(Topic ID: 242282)

Taxi Hum from MPU Sound

By Cheddar

4 years ago


Topic Heartbeat

Topic Stats

  • 13 posts
  • 3 Pinsiders participating
  • Latest reply 4 years ago by Dragon
  • Topic is favorited by 1 Pinsider

You

Linked Games

  • Taxi Williams, 1988

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taxi map darkmint (resized).png
soundtaxicolor (resized).jpg

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-1
#5 4 years ago

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?

#7 4 years ago

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

soundtaxicolor (resized).jpgsoundtaxicolor (resized).jpg

#8 4 years ago

oh just to be clear , i'm only addressing the electroltic caps not all of the caps

#11 4 years ago

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

#12 4 years ago
taxi map darkmint (resized).pngtaxi map darkmint (resized).png
#13 4 years ago

the power supply places grounds to all the boards , the blue section of the interconnect board had some burned traces and cold solder joints from the gi circuit in mine i had to reflow everything in there

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