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Saturday, July 28, 2012

Beomaster 8000 Microcomputer PCB Damage

Back to the Beomaster 8000. Today I looked at the missing display segments. First, I thought it is the usual dead LEDs on the boards. The display looked like this:
So I removed the three main displays. Luckily, I decided to plug them into the display 'harness' I made with a breadboard for the last display repairs. Powering them up yielded this:
This is the first Beomaster 8000 display I see that fully works. This told me the problem is elsewhere. I soldered the displays back in and looked at the circuit diagram. The particular LED segments that were out suggested a problem in the drain path to the SN74247 7-segment driver responsible for these segments (IC 2). I checked the signals on plug P-75 and indeed the pins responsible for the c and d segments were at 5V and did not show any strobe signal. This suggested a contact problem. I removed the display board and cleaned the plugs and resoldered the p-75 plug and socket. To no avail. The segments remained dead.
I followed the current path towards the SN74247 and found that the plug pins did not connect to the respective current limiting resistors R21 and R22. This meant that the PCB traces between P-75 and these resistors were broken somewhere. 
A closer look revealed that there had been the same problem at some point in the past with pin 3 as indicated by the white jumper soldered between capacitor and resistor:
I did the same for pins 7 and 8:
This brought the segments back:

The question now is whether to refurbish the displays anyway or leave them as they are.


  1. Are you going to be selling one or more of these 8000s once you are done?

  2. Yes. These babies need to be used. I am still perfecting a few techniques, but soon some will become available. Alternatively, I would also consider refurbishing these models on contract.

  3. Ah, this is a great find as my Beomaster 8000 is having the same problems. Do you have a schema or a small write-up for the display harness? I'd like to copy yours as it seems to work brilliantly!


Comments and suggestions are welcome!