Beogram 4000: Replacement of Sensor Arm Bulb with an LED Assembly

The final light bulb to replace with an LED in this Beogram 4000 was the sensor arm bulb. I recently developed a flexPCB based assembly with a warm-white LED that folds into the bulb compartment of the sensor arm. This Beogram 4000 has a sensor arm where the bulb/sensor compartment connects via a socket. Therefore, one can simply pull out the entire assembly and perform the LED conversion on the bench:

This shows the LED assembly together with the extracted bulb compartment:

and with LED installed:

The red 3D printed wedge serves to position the LED in the same spot where the filament of the bulb previously resided. Before installing and testing I measured the voltage at the collector of TR14, which amplifies the sensor signal before it is fed into the record detection circuit. In the Beogram 4000 2.2V DC should be present at the collector without a sensor signal. This was the case for this Beogram,

and so the installation could proceed:

The warm-white LED has enough red photons to present the B&O logo in its original red color. The final step of any light source replacement in the sensor arm is to measure the sensor response when no record is present. This is also measured at the collector of TR14 and an amplitude of 2.2V should be achieved with the sensor over the spinning platter to ensure proper disabling of the arm lowering circuit if no record is present:

As you can see this Beogram 4000 passed with flying colors. Each dip represents a platter rib passing under the sensor. A 'real-life' test with and without record confirmed the proper working of the circuit.

Beogram 8000: Repairing The Aluminum Deck and Front Trim

It is time to clean off the old double-sided tape residue that held the Beogram aluminum deck in place and re-glue the deck down on the cabinet

These pictures show the old tape residue.

I like to use GooGone to soften up the old tape residue where I can just scrape it off. I let the GooGone set in place on the old tape for a couple of hours. This has to be done on the chassis under deck and the underneath side of the aluminum deck panel.

 When the old residue is scraped away I clean the surfaces so they will accept the new glue.

I have used new double-sided tape before in re-attaching these panels before but we have found the fit is better using epoxy with some #4 aluminum washers (used as spacers).  I placed them at the same attach points as the original tape.

I use thirty minute epoxy so it gives me time to apply the glue and adjust the panel correctly. I can clamp the panel down in three of the attach points but have to use some weights on the fourth.

The gluing procedure went well but I noticed that the front trim piece of rosewood veneer was separating from the Beogram case.

So much of the front veneer was separating that the only good solution here was to remove the veneer trim completely and clean all of the old contact cement off.

Once the trim piece and the cabinet base, where it fits, were clean I could reattach the trim piece with some new contact cement. This next picture shows the two parts clean and ready for contact cement to be applied.

Here is the cabinet with the aluminum deck and reattached trim piece.

I also cleaned out the base of the cabinet good and applied some anti-corrosion oil on the deck hinge.

Before adding the Beogram main components back into the cabinet I performed the service manual adjustment for the platter to arm height. The distance from the top of the arms to the surface of the platter should be 23mm. This turntable was initially at 21.9mm so it required some adjustment using screw "A" as shown in the picture here.

Now I could move the main Beogram components into the cabinet and give them a quick test.

Very nice. Now on to the tracking force calibration and the tangential arm tracking adjustment.

Beogram 4004 (5526): Some Odds and Ends, Final Adjustments and a Test Drive with Dave Brubeck

My restoration of the Beogram 4004 that I was restoring in the last few weeks has come to an end. I procured a near-pristine platter, a nicer looking arm cover, as well as a nearly new looking keypad and installed them. I also added springs to the hood screws that prop them up when the hood is taken off. This makes it much more pleasant to install the hood. They often get lost when people open up the turntable and then forget to put them back in when they built things back together. I sourced them from McMaster. Here are a couple impressions:

this is where the springs belong:

I also fixed the cracked veneer at the back of one of the sides of the plinth:

I used a bit of Alene's Tacky glue and a carpenter's clamp to get things back together:

Almost like new:

Beolovely! 

On to the final adjustments. I adjusted the tracking sensor feedback

and then moved on to the arm lowering limit:

When the arm is down, the needle should miss the lower parts of the platter ribs by a mm or so to avoid calamitous events should the record detection system ever fail (due to a dead photosensor, for example). Then it was time to calibrate the tracking weight:

And then, finally, it was time for a first test drive! I selected a recently acquired Dave Brubeck record "Angel Eyes" on the Columbia label (CL2348). I admit, I partially bought it for the cover during one of my strolls through local vinyl stores...;-). This is an impression of the finished Beogram and the angel eyes:

A very nice album, well suited for casual listening on a Sunday afternoon...The Beogram performed admirably, and it may finally be time to send it back to its owner!

Beogram 4002 Type 5513: Finishing Up The Tangential Arm Position Sensor & The Fixed Arm Platter Sensor

It was great to see Beolover has the platter motor restored for this Beogram 4002 project. While it is being packed up for shipment back to me, I adjusted the voltages for two of the new sensor lamps in the Beogram.

The new LED replacement light source for the tangential arm position sensor was a little weak for me to leave it installed. It appears to work fine but with the 1R88, 25K ohms trimmer set for the highest LED output, the maximum voltage I could get on the 1TR17 base was 0.695 VDC. I would prefer for the 4D1 light source to have enough juice to max out the sensor then use 1R88 to dim the LED down where the base voltage of 1TR17 is 0.7 VDC.

That meant ordering a brighter LED. Here is the circuit as shown in the service manual. The 1R88 trimmer controls the current to the lamp source, 4D1.  That increases or reduces the LED light output to the sensor, 4IC1. The sensor controls the base of 1TR17.

With a new amber LED installed (Cree C503B-AAN-CY0B0251) for 4D1, there is plenty of light on the sensor now.

In operation the black bars on the tangential arm transport scale affect the light source on the sensor causing a change in voltage at 1TR17.  The output of 1TR17 drives analog control logic for the operation of the Beogram. It is quite interesting how the control system works on these analog 400x turntables compared to the 800x series which use a microcomputer for their control.

Here is a picture of the tangential arm position scale.

Here everything is put together.

The new LED does max out the sensor where I get a voltage of around 0.75 VDC at the base of 1TR17.  Using the 1R88 trimmer to adjust 1TR18 so the LED has less current, I got the desired 0.7 V at 1TR17's base.

Next up is the voltage adjustment on 1TR3 for the fixed arm sensor - the detector arm circuit. Two reference posts by Beolover show the relevant circuit and the installation of a 2M ohm trimmer in place of the 1R26, 1M ohm resistor (between the base and collector of 1TR3).

Here is the 1R26 resistor on the main board before switching it out with the 2M ohm trimmer.

Following Beolover's same procedure I temporarily installed the 2M ohm trimmer on the trace side of the board to make it an easier task of adjusting the trimmer setting while operating the Beogram.

Measuring the collector of 1TR3 to ground I adjusted the trimmer until I got to 4 VDC.

After the adjustment I de-soldered the trimmer and re-installed it on the component side of the board so it will be out of the path of the platter.

As a final confidence check of the detector arm circuit I connected a probe wire to the 1TR3 collector so I can view the resulting signal when the detector arm is over the platter.

I get a healthy 5.66 Vp-p minimum voltage when the arm is stationary over the platter and 6.16 Vp-p when the arm is moving across the empty platter.

These two sensor circuits are ready to go. The next tasks will be to run through some of the arm and platter mechanical service manual checks. After those I will adjust the tangential arm tracking of a vinyl record.

Beogram 4004 (5526): Replacement of Convertible DIN7 Plug with DIN7 Socket and an RCA Adapter Cable

Any restoration of a Beogram should include the replacement of the DIN connector to ensure a low impedance signal path on the way to the amplifier. In the case of a Beogram 4004, the DIN connector is a convertible male DIN 7 plug, where the two additional pins can be unscrewed from small sockets. These two pins would carry the remote control signals from a Beomaster 2400, which is designed to control the 'start' and 'stop' operations of the Beogram 4004. An early form of Beolink, so to speak. But of course, if one does not connect to a 2400, these pins are not needed, hence the convertibility. This shows the plug with the empty two sockets at the ends of the 5 pin half-circle:

This type of plug is no longer available, and so I resorted to replacing it with a DIN 7 female socket, which then connects to a jumper cable with the desired number of pins or RCA termination. In essence this approach converts the Beogram 4004 output scheme to that found in the later Beogram 800x models, who have a DIN 7 socket mounted in the back of their enclosures. That way one can connect with both DIN 5 and DIN 7 jumper cables to a non-B&O or B&O amplifier.

Even cable mount DIN 7 sockets are difficult to find these days, but Steve at soundsheavenly.com can provide them. Here are a couple impressions: This shows the socket soldered on:

and with its shield in place:

And finally with a DIN 5 to RCA adapter cable:

Beogram 4004 (5526): Replacement of Sensor Arm Bulb with a LED Fixture

The final incandescent bulb to be replaced with a LED in the Beogram 4004 (5526) that I am restoring right now was the one in the sensor arm. It is the most difficult one to replace due to the tight space and the requirement that it is positioned perfectly to yield a good sensor signal. I removed the original bulb

and then implanted the LED assembly. This shows the part, which I recently developed in comparison with the original bulb:

The flexPCB based LED fixture folds precisely into the bulb compartment. This shows how I soldered it in:

The red wedge holds the LED in the place where the filament of the bulb was located. This is important that the optics in the arm focus the light into a spot on the platter where the light sensor can 'see' it.

Once the LED assembly was installed, I upgraded the detection circuit with a 2M 25-turn trimmer that allowed me to precisely calibrate the working point of TR3 (which amplifies the sensor signal). TR3 needs to be adjusted that the collector is at 4V DC when no signal (record present) comes from the photosensor. In order to adjust the trimmer with the board powered up, I temporarily installed it on the solder side of the board so I was able to adjust it:

After the adjustment, I removed the trimmer and inserted it from the component side and soldered it in place permanently:

It perfectly fits in the space of R26 that it now replaces. After this was completed, I measured the sensor signal at the collector of TR3 to make sure everything was up to spec:

The Vpp signal should be between 5-6V in order to ensure reliable operation of the record detection mechanism. A test with no record and spinning platter revealed that the the arm would not drop, i.e. the mechanism was working.