Beogram 4004 Type 5526: Finalizing the Fixed Arm Record Detection

In the last post comments I mentioned learning something new every time I work on these Beogram turntables. This is certainly one of those cases. I installed the Beolover LED module in the fixed arm sensor assembly that I had swapped with a spare Beogram 4004 turntable. That sensor measured exactly as I wanted using the original incandescent lamp so this new LED light source should work just as well. To my surprise the LED light source produced a weaker sensor signal than the incandescent lamp. The signal was better than with the original sensor assembly but it still wasn't at the level I need it to be in order to continue with this restoration.

The peak-to-peak voltage target is 6 V and the above signal is still short.

Even though the LED light source looks brighter than the incandescent bulb there are other factors involved like the angle of the light on the spot that the sensor is focused on and the wavelength of the light. I discussed this with Beolover and determined that I needed to adjust the LED position on the Beolover light source.

Here is the Beolover LED module. The blue arrow shows the direction I am moving the surface mounted LED. How convenient that Beolover designed the solder pads for mounting the LED so that its position can be adjusted.

That was all it needed. The new position of the light source provides the necessary light stimulus to the sensor where it can now produce a healthy 6 V peak-to-peak record detection signal.

I am now confident in this circuit so I can continue on to the Beogram 4004's platter height adjustments.

Beogram 4004 Type 5526: Record Detector Testing and Adjustments

This Beogram turntable has a new LED lamp source for the record detector circuit that is in the fixed arm. Part of that installation involved switching out a fixed 1MOhm resistor (1R26) with a 2MOhm trimmer resistor to allow better tuning of that detector circuit voltage. With the turntable parts re-installed I started the procedure of making an initial setting of the new 1R26 trimmer so the collector of 1TR3 is at 4 VDC when the fixed arm lamp is on.

I made that adjustment then moved the 1R26 trimmer resistor to the component side of the board where it belongs (so it is out of the way of the platter).

A quick test of the Beogram Start, without a record on the platter, passed with the sensor circuit correctly recognizing there was no record on the platter.

To complete this test however it is always a good idea to actually see the signal on an oscilloscope. I connected a probe wire to the 1TR3 collector and then to my scope. When I ran the fixed arm and tonearm over the platter I paused it so I could measure what the detector circuit is seeing.

I was surprised and disappointed to see that the peak to peak voltage of the signal was not from 0 V to around 6 V as expected. It only drops to about 2 V and goes up to 4.72 V.

So while the Beogram appears to detect an empty platter as it should, the signal is not ideal and could lead to problems later on. A little bit less peak to peak voltage and this circuit will not detect an empty platter.

I wanted to know if the problem was in the new Beolover light source assembly or with the fixed arm light sensor. The easy thing to try first was to use a powerful LED flashlight to enhance the light on the platter. When I did that I could make the 1TR3 collector signal go to its full range. But look how much light it took for the sensor to register that. Something must be wrong with the sensor.

As a second part of the test I pulled out the fixed arm lamp and sensor assembly a ways so I could move the Beolover LED module and watch the effect on the 1TR3 collector voltage.

Moving the LED light source could not make the signal peak to peak range any better. It could make it worse though and confirmed my fear that a little less voltage would cause the platter detection to fail.

Next step then is to swap out the fixed arm sensor. I decided to swap out the entire assembly first with one of my spares (from another Beogram 4004 turntable).

Here is the removal of the current sensor assembly that has the Beolover LED light source.

Here is the replacement sensor assembly. It still has an original incandescent light source.

Just like that, the 1TR3 collector signal went right to what is expected. The peak to peak voltage is from 0 V to about 6 V.

That voltage is with a dimmer light source as the Beolover LED light source is brighter than the incandescent lamp. The original fixed arm platter sensor is the problem and will have to be replaced.

Beogram 4004 Type 5526: Tangential Arm Position Detector

Just a quick and short update...
I didn't get as much bench time as I wanted the last couple of days but I did sneak on long enough to connect power and adjust the Beogram's tangential arm position detector voltage.

Just like I did on the previous Beogram 4002 (Type 5513) project, I needed to verify that the base voltage of 1TR17 is at a nice 0.7 VDC when the light source 4D1 is illuminating the tangential arm position sensor, 4IC1.

It was a little off so adjusting trimmer resistor 1R88 was necessary to bring the voltage down. This picture shows the initial adjustment.

Once I re-attached the position scale I re-checked the voltage at 1TR17 base with the 4D1 LED at a transparent part of the scale.

I then ran the Beogram tangential arm through some of its operations. Everything worked great.

Later tonight I will measure and adjust the new 2M ohm trimmer for the fixed arm record detector.

Beogram 4004 Type 5526: Starting the Re-assembly

Re-assembly time for these Beogram 4004 components. It is time to get the floating chassis put back together. The first pieces are the transport lock assemblies.

Set the lower lock at the very end of the threaded screw. Later, the upper lock will be set to the top end of the threaded screw. That is so there will be the maximum gap for the floating chassis to operate when the turntable is unlocked.

Three washers go between the lower lock and the Beogram chassis. The washers are concave so the washer arrangement is as shown below.

The Beogram floating chassis with its components fit over the transport lock screws. New Beolover 3D printed bushings have been installed to replace the original ones that had disintegrated into a million pieces (that keep showing up).

The top lock threads into place followed by two more of the concave washers. Again, the top lock is threaded so it is just on the top part of the locking screw.

The top plate goes on next. It is secured by two screws. It is important to note that the two screws should easily screw into place. No extra force needs to be applied. If you find they don't screw in easily then do not force them. It means the screws are not aligned correctly. Re-adjust them until they easily screw into place.

The locks are in place so the leaf springs are attached and the floating chassis is back in business.

I installed the Beolover rebuilt (re-infused bearings) DC platter motor and the two circuit boards. It is just about ready to apply some power and start making electrical adjustments with the trimmers I installed earlier.

Beogram 4004 Type 5526: New Damping Material for Tangential Drive Motor

I mentioned in the post about Tangential Drive Work that the drive screw motor on this Beogram seemed to have a lot of play in it. I checked some other Beogram units I have and they have very little to no play. The DC motor feels solid inside its black plastic housing on those other Beogram units. Not so on this one.

I needed to do something about the drive motor before I continue so I took the housing off the motor. Sure enough, the foam damping material was all but disintegrated inside.

No saving or restoring this mess. It needs to go so I gathered up the old foam material for disposal.

There are quite a few options available today for a substitute. I decided to run down to my local RC hobby store and see what they use for motor vibration damping in RC cars, air planes, helicopters and other RC vehicles.
I found a 1/16th inch thick dense foam padding material that is sticky on one side. I cut it to size and applied it as two bands (front and back) around the DC motor. I also put a couple small rectangular pieces on the front and back.

It looks quite good and a test fit proved that it should do a good job.

I re-assembled the motor housing and this Beogram drive transport is ready to put back in the chassis.

Beogram 4004 Type 5526: Initial Tonearm Adjustments

The work on the tangential drive components so far involved unbolting the fixed arm. When I re-installed it I made sure it was perpendicular to the rear rail of the tangential arm assembly. That leaves the tonearm needing adjustment to make sure it will operate correctly in relationship to the fixed arm reference position.

The service manual says the distance between the fixed arm and the tonearm bars should be 7.7mm. That matches up with the Beogram 4004 transport fitting for the arms. I find it helpful to attach the transport fitting while I make my initial adjustments of the tonearm positioning bar.

The tonearm needs to be parallel with the fixed arm at the prescribed 7.7mm distance and the tonearm raise/lowering lever needs to meet the positioning bar right in the center of the triangular slot (circled in red above).

These adjustments take quite a few iterations even with the nice head-start  provided by the transport fitting. One reason is I am also adjusting the initial position of the counter weight and that adjustment affects the tonearm positioning bar.

The goal is to have the arms parallel and the tonearm balanced so when the lowering lever lifts up from the triangular slot so the tonearm is free to lower, it lowers straight down without any sideways movement. When the tonearm lever lowers into the triangular slot to raise the tonearm, the arm must raise straight up without any left or right movement.

I was able to eventually get this arm assembly adjusted where I wanted it. The final adjustments of the lowering limit and tracking weight fine tuning will be done later when the Beogram is re-assembled.

I checked over the arm position from the front and I think this assembly is ready to move on to the next stage.

Beogram 8000: Opening the Beogram to Service Position

I don't think Bang & Olufsen intended for the Beogram 8000 owners to ever open their turntable into its service position. The factory authorized service centers would take care of that. These days however, owners that want to do simple maintenance on their Beogram 800x model turntables must open their turntables up themselves. A common task like installing a new belt for the tangential arm drive requires putting the Beogram into service position.

For the technician though, the Beogram service position is great. It really isn't that difficult and like most things, having the correct tools makes a big difference.

The first step (not pictured) is lifting off the platter.
Next, there are some important Beogram 8000 components to be familiar with in opening up the turntable to its service position. For reference this picture points out the suspension lockdown screw. There are three lockdown screws and they must not be in the lockdown position to open up the Beogram. There are also three leaf springs and hangers that make up the floating suspension of the Beogram. The suspension hangs from the Beogram deck lid that needs to lift up so you must disconnect and lower the suspension before the Beogram can be opened.

I use a special hook tool that makes it pretty easy to grab the suspension hanger. Holding the leaf spring down with a thumb or finger allows me to lift the suspension hanger up and over the mounting hole of the leaf spring.

Another first step (you could do this step prior to lowering the suspension) is to remove a couple of plastic trim pieces inside the Beogram deck. For the piece covering the dust cover hinge mechanism I use a small, plastic pry bar tool like you find now days for working on cell phones and tablets. It can pry pieces that snap together open without marring the finish.

The other trim piece is the plastic cover over the tonearm and fixed arm.

Although the suspension is disconnected (and lowered) at this point the Beogram lid cannot open yet because of two screws under the left side of the chassis that lock the lid down.

It is kind of awkward to get to those as you can see. I hang the Beogram over the edge of the table a bit to get to them.

The Beogram lid can now start to lift into service position but there are still a couple of very important steps. The lid must carefully be maneuvered at the left edge where it is free from the chassis lip. Once it is free to tilt (lift) you have to make sure that the deck will clear the fixed arm of the Beogram.

When the Beogram deck is in its normal position for use, the distance between the edge of the deck and the fixed arm is not enough for the deck lid to open. It will surely clip the fixed arm (which would not be good).

By design the turntable floating deck, that the tangential arm assembly is a part of, can be slid to the right a little bit so there is clearance for the deck lid to tilt.

If you look at the posts in the pictures below the bottom picture shows the posts have been slid over to the right.

Now there is clearance between the deck and the fixed arm.

The deck lid now tilts up from left to right into the Beogram service position.

Now it is easy to change the tangential arm drive belt or whatever you need to get to inside the Beogram.

One extra note...Be very conscious of the dust cover lid and tonearm compartment lid with the Beogram deck tilted up. Sometimes I secure them with painter's tape to make sure they don't accidentally get damaged. For major work on the Beogram I remove the dust cover and tonearm compartment lids completely. However, you also have to be very careful in doing that as I have come across quite a few Beogram 800x units where the mounts for the dust cover have been damaged by people not careful with their removal. Perhaps that would be a good blog topic by itself.