Beocenter 7700 display faults

Following from a good post in the Beoworld forums by The Tinman (Marc) in 2008:

Marc said:

"As for the LED segments, they can get intermittent or go out altogether.  The problem is in the gold wire that connects the LED wafer to the PC board.  These were not potted yet like modern 7 segment displays.  As far as I know, there is no replacement for these.  But I have found that the "wire" can be glued back down using a microscope, a very fine pin, and silver paint."

Current examination by Ultrasystems reveals the following:

The display (FCS8499 -numeric and the Alpha - PN unknown) are arranged as common cathode arrays (Vcc to cathodes) with the annunciator segments reverse wired.

To service, remove the display assembly from the 7700.

Test the segments by using a current limited DC supply, for example 1.8 VDC through a 36 ohm resistor will limit the LED current to approximately 8 - 10 mA depending on the exact forward voltage of the LED.

The forward voltage of samples tested was around 1.5 volts.

The pinout from the original B&O schematic is for the LED module pins, not the PCB board header pins.

If the LED segments are confirmed faulty, then remove the LED assembly from the PCB as required (desolder pins).

Next remove the red filter housing and the light pattern guide.

This is done by slicing off the melted over tabs of plastic from these parts which protrude through the PCB.

Use a jiffy knife or razor to slice off the plastic tabs and then carefully lift off the plastic filter and light pattern guide respectively.

Take care to not damage the PCB traces when slicing off the plastic tabs.

For the 7 segment sections the cathode connections are glued in place with conductive cement.

The anode connections are a spot welded wire bond to a gold plated pad on the board and the likely failure is a break at the weld point.

Re-solder the wire bonds with silver solder to repair the defective LEDs.

If the break is the anode connection or if the LED itself has failed then fit an appropriate red SMD LED in place of the original.

The annunciator sections (dots) are reverse connected with the anodes common and the cathodes connected by spot welded wires.

The repair strategy is the same.

When repaired resolder the LED module to the PCB and re-test all segments.

If re-work is required then the previously loosened filter and light pattern guide can be lifted off leaving the LED module on the PCB so de-soldering the LED board is then not required.

NOTE: The wire bonds are extremely fragile and are easily broken so take great care in locating the pattern housing to the LED board.

 

The best strategy is to re-solder ALL the joints the first time.

Thanks for that. I thought the led-chip itself is failing. My center 7700 is suffering from dead segments also. And some other loose contacts: sometimes I cannot start phono and sometimes parts of the keyboard don't work...

When the switches do not work or are intermittent in function does the remote control still operate the same function reliably ? If so then the fault is likely the switch contacts or possibly a loom header to the PCB. You can easily access the loom headers and treat the contacts to eliminate those.