Beogram 4004 Type 5526 - California Project - Initial Assessment and Cleaning

Here is an initial assessment and cleanup of a nice Beogram 4004 Type 5526 turntable sent to me from California.

 

The platter was packed separately so it isn't in the photos.

The keypad and wood trim look great.

You can see that the tonearm has a problem.  Fortunately it doesn't appear serious.  It is just missing one of two M2 screws that attach the arm to the base.  I have spares for that.

The dust cover is in decent shape although the aluminum trim piece has a buckle.
The owner sent me a new dust cover to replace this original one however so this Beogram will have the new one installed when I am finished.

The inside looked good at first glance.

When I removed the circuit boards and floating chassis I discovered some sort of spillage had occurred in this Beogram's past.  Some sort of liquid was spilled?

Amazingly, this Beogram has original transport bushings that are not dried out and crumbling apart.
They are intact and still soft.  I will leave them in place and coat them with a rubber treatment to make sure they stay soft.

Also note in this photo that I installed the missing M2 screw on the tonearm.  It still needs adjusting of course but that will come later.  For now the arm isn't dangling around anymore.

As I removed components from the floating suspension frame I looked for more remains of whatever was spilled into the Beogram.

Besides a dry film on the Beogram part surfaces, there were a few places where there is corrosion.

The suspension wire assembly at the back (where the spillage must have occurred) has corrosion so I will replace that part in the restoration process.

Not surprising the pivot posts for the tonearm operation were gummed up from the liquid that spilled.
The lever that lower and raise the arm would barely move.

I didn't want to wait until later to start cleaning this residue up so I decided to clean as I went through the initial assessment.

The cleaning up wasn't too bad.  I only had to apply corrosion cleaner to a few places.  Most of the residue scrubbed clean.

The arm lowering and raising assembly is back working smoothly again.

The pulley belt for the tangential arm is all stiff, dried and broken. That isn't unusual but is kind of surprising seeing how the transport lock bushings remained soft over the years.

Really amazing is the condition of the phono DIN plug.  It is corrosion free.

The old tangential arm drive pulley is going to be replaced with a new, aluminum pulley so I removed it and cleaned up the tangential arm drive spindle.  I also cleaned the guide rails, spindle nut and cover.

This Beogram 4004 is now at a point where I can begin working on the restoration of the mechanical parts for the turntable.

Beogram 4000: Installation of New Dust Cover and Aluminum Trim

The customer of a Beogram 4000 that I restored in January-February 2025 recently let me know that the new DKsoundparts hood that I installed during the restoration broke off one of the hinges. He included this sad picture:

I decided to replace the cover under warranty since it cannot be ruled out that this was caused by mounting screws that were tightened a bit too hard.

He sent me the hinge. And I installed a new hood and aluminum trim. See here if you are interested in this service. This shows the parts before installation:

First I improved the design of the plexiglass cover by installing small 3D printed plastic bits into the top ends of the 'channels' they designed into the side areas where the aluminum trim is supposed to adhere: 

These channels make it difficult to achieve a perfectly sharp bend with the trim, since it is not supported when it is bent, causing a more rounded corner.

The next step was to check whether there is any interference between the metal back and the plexiglass part when the hinges bend. The new hoods seem generally a bit tighter than the original ones and bent metal hinges can cause interference between metal and plexiglass. The resulting stress can crack the hinge areas of the plexiglass cover. I bolted the metal back onto the hood and checked if the metal would touch the plastic in any way during bending the hinge. On both sides I saw a minimum 1 mm spacing between the parts, i.e. this was a pass:

I removed the metal back again and centered the aluminum trim using my alignment pieces:

I usually use a strip of blue tape to make a pencil mark between strip and the hood:

Then it was time to bolt the metal back in. I put some white wood glue on the threads of the holes in the hinges:

This holds the bolts in place even if they are not tightened very hard.

The final step was applying the aluminum trim

and bending it down on both sides:

Nice precise corners due to my little plastic inserts! Beolovely! Here an impression of the entire dust cover assembly:

Time to send it back to Australia!

Beogram 4002: Installation of a New Dust Cover and Aluminum Trim

I recently received the back of a Beogram 4002 dustcover from a customer in California for the installation of a new dust cover and aluminum trim (Beolover now offers this as a standard service). This shows the received parts on the bench:

My customer already had the hinges removed. This made it easy to check if they were not bent using a special tool that I made a while back when a new hood cracked due to bent hinges (they caused it to chafe on the metal back, and after a while it simply cracked off the hinges due to the stress that occurred every time it was opened or closed). In this case the hinges were perfectly 'spec':

I re-installed them on the black back part. This one was an earlier design that still had the nice set screws for the dowel pins that hold the hinges in place. The set screws are located under separate screws that cover the threads for the set screws:

After removing the screw the set screw can be screwed out. This shows it all the way out for clarity:

Normally, they only need to be loosened a turn or so and then the dowle pin can be stuck in from the side. This shows one of the hinges installed

and here the other:

This shows the components before putting them together: DKsoundparts hood, Beolover aluminum trim and the original hinge:

I strongly recommend doing a quick test with a new hood if it really works with the assembled dustcover back. The new DKsoundparts hoods are a bit 'tighter' than the original ones and the metal back gets closer to the plexiglass than an original one would. Therefore, it is crucial to make sure that there is no chafing. If there is the plexiglass will likely crack after a while. So I now always bolt the metal part in for a quick check if there is any contact between metal and plexiglass:

Once the parts are bolted together, I manually crank the hinge and watch closely if there is any issue. If there is interference, it will most likely be close to the sides. This shows the right hinge in the position with closest metal to plexiglass proximity. A satisfying ~1mm gap can be observed:

And there the other side with a similar gap:

This was no surprise since both of the hinges had passed by calibration tool at the beginning of my process. 

Sadly, later Beogram 4002/4004 do not have the set screws for the dowel pins anymore. Their pins are pressed in and in many instances they cannot be removed without the risk of damaging the black metal back. In such cases the only remedy when the hinges are bent enough to cause interference is Dremeling the plexiglass a bit until friction-free operation is achieved. Luckily, the 'region' that usually needs to be Dremeled is not visible during normal use of the turntable, i.e. this can be done without harming the appearance of the unit.

On to the installation!

I removed the hinge part and installed small 'filling pieces' in the channels that they designed in the back of the hoods where the aluminum trim needs to be installed. Not sure why there are these channels, maybe a requirement for the casting process. The original hoods do not have it. These channels are a bit annoying in that they make it difficult bending the aluminum trim smoothly. There is nothing under the trim when it gets bent, and so the bend is less 'sharp' in the area of the channels. These little 3D printed pieces try alleviating this issue. I epoxy them into place and then trim them with a razor blade:

The next step was determining the perfect center position of the aluminum trim. I do this with my alignment tools that clip to the hood. They make it very easy centering the trim:

Once the trim was centered I made a pencil mark across the trim and a piece of blue tape on the hood:

Then it was time to bold the back part in again. I usually put a bit of white carpenter glue on the threads in the hinges:

The idea is to hold the screws firmly in place since they cannot be bolted in very tightly. If they are bolted in too hard, the hood can crack again after a while. The white glue is soft enough that the screws can still be removed if need be, but it will hold them in place against vibrations etc...

Then it was time to glue the aluminum trim on and bend the side pieces. I use a specially designed bending block. It makes it easier to get a good bend around the corner:

The final step was inserting the hood bumpers on the front corners:

These bumpers come with the DKsoundparts hoods. And here a picture of the final result!

Beolovely! This hood assembly is ready to be returned to its Beogram 4002!

Beogram 8002: Carriage Runs into Mechanical Limit During Carriage Return

A Beogram 8002 that I am currently working on exhibited a strange issue after I had it fully restored and mechanically adjusted (i.e. the arms were perfectly orthogonal to the carriage rods and the end switch was set to a position where the arm would drop properly at the LP setdown point): Whenever I activated STOP or the arm auto-returned after playing a record, the carriage would bump into its mechanical limit and tilt upwards a bit. Not very beolovely! This meant the carriage motor somehow did not shut down soon enough after the end switch activated. 

This was odd, since the end switch was adjusted for a perfect arm drop onto an LP run in groove, i.e. this issue could not be fixed by changing the end switch position a bit inward so the carriage would travel less far towards its home location. On another 8002 that I have currently on the bench everything worked nicely, i.e. the carriage traveled not as far after triggering the switch and the carriage did not bang into the rod support.

After a bit of head scratching I set up my portable DS-213 oscilloscope to do some measurements on these two Beograms. I measured one of the rotary encoders (blue) vs. the carriage motor voltage, hoping to learn something about the microcontroller timing of the carriage motor operation.

This figure compares the measurements for the problematic unit (top) and the normal one (bottom):

The problematic unit has a 100ms delay between switch activation and shut down of the carriage motor. I should point out here that the stop of the encoder signal coincides with the switch activation since the switch not only interfaces with pin 32 on the uController, but also directly turns off the power to the IR diode in the encoder.

A good question here is 'why did they set it up like this and not just use the encoder steps to determine an exact end of travel for the carriage?' Oh, the mysteries of vintage B&O!..;-). I would certainly not have designed it like this. Maybe a constraint was that not enough I/O pins were available and they had to combine a number of control items on a single pin.

Anyway, in contrast to this measurement the 'normal' 8002 only shows a ~70ms delay. In other words, the problematic 8002 runs the carriage motor about 50% longer than the good one after the end switch is triggered. This suggests that the two microcontrollers have different programming since the motor shutdown is directly controlled by pin 26 (">>") of the controller. In other words there was nothing to fix. Everything was mechanically and electronically in good shape, just the controller did some slightly different controlling!

My big question was how to alleviate this issue without the ability to change the firmware on the chip. In the end I saw only one way to deal with this: Slow down the carriage speed a bit during return to the home position. Luckily, there is a pretty straight forward way to do this!

This shows the relevant portion of the circuit diagram:

The shown circuit snippet is the half of the motor H-bridge that takes care of rightward (>) motion of the carriage. The opamp IC2 drives the TR3/4 push-pull stage that provides the motor current to OM1. The opamp itself is controlled by comparing the voltage at the + input with the voltage at the - input. The + input is connected to pin 26 of the uController via D7 and to the >> photoresistor in the black box on the keypad PCB via D8.

The voltage coming from the >> photoresistor ranges from 0.62V (if the calibration screws are adjusted properly) to about 3.2V depending on how hard the >> button is pressed.

This told me that the voltage control range tops out at about 3.2V, corresponding to full speed of the carriage. When carriage return is activated by pin 26, however, it applies the full logic voltage of about 4.8V or so, reduced to about 4.2V by D7. This means the the opamp is driven into saturation and puts out its maximum voltage.

My goal was to reduce the carriage speed enough so the carriage would stop slightly before from the mechanical end of travel during the 100ms after the end switch is triggered. This meant I had to reduce the voltage after D7 below 3.2V. I added a 1MOhm trimmer between D7 and the 1MOhm resistor R34, thereby forming a voltage divider that would allow me adjusting the voltage at the + input down to ~50% with the trimmer. For this I removed D7 and put it on the solder side of the board together with the trimmer for better access. Here an impression of this temporary experimental setup:

As expected the voltage yielding a sufficient speed reduction to prevent crashing the carriage was below 3.2V:

This voltage corresponded to ~840kOhm trimmer resistance (trimmer needs to be disconnected for an accurate measurement). With this resistance in place the carriage stopped about ~1mm before the mechanical end of travel, which in turn corresponded to a 9 mm distance between carriage and encoder housing:

All this was measured with the switch set to a position where the arm would drop perfectly into the center of the run in groove allowing time for the output relay to open before the needle would meet the music:

After this successful experiment, I replaced D7 with a small replacement circuit composed of a 820kOhm 0603 SMD resistor and the original diode on a Dremeled 1x6 piece stretch of prototyping board (it seems despite the 'analog' appearance of these 1980s boards they actually put the solder points on a 0.1" matrix!..;-):

Here an impression of the little board replacing the originally lonely D7 on the component side:

After this I tested the unit again and everything still worked! A promising sign!...;-). All good again in Beogram 8002 land! I should point out here that the 'apparent speed' of the carriage return only changed little, i.e. the casual observer will most likely not notice that the return speed was reduced a bit.

On to writing the main post about the restoration of this unit!