinge represent a mechanically weak spot. Combined with the brittle nature of plexiglass and the torque acting on the hinge region this can lead to stress fractures. I recently developed a method to repair such cracks when I needed to save a rare Beogram 4000 hood. For this project I developed laser cut patches that are thin enough to still permit mounting the hinge after the repair. Before applying the patches, however, the first step is to glue the cracked parts back together to maximize strength of the repaired area. This can be done by simply pressing them together and infusing a bit of Weld-On #4 acrylic solvent into the crack. Capillary forces draw the water thin liquid into the crack where the abutting sides are dissolved in the interface region. Evaporation finally cures the dissolved regions into one piece of plastic:

After 5 min of holding the parts together one can let go and add the patches to further stabilize the part:

If the hood area is smooth enough one can also use #4 for gluing the patch on. If the area is rough, higher viscosity #16 is better for the task. Here #4 did the trick. I also applied a patch to the outside and moved on to the other side of the hood where cracks are already developing:

On this side I only applied a patch on the inside, since the crack was not fully developed yet:

After letting the acrylic harden for 24 hrs, I bolted the hood hinge back in and then glued the side parts of the aluminum strip to the plexiglass with ultra thin double sided tape. This shows the side that had the fully developed crack:

The outside patch made the hood a bit wider, i.e. the aluminum strip is slightly offset from the hood surface....Unfortunately, the aluminum strip already had developed a small crack where it is bent, but I was able to catch it in an early stage and it is hardly visible. At least this hood can be used again!

Beogram 4004 (5526): Restoration of DC Motor - Oil Infusion of the Bearings and New Spark Snubbers

The DC Motor of the Beogram 4004 (5526) that I am restoring right now was in need of restoration. It seems almost all Beogram DC motors need this treatment at this point in time. First I did the usual oil infusion of the dried out Oilite bearings. This requires completely disassembling the motor. This shows the motor extracted from the enclosure:

And taken apart:

The bearings are the two small 'donuts' on the back pad. I immersed them in motor oil and pulled a vacuum:

Immediately, bubbles emerged from the bearings indicating leaving air to make room for oil that was now able to diffuse into the bearings. After about 48 hours this process finally stopped, indicating that this Beogram DC motor had very thirsty bearings (often it is already over after 24 hrs). I installed the bearings in the motor housing. I used a specially designed tool to press in the ring that holds the upper bearing:

This shows the bearings back in place:

After completing assembly, it was time to give this motor a 24 hrs RPM stability test using the BeoloverRPM device:

Aside from precisely adjusting the RPM, the BeoloverRPM also allows the capturing of the RPM in 10s intervals during extended periods of time. The blue curve in the graph below is what I measured over approx. 24 hrs after the oil infusion:

While not too bad in terms of absolute deviations (they would not have been audible to most people with 'normal ears') this curve was not acceptable. The downward spikes indicated that the spark snubbers of this motor needed replacement.

I took the motor apart again and extracted the rotor again:

The three yellowish 'devices' around the rotor are the spark snubbers. Essentially they are semiconductor devices that become conductive after the voltage across them exceeds a certain value.  They are connected in parallel to each of the three coils of the rotor. It appears that they fail in an initially intermittent way, while usually culminating in a full short circuit at the end of the decay process. The latter causes the motor to be very weak, while the former causes occasional RPM drops, similar to dry bearings. I removed the snubbers ring and installed new modern TVS diodes (small black blocks around the shaft). These diodes need to as close as possible to the coils to not interfere with the brushes when the motor is assembled:

After assembly I tested the motor for another 24 hours and the result was much better (red curve in the above graph). This motor is in business again!

Beogram 4004 (5526): Replacement of the Sensor Arm Bulb with a LED Assembly and Optimization of Detection Circuit

The final incandescent bulb to be replaced in the Beogram 4004 (5526) that I am currently restoring was the bulb in the sensor arm. This bulb enables the record detection circuitry and therefore is essential to the proper working of the turntable. I recently developed a flex-PCB based LED replacement assembly, which 'folds' into the small bulb compartment. This places the LED in the right spot to be projected through the lens onto the record that is being played. This shows the original bulb in place together with the LED replacement:

I removed the lightbulb:

and installed the LED assembly:

The assembly is drop-in, i.e. directly replaces the lightbulb pulling the same amount of current. This is important to not confuse the record detection circuitry. This shows the LED in action:

Since it is a 'warm white' LED, it has enough red light components to accurately illuminate the red-orange B&O logo at the end of the arm.

The design of the detection circuit has one flaw, that sometimes manifests itself by neutering the detection circuit allowing the arm to drop even if there is no record present. This is caused by biasing the TR3 transistor that is responsible for the initial amplification of the sensor signal too low. This can be fixed by replacing resistor R26 with a 2MOhm trimmer that can then be adjusted to yield 4V at the collector of TR3 when there is a record present (i.e. no sensor signal coming in from the black ribs passing underneath the arms). A more detailed description of the circuit and its improvement can be found here).

Once R26 has been removed it is a good idea to install the trimmer temporarily on top of the circuit board, so it can be adjusted while the board is in place:

After adjusting the trimmer to yield 4V at the collector of TR3, it can be installed 'below deck':

It fits neatly between the other components around TR3, as if the designers of the boards foresaw that the Beolover would come up with this fix in 2017!..;-). 

After this upgrade it was time to measure the sensor response at the collector of TR3. This shows the signal trace while the platter was spinning under the sensor:

Each dip corresponds to a black rib passing under the sensor. The amplitude of the signal should be close to 6V to make the sensor work properly. Generally it can be said that if the dips are not going all the way to zero, the position or brightness of the light source (LED) is not good enough, and if the amplitude is not reaching 6V on the upper end, the collector voltage at TR3 is too low and needs to be adjusted.

Beogram 4004 (5526): Replacement of Bulbs in RPM Trimmer Panel and Installation of New White Background Strips

After restoring the electronics of the Beogram 4004 (5526) that I am working on right now it was time to replace the light bulbs in the RPM adjustment panels with LEDs. This is not only good for the longterm stability of the unit, but it also helps alleviating potential RPM stability issues. The light bulbs heat up the trimmers in the panel and that can cause abrupt RPM changes while listening. LEDs run much cooler, and this eliminates this issue. This shows the panel removed from the Beogram:

The first step is to remove the bulb covers:

Then I unsoldered the bulbs and removed them:

Now it was time to solder in the replacement LED assemblies:

They are direct drop-in replacements that run on the same voltage as the bulbs. They are available to other enthusiasts, just send me an email or use the contact form on the right.

After replacing the covers

I fired the LEDs up for the first time. Unfortunately, this showed that I was not done yet, since it turned out that the white background strips behind the RPM indicators were wavy, which created a wavy illumination pattern:

This happens maybe in about 10% of the Beograms...not sure why some have it and others not. I had a look by opening up the RPM panel. This requires cutting the 4 locking washers that hold it down. One has to destroy them to get them to release the part:

You can see the wavy edges on both white strips. I removed them

and implemented cut to size replacements fashioned from white electrical tape:

Then I reassembled the panel. This required replacing the locking washers with new ones. I also use nylon washers that I put underneath:

After installing the RPM panel, the scales looked pristine again:

Beolovely! On to restoring the motor!

Beogram 4004 (5526): Restoration of the PCBs and a New Reservoir Capacitor

After rebuilding the arm lowering and tracking systems of the Beogram 4004 (5526) that I am restoring right now it was time to rebuild the electronics. I started with the main PCB:

This shows it in the original condition:

I replaced all the electrolytic capacitors, the RPM relay and the RPM trimmers:

This shows the RPM section in detail before

and after:

The RPM relay was replaced by a SMD relay on a breakout board that I developed a while ago:

The replacement relay board has exactly the same footprint as the original National relay. This part is available to other B&O enthusiasts. Just send an email or use the contact form on the right.

The original RPM trimmers were replaced with modern encapsulated 25 turn trimmers. Since they need to be adjustable from the solder side of the board, they need to have their solder terminals extended that I could install them upside down:

On to the output PCB. The 4004 has a remote control function when connected to a Beomaster 2400. This is the purpose of the additional circuitry on the output board (when compared to a 4002). This shows the board in original condition:

Upon closer inspection I found a burned out tantalum capacitor:

When I removed it it completely broke into small ash pieces:

An impressive demonstration why it is a great idea to replace all electrolytic capacitors at this point in time. I replaced all electrolytic capacitors and installed a new output relay:

This shows the output circuit in detail after the restoration:

The red switch in front of the output board header allows making a connection between signal and system grounds. This is a convenient feature to prevent humming if the Beogram is connected via an RCA adapter that does not have a broken out ground lead.

The final step was to replace the original reservoir capacitor:

They are also frequently out of spec at this point in time. The modern replacement is a bit smaller, and I used a 3D printed adapter to fit it into the same space like the old can:

All good in the capacitor department now! On to replacing the light bulbs in the RPM trimmer panel with LEDs.

Beogram 4004 (5526): Restoration of Arm Lowering and Tracking Systems

As usual, my first step of the restoration of the Beogram 4004 (5526) that recently arrived was to rebuild the arm lowering and tracking systems. This shows the arm lowering mechanism of the unit:

I removed the linkages and the damper for cleaning and lubricating:

This shows the parts spread out:

After reassembly it was time to get to the linkage that connects the damper with the tonearm. This shows the back of the arms:

Removal of the two screws at the bottom of the sensor arm enabled its removal:

I removed the retaining washer and the small spring underneath (careful here - this tiny spring likes to vanish very spontaneously) and removed the linkage:

After lubricating the pivot shaft I put everything back together and adjusted the arms to a parallel alignment:

This concluded the restoration of the arm lowering system. I moved on to the tracking sensor and the pulley that drives the carriage spindle:

I like to replace both since the light bulbs in the tracking sensor often break and most of the pulleys seem to crack at some point, which makes them wobbly and this causes unbeolovely vibrations when the carriage is driven. I removed the tracking sensor light bulb assembly. This shows the Beolover LED replacement assembly in comparison with the original unit. The LED is in the same spot where the bulb filament is located.

Like all Beolover parts featured on this blog, this part is available to other enthusiasts. Just send an email or use the contact form on the right. This is how it looks after installation:

Next was replacing the plastic pulley with one of Nicks awesome precision machined aluminum units:

I'd be happy to put you in touch with Nick if you want to get such a pulley for your own Beogram! I also put in a new belt, and that concluded the restoration of the tracking system. On to the electronics!