Beogram 4000: An Unusual Keypad PCB

This post is about an unusual keyboard PCB of a Beogram 4000 that was recently brought to my attention. Normally, the two position scale illumination bulbs in a Beogram 4000 are connected in series between the 24V rail and GND. This shows the bulbs on a standard board.

Below the PCB is the circuit layout as shown in the service manual. The way the bulbs are connected in series creates a situation where the polarity of them is reverse.

This is perfect for the installation of my (polarized) LED boards for replacing the bulbs:

When installed, these boards need to point in opposite directions to illuminate the entire position scale, i.e. their polarity is reverse relative to each other, which matches the above standard circuit layout, and both boards light up:

BTW: The red LED is the back illumination for the 33 RPM trimmer.

However, recently I received an email from another B&O enthusiast whom I supplied with two of these LED boards for his Beogram 4000 restoration, stating that only one of the boards was working in his Beogram! Very unbeolovely! After some more communication it turned out that his Beogram 4000 had a different keypad PCB, where two 24V bulbs were connected in parallel between the 24V rail and GND, instead of the normal two 12V bulbs in series.

This arrangement resulted in the same polarity for both of the bulbs, and this caused one LED board to be connected in reverse polarity, i.e. the LEDs did not light up. This situation is shown in the top-right picture in the slide below. 

He sent me a picture of this strange PCB (shown on the left). After a bit of head scratching it occurred to me that my LED boards could still be used if a small modification is made to the board:

By (1) removing one of the jumpers that connects one of the bulbs to GND, and (2) moving the 24V terminal to the solder point where the jumper originally connected towards the bulb, the standard in-series circuit was restored.

After this change was made, both LED boards started working (bottom right picture).

Beolovely again!

Beogram 4002 (5521): Replacing the RPM Scale Illumination Bulbs with SMD LEDs

After replacing the broken light source of the tracking sensor, The next step of my current Beogram 4002 (5521) restoration was to replace the incandescent light bulbs in the RPM trimmer panel. The 33 RPM bulb was burned out, so we decided to replace both bulbs with my drop-in SMD LED assemblies. I am using red-green LEDs to ensure that there is a red light component to ensure proper illumination of the red RPM indicators. While amber LEDs can result in an incandescent-like look, they are not able to properly illuminate red surfaces due to their narrow wavelength distribution. Here is a picture of the small circuit boards that carry the LEDs as well as two current limiting resistors, one for the red and one for the green LEDs. This allows to tune the relative intensities to achieve an authentic looking illumination:

I recently made a video that shows how to exchange the bulbs with these boards. They are available to other enthusiasts. Just send me an email. 

Here are a few impressions of the installation in this particular Beogram. This shows the trimmer panel after removal:

The bulbs can be accessed after removing the two covers:

After unsoldering the leads the bulb assemblies can be slid out:

Now the PCBs can be installed. It is important that they sit flush with the aluminum surface:

And then the covers can be replaced:

And that is it!

Beogram 4000: Redesigned Drop-In Tracking Sensor Light Bulb Replacement

It was time to replace the tracking sensor light bulb in the Beogram 4000 that I am restoring right now with the SMD LED based replacement light source that recently I developed. I used this occasion for redesigning the part with the aim to incorporate the intensity adjustment trimmer into the light shield of the source to hold it firmly in place. I did not like that the trimmer was only held in place by the leads in my previous design, which resulted in a slightly wobbly appearance when adjusting it. In other words, this part was not yet fully up to Beolover standards!

So I designed a new circuit board which allows to directly solder the trimmer to it. This, however, required the concurrent redesign of the 3D printed housing part to enable the trimmer to protrude from the PCB. Here are some pictures. This shows the original light source.

This Beogram had the optional 100R trimmer installed that is mentioned in the supplement of the service manual. This allows the adjustment of the intensity of the light bulb which is a great feature that much simplifies the adjustment of the tracking feedback circuit sensitivity. 

This shows my redesigned part from the light source side. The LED is powered by 6V as it would in the Beogram 4000:

This shows the part from the other side prepared for insertion into the tracking sensor

The potentiometer protrudes through a opening in the plastic part and looks now fully integrated. Lovely! The negative lead is curled around one of the bolt holes. This makes the ground contact (the Beogram chassis is tied to the system ground). This setup essentially copies the original setup with the 100R potentiometer. The other lead is bent up to be able to go into the 6V rail solder point on the PCB from below:

After the installation I adjusted the tracking sensitivity to the prescribed 4 rounds of the record before the tracking mechanism kicks in after the needle is put into the groove. A few months ago I made a video about the tracking mechanism and the adjustment process:

Beogram 4000: Replacing the Keypad Light Bulbs with LEDs

The next step of my restoration of the Beogram 4000 that is on my bench right now was to replace the light bulbs in the control panel assembly with LEDs. I like to do this to increase the longevity of the unit and to guard against shipping issues. Incandescent light bulbs do not like vibration...Here is a picture of the opened up keypad:

The two bulbs at the top end are the bulbs that illuminate the unique position indicator feature of the Beogram. The vertically implemented bulbs below illuminate the RPM trimmers. I recently developed SMD LED based PCBs that replace the two position indicator bulbs (these parts are available - just send me an email). This shows the boards before installation:

The RPM trimmer bulbs can be replaced with standard red LEDs and current limiting resistors. Here is a picture of the implanted LEDs:

And after starting up the Beogram:

The position indicator LEDs are red-green LEDs tuned to appear with an incandescent sheen. This ensures that there are red photons to properly illuminate the red scale indicator.

This shows the LEDs at work after closing the panel back up. The red indicator is missing, since I did not reinstall the panel since I still need to restore proper functionality of the keypad switches. The unit has some issues with the slow scanning functions, which is an indication that the switches are corroded.

Very beautiful! What a nice design.

While I was in there I also glued the strobe light mirror back on (it often does off at this age):

On to the keypad switches!

 

Beocord 9000: Restoration of the Peak Program Meter Scale Illumination

While the scale illumination light bulb was still working in the Beocord 9000 display that I am rebuilding right now, my customer decided to replace the bulb anyway to make sure that there are no unwanted surprises during return shipping. I can only agree since these light bulbs have a tendency to break their leads of during high vibration-situations (think speeding FedEx truck and a bunch potholes...;-). Once one is in there, I think it is best to simply replace everything and be done with it.

I did this procedure already once in the last Beocord 9000 that I rebuilt, which had a broken bulb.

Here are a few pictures to document the process for this particular unit:

This photo shows the original light bulb held in place by the 'cork' that sticks through the PCB

and from the back:

This is the PCB that I implanted. It is the same that I use in the Beogram 4000 to replace the position-scale incandescent bulbs.

It has almost the same width as the 5 mm bulb that they used in the Beocord 9000. I only use one red-green LED on the board for this application, i.e. I bridged the second LED position with SMD jumpers ("000"). It is important to replace incandescent bulbs with red-green LEDs if the light is being filtered or reflected by red or green features (like in this case where the scale is both green and red).

This picture shows the LED board inserted into the bulb space.

And from the back. It can directly be soldered to the solder points of the bulb, the polarity matches:

Here is a picture of the test drive:

On to the 7-segment display.

Beocord 9000: Restoration of the Peak Program Meter with SMD LEDs (III)

This is the third installment of my report on how to rebuild the peak program meters (PPM) of a Beocord 9000. The first two posts report on the installation of SMD LEDs to replace the original faulty LEDs on the PPM PCB, and the re-attachment of the plastic covers. This post demonstrates the installation and brightness adjustment of the rebuilt PPM.

The first step was to replace the original current limiting resistors with the correct values for the modern SMD LEDs. Modern LEDs typically need much less current and achieve higher brightness than the original LEDs. This is reflected in the much larger resistor values that my initial experiments yielded to achieve similar or slightly increased brightness with the new LEDs. The original values were 180 Ohm for the red and 120 Ohm for the green ones. The SMD types required 470 Ohm for red and 330 Ohm for green for slightly enhanced output (see first post). The resultant currents were in the 10 mA range, while the original LEDs easily draw 40 mA, while having a lower intensity.

Here is a picture of the original resistor bank (the 8 leftmost resistors)

This shows the new resistors installed. Despite the PPM having 16 LEDs there are only 8 resistors. This is due to the fact that the two left and right PPMs are multiplexed at 60 Hz, i.e. only 8 are on at a time. This is seen in the signals of the two common anodes (pin 1 and 2 on the display board) which each drive one of the PPMs:

This shows the new resistors installed:

And here after installation of the rebuilt PPM:

And in action:

It turned out to be a good idea to fix the two display boards in sequence, i.e. leave one in while taking out the other. this allows to realign the one that was out with the one that is still in when putting it back. That way it is easy to get it back into the right position that it fits perfectly into the plastic bezel. Another helpful thing to do is to mark on the headers of the displays how far they were inserted into the PCB. This is important to get the right slant of the displays that matches the cover.

On to replacing the scale illumination bulb with a red/green SMD LED.

Beocord 9000: Restoration of the Peak Program Meter with SMD LEDs (II)

The 24 hrs test of the rebuilt Peak Program Meters (PPM) of the Beocord 9000 display that I am rebuilding was successful:

So I set out to reattach the red-green covers. Encouraged by my recent rebuild of a Beocord 9000 counter display, I decided to rely on melting the plastic posts of the red-green covers with my soldering iron set to 200C to recreate the original attachment method. First I needed to clamp the display together, however, to ensure that the covers would make a tight bond with the PCB to avoid light escaping through cracks at the bottom:

I applied the clamps in a way that the cover posts were still accessible and then I melted them by rolling the solder iron tip cone over them. This worked quite nicely:

When I cleaned the PCB with ethanol after soldering the SMD LEDs, the glue that held the light guide down dissolved. This told me that this clear glue was most likely some kind of polyvinyl ester based carpenter glue. So I used 'Aleene's Original Tacky Glue', which is my mainstay for bonding speaker foams to the cones. This glue dries clear, recreating the original look of the bonds:

After the glue was hardened, I put the display again to a 24hrs test to ensure that nothing bad happened during the reassembly:

What a beautiful design! I can't wait until I can make a test-recording with this deck using these lovely big rebuilt PPMs.

Beocord 9000: Restoration of the Peak Program Meter with SMD LEDs (I)

While I am waiting for the pinch roller for the Beocord 9000 I am finishing up, I started working on another 9000 that needs the displays rebuilt. Cassettes are definitely back! I hope they will start manufacturing new tapes again (and SONY decides to restart the production of the Walkman II...but that is an entirely different story...;-).

Anyway, here is a picture of the sad state of this display:

Not shown here, but the TIME CAL bulb is also dim, like on the other 9000. I decided to attack the VU or 'peak program' meter (PPM) first, since I never did this type of display before. Beolover excitement, no doubt! After opening the Beocord up, the first step is to remove the display bezel that covers the boards and also holds the translucent windows:

After taking the bezel off:

The I unsoldered the PPM board:

The next step was to remove the plastic covers. It turned out that there are black cardboard strips inserted between the covers and the light guide for the scale illumination:

The plastic covers were fixed to the PCB by melting their protruding plastic feet and flattening them, which creates round shaped 'seals':

These must be removed before the covers can be lifted:

I decided to leave the light guide in. Here is a picture after taking the covers and strips off:

For the fun of it, I piped some light in from the side where the scale illumination bulb sits. One can see nicely that the triangular cutouts in the bottom act as mirrors that reflect light to the positions where the scale numbers are:

After playing with this I started working on the rebuild. The first task was to determine the right resistors for the new SMD LEDs. First I determined the values needed to achieve an identical brightness to the original LEDs. For this I replaced a red and a green one of the four broken LEDs with SMD LEDs (Newark 75K1441 (red 14MCD 2.0V) and 75K1440 (green 18MCD 2.1V):

then I rigged the setup with resistors and ran it:

And with covers on:

It turns out that values of about 1k Ohm for red and 670 Ohm for green at 6.2V yield similar brightness. Note that the red LEDs had a similar brightness difference as the green ones, but the camera was not able to show this very well.

My client decided to ramp the brightness a bit up that the displays are better visible in brightly lit rooms. So I determined values that will make them match well with the higher brightness 7-segment display of the counter and indicator light SMD replacements that I usually use.

Here is a picture of the higher brightness version in comparison with the original LEDs:

The values that do the job are 330 Ohm for green (yielding 12.6mA) and 470 for red (9.4mA)

After this tests, I replaced all LEDs with SMD versions:

And running at 6.2V with the above resistor values in series:

Beautiful!! I will let this run for 24hrs before I reattach the covers to make sure that there are no problems.