Amidar Repair Logs

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Repairer: Paul Swan

BOARD NO: 0054
GAME: Amidar
DATE: 1981

SYMPTOM (19/05/96)

(1) No initialization.

(2) No sound.


(1) Using a logic probe to check on CPU activity indicated that the video Z80 wasn't running. The reset line was inactive and the clock was running. Using a pulse injector on the reset line started the game, but the game wouldn't start from power on. The board set was a converted pirate Scramble set, and the watchdog circuit had been disabled. Re-enabling the watchdog reset the game continuously. Presumably this game does not implement the watchdog. Using a pulse injector on pin 6 of IC5C (the buffered output of the power on reset RC time constant) always started the game suggesting the digital section of the reset circuitry was working properly. Examining the RC input at power on with an oscilloscope seemed to indicate that the RC delay was over before the power had settled, and the buffered output at IC5C pin 6 never got the chance to go high and generate a reset. According to a multimeter, the resistors were as indicated. Changing the capacitors had no effect. The two transistors in the reset circuit were removed, verified as OK in transistor checker and then put back. Changing IC5C made a slight improvement (game started one in five power cycles). The RC time constant as fitted gave a reset pulse from power on of around 10ms (10K & 1.0uF). Changing the 1.0uF capacitor to a 470uF capacitor to give around a 5 second reset pulse fixed the game.

(2) There was no digital noise from the amplifier and the metal tab had been broken off it leaving the die suspended in free space by it's bonding wires. 'Nough said.