Double Dragon Repair Logs
Board would power up and display yellow and white garbage. Checked the ROMs and found garbage on the data lines. Replaced a bad 74LS244 at location IC12 and the CPU would try to boot, but still would not. Found that pin 2 of IC21, a 6264 SRAM IC was stuck high. Traced it back to IC 1, a 74LS157. The LS157 at location IC1 and IC2 were missing their select signal from pin 1. Jumpered pin 1 of IC2 to pin 1 of IC3 to fix the board. Replaced 2 broken caps and tested.
Board would power up and display yellow and white garbage. Checked the ROMs and found that pin 22, one of the control lines, was dead on IC 25, 24, and 23. Replaced a bad 74LS139 at board location IC72. Board would then power up and run, but would not recognize any inputs. Replaced a bad 74LS138 at location IC90 which allowed it to read the DIP switches, but still no inputs from controls. Checked signals against a known good board and discovered that pin 4 of the CPU, FIRQ*, was not going low when a coin was added. Replaced the 74LS74 at IC73 to fix the board. Replaced 2 broken power decoupling caps and tested the game.
Symptom: No sound
CPU checked good by substitution. Cleaned the legs on the sound program EPROM. Chip select signals looked good but the SRAM was never going into write mode so the subsystem was never fully booting. Checked continuity on the data and address lines and found a bad connection between the sound ROM and RAM on D3. Patched bad trace and tested the game successfully.