beolover: Beogram 4002 (Type 5503): Full Restoration and Testing

This post describes the work done during the restoration of a Beogram 4002 (Type 5503 with AC platter motor) that I was able to purchase from a gentleman in Illinois recently. The unit is a single-owner unit and came out of long-term storage.

This shows the final result of my labors:

Looks nearly new and also plays like new! Beolovely! This Beogram is available for purchase at the DKaudiolover store.

This is how the unit presented itself after I received it:

Like most units that come out of storage, it had some serious scratches on the plexiglass cover (not sure why people cannot put an old towel on them when they go into the basement!...;-):

Luckily, this is not an issue anymore since new reproduction hoods are now available. I removed the hood and had a look at the aluminum surfaces and the keypad:

The keypad has the usual worn-down coating where it was touched for operating the turntable. I will replace it with a new keypad plate.

On the positive side, the platter is in very nice condition:

This unit also has a very nice original rosewood plinth with sharp, almost like-new corners:

I removed the aluminum plates and the platter and had a look 'below deck':

It all looked pretty original, which is a great starting point for a restoration. Of course, it had degraded plastic plinth guidance washers

and completely deteriorated transport lock bushings:

These plastic parts can be replaced with new Beolover reproduction parts. See here for our parts for the 4002 AC motor models.

In the following, I will discuss all the work done to this unit to bring it back to a like-new condition:

While the synchronous AC platter motors are less troublesome than the later DC motors,

they also have issues with drying out Oilite shaft bearings. Dry bearings often cause them to 'knock' while driving the platter. Therefore, I also generally re-infuse the motor bearings with oil under vacuum when I restore this Type. I usually do this right at the start of the project since it can take several days until the oil has been replenished in the bearing material.

This shows the AC platter motor together with the original big capacitor cans that serve as motor coupling and phase shift capacitors, as well as reservoirs for the main circuit:

I removed it all:

This shows the extracted motor:

It has four leads since it is a two-phase synchronous motor. I took it apart to prepare the motor enclosure halves for oil infusion:

It seems impossible to remove the motor bearings without doing significant damage, so I simply submerge the entire motor housing in synthetic oil, making sure that both bearings are completely in the oil. After pulling a vacuum, one can usually see air bubbles coming from the bearings in the center of the pieces. :

This indicates that air is drawn from their porous bearing material, making room for oil to diffuse into the material. This motor had pretty thirsty bearings, and so the oil foamed up vigorously a few minutes after the vacuum was established:

When the bubbling stops, the process is complete, and the bearings are ready for another tour of duty.

While this process was underway, I focused on the other restoration tasks. First came the restoration of the carriage. The arm-lowering and carriage motion mechanisms usually suffer from hardened lubricants. Therefore, it is a good idea to take it all apart for a good cleaning and re-lubrication. This shows the original encrusted setup:

A detail shot of the cracked black plastic carriage pulley:

I removed all moving parts for cleaning in my ultrasonic cleaner:

Aside from cleaning and re-lubricating, it is also a good idea to demagnetize the solenoid. If the plunger is magnetized, it can lead to delayed arm raising, which can cause the needle to drag across the platter during auto return after playing a record. Not a fun moment when a newly rebuilt $800 cartridge is mounted on the arm! This shows the extracted solenoid:

I removed the plunger:

The removed plunger was indeed magnetized. It was able to attract a small M3 iron set screw:

After treating it a bit with my tape head demagnetizer, the setscrew was able to stay put even at very close distances:

In the meantime, the parts came back from the ultrasonic cleaner nice and shiny:

I started re-assembling everything. It is a good idea to install a new damper gasket:

The old ones are often hardened and/or deformed, which causes inconsistent arm lowering speeds. It is a bit hair-raising when the nice new cartridge occasionally crashes on the platter without damping!

This shows the entire setup back together:

Here is a picture of the new Beolover aluminum carriage pulley:

Beoshiny! There was one more pivot point left in need of cleaning and re-lubrication: the damper-to-arm linkage! It is mounted in between the tone- and sensor arm assemblies. You can see it stick out from the V-cut in the small metal linkage that is bolted to the back of the counterweight:

In order to get to it, the sensor arm needs to be removed. This shows the sensor arm lying on its side

and with the linkage removed:

Of course, the small copper plate that helps the arm move laterally when up came loose with only a light tug with my tweezers. I epoxied it back into place after reassembling the arm:

Next, I replaced the original incandescent bulb of the tracking sensor with a Beolover Tracking Sensor LED Light Source (Beogram 4002 and 4004). This shows the original setup with the black bulb housing still in place:

I removed the bulb housing, which revealed the tracking aperture:

It permits more or less light onto a photoresistor, which drives the carriage motor to keep the light intensity constant. This shows the original and new parts next to each other:

The small SMD LED of the Beolover part is in the same spot as the filament of the bulb, enabling a smooth drop-in replacement process. This shows the part installed:

This completed most of my work on the carriage.

Now came the moment for removing the entire setup from the enclosure. This gives better access to several components that need to be replaced. In contrast to the later DC motor models, the AC motor Beogram 4002 is mostly hard-wired, and it is not easily possible to remove the main PCB, for example, to work on it. This shows the emptied enclosure:

And the removed 'guts' spread out for working on them:

First I restored the main PCB. It usually has two 'afterthought' capacitors installed on its solder side:

I replaced these two tantalum capacitors with modern units of like value

and then I flipped the board over:

I replaced all electrolytic capacitors and power transistors, as well as the RPM trimmers and the sensor arm transistor (which is usually out of spec and does not have enough gain anymore). This shows the rebuilt board together with the extracted original components:

Next came replacing the two TIP31/32 transistors of the AC platter motor push-pull amplifier stage:

I replaced them with new and stronger TIP 41/42 packages:

On to the solenoid transistor, which is usually a TIP41A

I usually replace them with a TIP41C, a higher voltage variant that I hope may last longer in this circuit configuration:

The next step was updating the solenoid switch. I now always replace these switches as part of my standard restoration Type 550x 'package' since I had an unnecessary warranty call last year when a 5503 that I sold earlier in the year came back with a zapped solenoid switch. These switches are a bit under-dimensioned for the task since they need to break currents of 3 to 4 amps every time the arm lowers, while they are only rated for 2 amps. I guess B&O accepted a bit of 'planned obsolescence' here, assuming the switches would not see enough switch cycles over the expected 10-15 year product lifetime to cause significant trouble.

This shows the original solenoid switch (black unit under the wiring) still in place:

This shows the board with the new (white) switch installed:

After installation of a new switch, it is a good idea to check if the solenoid arm actually fully actuates the switch when the solenoid plunger is extended:

Now it was time to restore the carriage position detection PCB. It is located under the plexiglass 'ruler' that is bolted to the carriage assembly:

This shows it with the ruler removed. The two black items are the carriage position sensor. The larger unit is the photoresistor, and the smaller one contains the bulb:

This shows the bulb after removal of the black cover:

These days, I completely remove the sensor assembly and replace it with the monolithic Beolover Carriage Position Sensor for Beogram 4002 (Types 550x). This shows the removed parts with the Beolover board:

The Beolover part uses a modern monolithic interrupter read out by an op-amp and Schmitt trigger, which makes this a much more stable setup. It also has a built-in switchable LED that is able to indicate trigger events for troubleshooting purposes.

This shows the board with the solder pads of the sensor components cleaned up:

The next step is placing the Beolover part so that its lower right corner precisely matches the lower right solder pad on the original board. This shows the part still slightly off to show the solder pad underneath:

Once the board was aligned precisely, I soldered the 22.8V connection (large circular via on the northern end of the board):

After confirming alignment one more time, I soldered the GND and signal connections in the lower left half of the board:

Now the board was firmly in place, and I focused on the rest of the parts that needed replacement. Those are usually the solenoid resistor (white rectangular brick) and the orange

solenoid capacitor. In this case, I also had to replace the 'off switch' (the right switch on the board that is triggered by the plexiglass ruler to switch the unit off when the carriage reaches home position). It had a bent return spring, probably from a previous repair attempt. This shows the restored board together with the replaced components:

And here with the ruler bolted back on:

If the Beolover board is placed correctly, the ruler should be neatly between the 'legs' of the black optical interrupter that is on the board. Note that the ruler alignment needs to be fine-tuned once the unit is functional again to make sure that the ruler has the same position relative to the interrupter legs throughout its entire travel.

The next task was rebuilding the output board. It carries the output grounding relay and its time delay circuit:

I replaced the relay with a new Beolover Siemens Relay Replacement for Beogram 4000, 4002, and 4004 and a new capacitor for the delay circuit. I also installed a (red) switch that allows connecting signal and system grounds if there is a hum in the audio signal. Connecting the grounds usually fixes such issues. This shows the rebuilt board:

On to fixing the decayed transport lock bushings. This unit had lost them completely. This shows one of the 'naked' transport lock orifices:

I implemented a new Beolover Transport Lock Bushing Set for Beogram 4000, 4002, and 4004. They install very easily since they are designed in two parts. Simply insert one half from the bottom

and the other from the top:

Note that some Beograms have slightly bigger transport lock orifices than others, and it can be a good idea to glue the lock bushing halves in with a small amount of super glue gel on the vertical parts.

At this point, it was time to reassemble the unit. First, I bolted the new power transistors into their places on the enclosure bottom. This shows the two motor driver transistors in place:

And here is a picture of the bolted-in new solenoid transistor:

Then I positioned the floating chassis onto the transport lock bolts. This shows one of the bushings with the transport lock assembly in place:

The next step was the installation of the Beolover Efficient 22.8V Power Supply and Main Capacitors for Beogram 4002 (Types 550x) board. It elegantly replaces the original big capacitor mess and also gives the Beogram a modern, precisely controlled 22.8V power rail that is much less wasteful than the hot original linear voltage regulator setup. This means the Beogram will consume around 30% less energy than before when this board is installed. The board fits into the space previously occupied by the big capacitor cans and uses the same bolt holes:

This shows it bolted in with all wire connections (except those from the motor) in place:

In the meantime, the motor bearings had completed their oil infusion process. I put the motor back together:

The red parts are 3D-printed brackets that hold the nuts in place that I use to replace the drilled-out threaded rivets that cannot be reused.

Then I soldered its wires to the terminals on the power supply board:

This shows everything bolted in. I usually put the original mounting strip that held the capacitors in place back in. It nicely keeps the wiring down so it does not scrape on the platter later.

There were still a few light bulbs that needed replacement with LED assemblies. First, I replaced the one in the sensor arm. This shows the small compartment pulled out with the original bulb still in place:

Next to it is the Beolover Sensor Arm LED Light Source (Beogram 4002, 4004 and 4000) together with its alignment aid. I implemented the LED board:

Two more bulbs that need replacement are in the RPM adjustment panel above the keypad. This shows the panel flipped on its front, which reveals the two bulb covers:

I removed the covers:

As usual, the 33 RPM cover had heat damage, as is evident from the brownish discoloration on its inside. I replaced the bulbs with Beolover RPM Panel LED Backlights for Beogram 4002 (Types 550x). They solder directly to the points where the bulb leads went:

The little boards essentially act as extensions of the original circuit board. The bulb covers still fit on the assembly after the installation of the LED boards:

I tested the LEDs, and as expected, the white background behind the 33 RPM trimmer scale was wavy:

The white background foils also suffer heat damage from the hot incandescent bulbs, and this usually shows up at 33 first. I removed the plastic assembly from the front plate, which revealed the foils.

Indeed the 33 RPM side was wiggly:

I replaced the foils with 3M white insulating tape cut to size:

A second test after reassembly showed a nicely homogeneous background:

Beolove is in the details!...;-).

The next step was adjusting the bias of the sensor transistor to yield the service manual-prescribed 3.0 V at its collector:

After the adjustment, I moved the trimmer over to the component side of the board:

Then I checked the sensor response with my oscilloscope. This shows the collector voltage with an empty platter spinning underneath the sensor:

Each dip in the curve corresponds to a black platter rib passing under the sensor. The measured amplitude of about 10V is an excellent result. The record detection circuit was working very well in this Beogram!

While I had the oscilloscope fired up, I also measured the motor signal. This shows how I connected my probe:

The measured signal was a perfect sine-wave. I adjusted the motor voltage trimmer to get about 10V peak-to-peak and the 33 RPM trimmer to yield about 43 Hz frequency:

I also adjusted the 45 RPM trimmer to get about 57 Hz:

The amplitude for 45 will always be a bit different from 33 since the motor driver is an analog oscillator. The signal frequencies are not precise, but they closely correspond to 33.33 and 45 RPM.

A note on the motor voltage: The service manual recommends adjusting to "min 4.5 V eff". I take this as 4.5V RMS, which corresponds to a Vpp of 12.73 V, i.e. a bit higher than the 10 V I normally use. In my opinion, the 'best' motor voltage is that which is high enough to satisfactorily accelerate the platter to full speed before the arm drops at the LP set-down point. Higher voltages only cause the motor to run hotter, which increases the strain on the transformer and the power supply and wastes energy. Maybe the biggest disadvantage of synchronous motors run at constant voltage is that they do not change their current draw much depending on the load on the motor. That means the current needs to be adjusted to a level that is sufficient for the highest load (i.e., during start-up). When the load on the motor is lower than this maximum load (i.e., when the platter runs at a constant RPM), energy is therefore wasted.

On to some finishing touches: This unit had a very grimy corroded DIN5 plug:

I put a new all-metal plug on it with gold-plated contact terminals:

Beogolden!

Then I checked the fuses:

When I popped them out of their holders, one came apart:

This is a common failure of the around 50 years old original fuses, and it is generally a good idea to replace them with new ones whenever a Beogram is restored:

It was time for the mechanical adjustments. In order to do this, the aluminum plates need to be installed, which made it the perfect moment to install a refurbished keypad to replace the badly damaged original one. This shows the original keypad (left) in comparison with a newly restored keypad:

Please see here if you are interested in fixing your keypad, or let us rebuild it for you.

I also reinstalled the original wood frame. I replaced the cracked original plinth guide washers with a new Beolover Plinth Guide Washer Set for Beogram 4000, 4002, and 4004:

This shows one of them installed:

The next step was aligning the sensor arm base and the sensor arm itself orthogonally relative to the rods the carriage travels on. I did this using my recently developed Beolover Arm Alignment Tool for Beogram 4000, 4002 and 4004:

It streamlines the process considerably compared to eyeballing the alignment, which I did before I came up with the tool.

Now it was finally time to do all the adjustments. In these adjustments, the arm travel across the platter needs to become parallel, the platter needs to be at the proper distance from the arms, and the platter needs to be flush with the surrounding aluminum panels. This can sometimes be a pretty iterative slog. Due to the little space the floating chassis has for movement, there is not much room for 'improvisation,' and everything needs to be perfect.

At this point it was a convenient moment for doing my usual 24 hrs RPM stability test. With the AC motors I usually do not expect much trouble in the RPM stability department, but it is always nice to see a very stable RPM curve!

I installed my BeoloverRPM device for the measurements:

The BeoloverRPM has two operational modes. In 'slow' mode (shown above), it measures the RPM in 10 sec intervals and sends the data to a serial port of a computer, where it can be recorded with any generic terminal software. This allows plotting the RPM over long periods of time using Excel or similar software. This shows the result of my 24 hrs measurement:

As expected, the curve is pretty flat and has only weak long-term drift due to the temperature changes between day and night. The Beogram AC platter motors are synchronous motors driven by a Wien oscillator, a great 'recipe' for rock-solid RPM behavior!

Next, I collected some high-resolution data in the 'fast' mode of the BeoloverRPM:

In this mode, the device sends an RPM measurement every time a platter rib passes underneath the sensor. This gives 24 RPM measurements per platter turn (there are 24 ribs). This allows visualizing short-term RPM changes caused by the feedback-based motor control circuitry. This graph contains the data covering ~35 platter turns (~70 sec):

The prominent wavy pattern is a measurement artifact that comes from the fact that the spacing of the platter ribs around the platter is slightly irregular due to manufacturing imperfections. This generates a repeating pattern for every platter turn (sort of a 'platter fingerprint'), which is superimposed on the real RPM changes that are introduced by the platter drive. This real RPM change is essentially the weak sine-wave-like pattern that modulates the zig-zag pattern. An evaluation of the wavy component yields a wow and flutter estimate of about 0.1%. Since the platter motor is a synchronous motor, such variations can only be introduced by frequency fluctuations of the Wien oscillator, or due to the elastic coupling between the motor pulley and the platter, i.e., oscillations that develop in the rubber belt due to motor torque or friction changes during its rotation. We may never know exactly.

It is interesting to note that 0.1% is double the 0.05% stated in the specs list in the service manual. This difference is most likely systematic due to the entirely different way wow and flutter were measured in the 1970s when these turntables were produced. I see it for all Beograms that I restore. Back then, the measurement was carried out with a test tone on a test record. In these measurements, deviations from the tone center were measured with an analog spectrum analyzer and then converted into a wow and flutter number.

It should be pointed out that this discussion is pretty academic in the first place, since humans typically start recognizing frequency fluctuations above the 0.7 % threshold; i.e., the RPM fluctuations of this Beogram are well below this threshold, whether the number is 0.05% or 0.1%. This motor is definitely ready for duty again!

After these measurements were completed, I did the arm adjustments. But first, I replaced the flimsy locking washer that holds the counterweight screw in place

with a square nut and a washer:

The nut allows me to lock the calibration in place so it survives shipping.

Then I put a cartridge on the arm and calibrated the counterweight to get 1.2g at the 1.2g setting of the little tracking force dial:

The next step was adjusting the arm lowering limit so that the needle would stop dropping about 1 mm above the lower platter rib sections:

This is an important adjustment in case the photocell in the sensor arm fails or another circuit malfunction occurs. The Beogram circuit only detects sensor lamp failures, which cause the arm lowering mechanism to be disabled.

Then I adjusted the tracking feedback

after which it was finally time to give this lovely Beogram a first test spin. I selected one of my favorite Return to Forever/Chick Corea records, "Hymn Of The Seventh Galaxy" which was released in 1973. I have a German pressing that I bought around 1985 (Polydor 2310283). A perfect contemporary to this Beogram, which may have been originally purchased in 1975 or 1976:

The Beogram played perfectly, so I started working on installing a new plexiglass dust cover. The first step was removing the badly scratched original cover from the metal hinge assembly. The mounting screws are easily accessed after lifting the aluminum trim strip on the sides with a razor blade:

I always epoxy small (grey) plastic bits into the strange channels they designed into the otherwise very nice replacement hoods from DKsoundparts in Denmark:

These channels make it impossible to bend the aluminum trip sharply down on the sides. Since there is a void under the strip in the channel area, the bend invariably gets a bit rounded. The plastic bits make it easier to get a reasonably sharp bend across the entire width of the trim strip. Once the epoxy has hardened, I use a razor blade to trim the plastic bits to match the plexiglass corner:

The next step was centering the new trim strip with my alignment tools

I usually use a piece of blue tape so I can make a pencil mark for the strip alignment:

Then I bolted the new hood to the hinge. I usually put a bit of white wood glue on the threads:

This helps keep the screws in place. One should not tighten them very much since this can easily lead to premature cracking of the plexiglass around the bolt holes.

The final step was gluing the new trim in place and bending it down at the sides: