Beogram 4004 Type 5526: Cleaning and Polishing the Dust Cover

The Beogram 4004 turntable has been test playing in my office listening room for a couple of weeks now. All that is remaining on this project are a couple of cosmetic issues. One is polishing out some minor scratches on the dust cover.

The procedure for this is time consuming because it takes a number of wet sanding steps using finer grit sanding material on each step. After each sanding step I rinse off the excess sanding residue and dry off the dust cover for inspection. Then I rotate the dust cover ninety degrees for the next step so each step is perpendicular to the previous step.

Here is a picture of the Beogram 4004 dust cover prior to beginning the work. It is hard to capture the true severity of the scratches in a photo but you can see there are scratches on the dust cover. I have seen much worse but these are still worth polishing out.

Here is the dust cover and all of the polishing tools laid out on my fabrication table.

This picture shows what to expect after the first wet sanding step. In this case I started out with 600 grit wet sandpaper because there are not any real deep scratches on the dust cover.

After going through 600 grit to 2500 grit sanding steps I reached the 3000 grit sanding step. That is the halfway point in my progression.

Here is the dust cover after the final sanding step of a 12000 grit Micro-Mesh pad.

It is starting to look good again but you can see it is a little hazy by looking at the sanding tool packaging text under the dust cover.

After a buffing polishing step using an eccentric orbiting pad on my power drill the dust cover clarity really comes back.

Here are two pictures of the polished dust cover similar to the two I posted of the dust cover before starting the work. The scratches seen earlier are gone.

This dust cover looks like it is ready to be re-installed on the Beogram.

Beogram 4004 Type 5526: Electrical & Mechanical Work Complete...First Play Test

I will begin this post with a picture of the Beogram 4004's first play test. It feels great to listen to some Stevie Ray Vaughan playing while I compose the blog report.

To get to this point from the last post I had to install the two Beolover replacement LED modules for the speed trimmer indicator lamps and run the 24 hour platter speed stability test.

Here are some pictures of the speed indicator LED replacement.

The LED lamp replacements on this speed trimmer circuit reduce the load on the Beogram motor control enough to show up as a final improvement on the 24 hour speed stability test. This of course goes hand in hand with the Beolover restoration of the DC platter motor, new trimmers on the main board and the new RPM speed selection relay.

Here are two plots of the 24 hours speed tests I performed on this Beogram motor. The first one, back in May, was with the Beogram before any restoration. The second plot, from the last 24 hours, shows the dramatic difference in the platter speed test results after the completed mechanical and electrical restoration work.

Beogram 4004 Type 5526: Finalizing Mechanical Adjustments

As usual with these Beogram 400x turntables it takes a good number of iterations on the mechanical adjustments before I am satisfied that the turntable is correctly aligned and ready to play records again.

The remaining final adjustments on this Beogram 4004 were the platter height adjustment, tracking force calibration, stylus tracking path check, stylus set down limit and the tangential tracking sensitivity.

For the height adjustment of 23mm between the Beogram platter deck and the top of the fixed arm I ran into a problem where the tangential arm assembly wasn't level enough with the floating chassis frame.

I stripped the Beogram chassis down again and moved the set screws for tangential arm assembly height so they could be adjusted from the top instead of underneath the floating chassis. This will save time in the future and not require taking everything apart again if that adjustment needs to be tweaked.

I ran into a second problem which was with one of the set screws on this Beogram. Someone previously had damaged one of the set screws where it could not be removed. I had to use a Dremel tool to drill it out, then re-tapped the hole for a replacement set screw from my spare Beogram 4004 unit.

Once I was passed that issue it was pretty straight forward to adjust the tangential arm assembly, using the set screws, to get the fixed arm level with the chassis.

I re-installed the floating chassis in the Beogram case and adjusted the platter so the top of the platter is 23mm from the top of the fixed arm. Sorry for no pictures but here is a link to some reference pictures from a previous project.

With the deck height set I went about calibrating the tracking force for the cartridge. The way I like to do that is to set the tracking force wheel to 1 gram, then adjust the counter weight and tracking force wheel as necessary so I measure 1 gram with my digital cartridge scale.

Since I haven't put the Beogram deck completely back together I needed a good base for the scale to rest on in order to measure the tracking force. The ridges on the Beogram 4004 platter don't allow room to put the scale under the tonearm so I borrowed a platter from a Beogram 8002 turntable which is completely flat.

I also left the Beogram 8002 platter in place for the stylus tracking path check.

For the stylus lowering limit adjustment though I switched the platter back to the Beogram 4004 platter as the adjustment is to make sure the stylus lowering limit clears the first ridge of the platter.

That is to protect the stylus if the Beogram ever fails to detect that a record is not present.

Everything is looking good so far. The final adjustment I make is the tangential tracking sensitivity. With the platter belt disconnected from the platter motor I can manually rotate the platter. The adjustment is to set the tonearm down on a record track, then adjust the tracking detector so the tangential arm drive motor begins rotating after a couple of (manual) platter rotations. After that the tangential arm motor should move the arm a little bit on every rotation. Obviously the exact results of this check can vary some depending on the record you use for the test. The Beogram service manual calls for a specific B&O test record (3621001) and track (5) that I have never been able to find. So I typically use a couple of old records I have and test the tracking sensitivity on several tracks.

Once the tracking is set I test play the old record and watch the tangential tracking system do its work.

Next step is to connect this Beogram to my listening room amplifier for the first record play test.

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.