Taxi
Williams, 1988Working on an early Williams Taxi with proto game and sound ROMs. This game was pretty hacked up and had some solenoid issues
Most issues are fixed but the sounds coming from the mpu board are just a loud hum. Unplugging the 3 wire cable from the mpu to the sound board removes the hum and let's the background sounds play loud and clear.
I have not.found anything that addresses this issue.
Thanks
you said the speakers are working clear with j4 removed from sound card , sound card feeds the speakers that sound ok when mpu is not connected . hum i assume sounds like 60 cycle hum , the story tells me the speakers are fine, the cable that interconnects the mpu to sound is a ribbon type if i recall correctly , do you have another one like it laying around?
Quoted from Dragon:you said the speakers are working clear with j4 removed from sound card , sound card feeds the speakers that sound ok when mpu is not connected . hum i assume sounds like 60 cycle hum , the story tells me the speakers are fine, the cable that interconnects the mpu to sound is a ribbon type if i recall correctly , do you have another one like it laying around?
I've only removed the 3 wire cable that takes the mpu sound to the sound card. The ribbon cable is still connected
sound volume control , question are the electrolytic capacitors original on the sound card ? is my first thought it may not be correct diagnoses to the gnd issue but original 30year old caps i would get a kit and replace them ,replacing the caps made a noticeable difference on mine and while your at it reflow the connectors and maybe that will do it
Update on the issue. Issue is isolated to the MPU sound section. Using another mpu with this sound card works great.
Swapped ROMs with new, swapped CPUs and ram and all are good.
Verified 12VDC, -14VDC and AC ripple of 35mv
Couldn't get my oscilloscope to give me good readings (operator error, need to learn how to use it) so I used a dmm to try and verify the sound section was running.
Put the game into sound test and I could see U9 2-5 stay high the go low quickly and return high. 6-9 stayed low. Also looked at U2 5-12 and saw some activity but nothing definitive. It seemed like the sound CPU was running but not 100%.
What's next?
the hum is likely a grounding problem , 60 cycles from your house wiring is being presented onto your speakers in a small way , this 60 cycle ac was supposed to be filtered away before it entered the audio circuit but some resistance in something like a connector or component has added an impedance to hold a little bit of the ac somewhere that for example might be riding on top of your + 12 or -12 volt or 5 volt adding noise to an amplifier circuit , make sure all of the metal panels in the back box are grounded well , all of the electronic pcb's need to be at the same common ground so as not to lift the voltages zero point ground on different pc boards to a slightly higher 0volt point ground . in my taxi i replaced every connector on the harness in my back box plus reflowed solder on edge connectors of boards i didn't replace and replaced my electrolytic caps on the audio card and replaced the power board with a switching power supply the reason i did all of that is because the machine is really old and i wanted it to work great all the time and didn't want to chase little intermittent problems down all the time
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Lodi, CA