Beogram 4002 (5503): Complete Functional Restoration of an AC-Motor Beogram

I am working my way through a stack of Beogram 400x and I just completed this Type 5503 Beogram 4002 unit from California, which is a 2nd generation AC platter motor design for 110V grid voltage, that has already the optical carriage position detection system that one finds in the latter DC motor types (551x and 552x). Please, refer to my initial post about my assessment of this unit before  the restoration. If you are interested in any of the parts that I used in this restoration, they are all available to other enthusiasts. Just send me an email or use the contact form to the right.

The first step of any Beogram 400x restoration is always the cleaning and re-lubrication of the arm lowering mechanism since the old lubricants are often hardened, impeding the arm movement.

I took the mechanism apart

and cleaned everything. I usually lubricate the damper with a bit of motor oil, and the pivot points with a synthetic grease.

Then I moved on to the damper to tonearm linkage, which is also often stuck:

This requires the removal of the sensor arm. After reassembling (make sure you do not loose the small spring that sits under the circlip that holds the linkage on its pivot shaft) it was time to adjust the arms orthogonal to the carriage and parallel to each other:

I continued the restoration of the carriage/arm mechanism with the installation of a new tracking sensor light source based on a 3D printed housing with an LED and a brightness adjusting trimmer:

Installed:

The final step of this part of the carriage mechanism restoration was cleaning the spindle and re-lubricating it, and replacing the old cracked pulley with a new precision machined aluminum unit (I get them from an enthusiast in Vienna, please, send email for contact info if you need one):

On to rebuilding the electronics. I replaced all capacitors 

and installed new 25 turn RPM adjustment trimmers

and I updated the record detection circuit with a new high-gain transistor and a 5MOhm trimmer for precise adjustment of the working point of the amplifying transistor:

From the solder side:

I adjusted the working point of the transistor to the prescribed 4V:

After this I replaced the reservoir and motor phase capacitors

with a 3D printed assembly that holds the modern, much smaller capacitors neatly in place:

After this a preliminary test with the platter installed revealed a knocking AC motor. I extracted it and took it apart to do an oil infusion of the bearings:

The bearings cannot easily be extracted from the can-halves and so I usually immerse the entire housing in oil:

After a day of immersion I put everything back together and re-installed the motor.

I took the keypad out and removed the output PCB to replace the output relay with a new encapsulated unit and to replace the capacitor that controls the relay delay. I also installed a switch with which one can connect the system and signal grounds if desired. Often, when there is humming, connecting the grounds cures the issue.

The next step was to install LED light sources in the RPM panel. The original incandescent bulbs can have an adverse effect on the RPM stability due to the temperature change they impart on the user adjustable RPM trimmers in this panel. This shows the LED replacements:

They are a drop in replacement for the light bulbs:

After testing the LEDs it turned out that the 33 RPM trimmer indicator had a wavy white reflective background:

This is also a result of the heat load the original bulbs put into this assembly. I tool the panel apart. This shows the wavy background sticker.

I replaced it with a cut to size piece of white insulating tape:

and put the assembly back together:

Beautiful...no more waviness! 

On to replacing the last light bulb, the one in the sensor arm. This shows the original bulb installed together with the flex-PCB based Beolover LED replacement circuit:

This part is based on a warm-white LED and a pass-by resistor that simulates the presence of the original light bulb (necessary due to the low power consumption of the LED).

This shows the installed component:

And in action:

These LEDs emit enough red photons to make the B&O logo show up in its warm red-orange color.

The next step was to replace the deteriorated transport lock bushings. This shows the new replacement bushings:

They consist of two halves that can be inserted into the mounting holes from below

and the top:

After this the locks can be reinstalled. Then I replaced the cracked cabinet guiding washers with 3D printed replicas. The black one is for front-center, preventing visibility through the gap between plinth and aluminum panel:

This shows one of the washers installed:

After this it was time to improve the tonearm counterweight mechanism by installing a M3 nut on the adjustment screw to replace the locking washer. The nut enables locking the weight position in place for posterity, greatly enhancing the stability of the weight calibration:

After positioning the counterweight I calibrated the scale and adjusted the tracking weight to 1.2g, which is appropriate for most cartridges that can be used on the 4002:

After this procedure, I adjusted the arm lowering limit:

This is important should the record detection mechanism ever fail. It makes sure that the needle misses the black ribs on the platter if there is no record.

The final adjustment was the tracking feedback, a process which is facilitated by the intensity adjust trimmer on the sensor light source:

Now it was time to test the RPM stability of the platter motor. I hooked up my BeoloverRPM device and ran it for about 24 hrs. This is the curve I measured:

An excellent result as one can expect from the AC motor Beograms. While the rapid fluctuations are pretty much similar to what the DC motors achieve, the longterm stability over several hours is better. Not that one would be able to hear the difference, these are very small fluctuations, but still, the AC motor is generally the better choice. Here we see the advantage of not having a temperature sensitive feedback system controlling the speed. Instead a strong (that is the main reason why the AC types are power hogs compared to the DC motor models) AC motor follows a stable voltage oscillation, and that keeps the speed very constant over time.

After the RPM test it was finally time for a test drive. I selected a CTI album that I recently discovered, "Concierto" by the guitarist Jim Hall (CTI 6060). His version of Concierto de Aranjuez is rapidly becoming one of my favorite record sides:

This record was of course ultrasonically cleaned with the Beolover's CleanerVinyl Pro System before it was played on this lovely Beogram 4002.

I will now play this Beogram for a couple weeks to make sure that there are no intermittent issues, and then it will be time to send it back to its owner.

Beogram 4002 DC Motor Restoration: Comparison of the RPM Stability Between Four Motors

Sonavor is working on four Beogram 4002 and he recently sent me their DC motors for restoration.

I did my usual restoration procedure on them: Extract the bearings and infuse them with oil. Then test the motor for RPM stability. This shows one of the motors:

After taking it apart:

The bearings are on the black pad up front. Insertion into oil and pulling a vacuum results in air bubbles coming from the porous brass material, making room for fresh oil diffusion:

After 1-3 days the bubbling typically stops, indicating that the diffusion process has come to an end and the bearing's oil reserves are replenished. After extracting them from the oil, I usually wick excess oil away on a paper towel:

Then it is time to reassemble the motors and do a RPM stability test with the beoloverRPM device. It allows logging the RPM for extended periods of time.

Luckily all four motors ran well after replenishing the bearings and none needed a replacement of its spark snubbers. These are the four RPM curves that I measured over about 24 hrs:

The curves are pretty similar, but there are some differences. I noted this already for a long time, that some motors seem a bit more stable than others. Of course one needs to be aware that these fluctuations are very minute and well below the audible range. But nonetheless there are differences. My hypothesis is that each motor has a certain friction profile, and this affects the analog feedback system that controls the motor speed. 

Another point is that most motors, over time, seem to get better. This may have to do with the fact that their shafts 'polish' the top bearing (which holds most of the torque exerted by the platter belt) in a spot directed towards the platter center. 

Since I realized that this is happening, I usually mark the bearing rotational position before I disassemble the motors, and then I try to get it in the same exact position during re-assembly, but of course this is not a 100% perfect process. So there may still be a bit of a 'burn in' process during the first few 100 hours of run time. Ah, the mysteries of analog technology!...;-)

At any rate, these motors are ready for duty again, and they can go back to Sonavor's bench.

Beomaster 4400 Type 2419: Reassembly and First Power On Test

I reassembled most of the Beomaster 4400 this evening. It is back together to the point it can be power tested and hopefully able to adjust the no load current.

The reassembly included re-attaching the output transistors to the Beomaster 4400 heatsinks. On the last Beomaster 4400 project I did I used the SIL-Pad product for thermal insulation instead of the typical mica insulator and transistor thermal paste. My reason for doing that is that it is less messy and easier to change transistors if necessary. I keep in touch with the owner of the last Beomaster 4400 project and he happily reports the amplifier is performing great and runs nice a cool. For me that confirms what I already knew...the SIL-Pads work great on these Bang & Olufsen amplifiers. Note that I have used the SIL-Pads a lot on the more powerful Beomaster 8000.

So on with the transistor reassembly and with the re-installation of the Beomaster 8000.

Here are photos of the reassembly process.

Here is the Beomaster 4400 ready to plug in and turn it on.

For a first turn on situation I like to use a variac combined with a dim bulb tester.  I built one quite a few years ago. Mine has a switch to put the bulb in the circuit or remove it. I also have an ammeter and voltmeter to read what is pulling through the unit under test.

As it turns out in this case...the dim bulb tester probably saved a fuse.

When I advanced the variac to apply AC current the bulb illuminated and stayed illuminated.  That is not a good sign. If the power circuit is working correctly the bulb should initially flash or glow a little, then go off.  It should not stay on.

In previous cases where this happens I found that there is likely a problem with one or both of the output amplifier circuits.

The Beomaster 4400 has separate power connectors for the left and right channel output amplifiers so I tried them one at a time.

The left channel allows line power to turn the Beomaster on.  I cranked the variac up to the full 125 VAC, 60Hz output of my house wall outlet and the dim bulb indicator remained off.

Next was a test with just the right channel output amplifier plugged in.  Sure enough, it is the culprit.
At just 25VAC, 60Hz the bulb illuminates and remains on.

So it is back to the bench and I will have to check out the components on the right channel output.
This may be the reason the original owner sold the Beomaster ;-).