SCORE DISPLAY REPAIR FOR FAINT OR FLICKERING 'F's - March 25, 2017
Written by Shaun Wood
The Dragon's Lair / Space Ace / Thayer's Quest score display uses two controller chips (Intersil's ICM7218D) to drive the 7-segment
displays. But the designers of this chip broke digital rules in order to save a control pin.
They wanted 3 modes of operation: Hexadecimal, Code-B (which the game uses), and Power Shutdown.
To get 3 modes of operation, they made it so that if pin 9 is held high, you get hexadecimal, if it is held low, the chips power down, and if it is not connected at all, it operates in code-B.
Well, that's breaking the digital rules. An input control pin should always be held either high or low. This not-connected (floating) condition is not normally allowed, and that is where the FFFFFF problem comes from.
The controller chip has an internal voltage divider that holds pin 9 at 2.5V when nothing is connected to it, allowing the display to operate normally.
But after 30+ years the internal resistors can vary to the point where the voltage on pin 9 is too high and the controller shifts into hexadecimal mode. Thus when a command is sent to blank a digit (off), the controller displays it in hex as 'F'.
The fix is simple enough. Build an external voltage divider to force pin 9 back to 2.5V. I use 1K resistors.
PARTS AND TOOLS NEEDED
Two 1K resistors
Short piece of jumper wire
Gently scratch away a small bit of the green protective coating from the back side of the score board at locations A and B (see picture).
Apply a small amount of solder to the exposed area.
Cut the leads on the two resistors to approximately 1/4 inch and solder them from each pin 9 to the exposed solder points.
Finally, install a jumper wire to connect U1 pin 9 to U2 pin 9.
That's it. Problem solved. Enjoy!
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Use at your own risk.
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