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Saturday, May 30, 2015

Beocord 9000: Display Restoration with SMD LEDs

Like most Beocord 9000 and Beomaster 8000 units the Beocord 9000 that I am rebuilding right now has a broken 7-segment display. In this case, only one segment was missing, but of course this needed to be fixed, these big early LED displays are one of the stunning aspects of these units. I can understand that Jacob Jensen was fond of using them:

This display is very similar to the frequency display of a Beomaster 8000. However the design is somewhat different, so they are not interchangeable. The restoration process however is the same. I made many posts in the past about it, see here for example.

Taking the display out is slightly different due to the bezel that is clamped to PCB #1 that holds them in place and also prevents any light from the back and sides of the display to escape. This is superior to the earlier design of the Beomaster 8000. The first step is to remove PCB#11 and then the bezel. Then the display can be unsoldered. A desolder gun is essential for this task since all pins need to be able to move freely when it is taken out:

After opening the display unit up, the first task was to determine the correct current limiting resistors for the SMD LEDs (Newark 75K1441). The Beocord 9000 circuit has no easy way like the Beomaster 8000 for tuning the display intensity, so the original resistors need to be exchanged with values appropriate for the much more efficient modern LEDs. I went at it by replacing the dead segment LED with a SMD LED and then I connected the old LED with a 100 Ohm resistor to 6.2V (the spec value from the diagram). Then I experimented a bit with various resistor values on the new LED until I achieved the same brightness:

It turned out that a 470 Ohm resistor did the job very well to achieve the exact same appearance.
However, I always felt that the original intensity of these displays is a bit too low for well-lit rooms, and I prefer to give them a bit more intensity. A 220 Ohm resistor, it turns out yields exactly the same intensity as my typical Beomaster 8000 intensity adjustment:

The difference seen on the photo is actually stronger than it appears to the human eye...a digital camera artifact. The higher intensity allows a very nice appearance if the room is bright, but still does not look overly strong in a dark room. A matter of taste at the end, since it is easy to do one or the other. After this experiment, I went ahead and replaced all the original LEDs with new ones and performed my usual 24hrs test to make sure that things are stable:

This shows the current being drawn by the display during the test at 2.2V:

My work passed the test, and I went ahead and closed the display unit up. I had to deviate a bit from my latest black-gluegun-glue solution in that the display sits very tight in the bezel, which makes it impossible to put the glue on both sides. So I put glue in those locations that were accessible and did not hinder the bezel fit:

To add a bit of strength to the seal, I used the soldering iron at a 225C setting to widen the remnants of the original plastic posts that they would also help to keep the display together:
I think this resulted in a pretty strong overall bond between cover and PCB. One advantage of this design is also that the bezel already holds things together due to its snug fit. 

Before I put the display back in, I replaced the original current limiting resistors. Here is a picture of the original setup:

My client decided to go with the brighter setup, so the 100 Ohm resistors of the seven segments were replaced with 220 values, while I used 750 Ohm for the 390 Ohm original values on the three dots of the display. Mathematically this is a bit too low (390 x 2.2 = 858), compared to the 100/220 pair, but it turned out that the dots look perfectly adjusted with it. They have a different situation in the display due to their much smaller aperture in the diffusor of the display. This is the new setup:

After this it was time to give the display a spin. I reinstalled it. It is important to put it at the exact right angle that the bezel can fit properly. This is easily achieved since the display aligns perfectly with the peak program meter unit. So I put it in, tacked it on with one pin, then adjusted it to align and then soldered the remaining pins. Here you see it with the bezel installed again:
Looks very nice and balanced! However, all this tugging and pulling on the PCB apparently killed the scale illumination bulb for the peak program meter. When I started out, it still worked:

Oh well...this would have happened soon anyway if a bit of wiggling and messing with the PCB broke the bulb. Another interesting repair task coming up! This is Beolove!

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