Phoenix by Paul Swan
BOARD NO: 0077
GAME: Phoenix
MANUFACTURER: Taito
DATE: 1980
HARNESS: CENTURI-A
SYMPTOM (08/06/96)
No ship explosion or shoot sound. Other sounds O.K.
SOLUTION
IC5 and IC6 (LS374) are the sound control latches. Using a logic probe to map out which circuit contributes to what sounds gave the following results:-
IC5 (Data outputs)
pin 19 -> Barrier tone (Bit 0)
pin 2 -> Barrier tone (Bit 1)
pin 16 -> Barrier tone (Bit 2)
pin 5 -> Barrier tone (Bit 3)
pin 15 -> Alien hit explosion
pin 6 -> Bird wow-wow
pin 12 -> Music select
pin 9 -> Music on
IC6 (Data outputs)
pin 19 -> Alien swoop/hit tone (Bit 0)
pin 2 -> Alien swoop/hit tone (Bit 1)
pin 16 -> Alien swoop/hit tone (Bit 2)
pin 5 -> Alien swoop/hit tone (Bit 3)
pin 15 -> Alien hit explosion
pin 6 -> Alien swoop
pin 12 -> Ship fire
pin 9 -> Ship explosion
Both the ship fire and explosion circuits were fed from a linear feedback shift register (IC46, CMOS 4006) that was in turn clocked from a NE555 oscillator (IC42). The oscilator was switched by TR1. Using a logic probe verified that TR1 was working correctly and pin 3 of IC42 was oscillating as was pin 3 of IC46. The outputs of IC46 taken at pins 8 and 9 were stuck high. These two outputs feed a an XOR gate (IC22, LS136) the output of which is inverted and fed back into pin 1 of IC46. The schematic for this circuit seemed to suggest that it was possible for this LFSR to "lock-up" if all the shift register latches end up all-one, and using a pulse injector on pin 1 of IC46 to add a few zeros into the LFSR chain started the sounds off and the LFSR maintained itself from then on. Power cycling the board always seemed to result in the LSFR locking up. Since this seemed to be a "design fault" rather than a circuit fault, I examined some of the other LPN Phoenix board sets and these had a modification on them to insert zeros into the LFSR at power on as follows:-
Wire link: IC9 (8085) pin 3 -> IC28 (74LS05) pin 5
Wire link: IC28 (74LS05) pin 6 -> IC29 (74LS05) pin 9
It looks as if this was a "field" modification and it isn't a particularly "neat" solution but it does seem to work.
