BOARD NO:     0110
GAME:         Vanguard
MANUFACTURER: SNK
DATE:         1981
HARNESS:      SNK-A

SYMPTOM (07/07/96)
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(1) Game worked OK in the cabinet but wouldn't initialize on the test rig.
(2) Game worked OK in the cabinet but wouldn't sync to the test rig monitor (Microvetec CUB). 

SOLUTION
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(1) Using a pulse injector on the 6502 reset pin started the game. The power on reset was provided by an NE555 timer circuit and changing the time constant didn't make any difference. I replaced the 220Ohm pullup resistor on RES with a DS1233 three terminal reset generator and this fixed the problem completely. I ordered a 10 pack of these devices and will use them from now to fix older games that have poorly designed reset circuits.

(2) Looking at the SYNC output with an oscilloscope revealed the logic zero was about 1.3V and hence not quite low enough for the monitor to lock on to. The SYNC output pin was driven by an open-collector OR gate pulled high with 1K and with 1K in series. The 1K in series seemed a little on the high side to me, especially since the monitor is probably terminated in 75Ohm, and so I replaced the 1K with 75Ohm to match. This fixed the problem.