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22Mar/210

Power Mac 6100/66 DOS Compatible – MIDI Out

After building the 'hydra cable' for this DOS Compatible machine, I quickly found out that the Joystick port could not be used for MIDI input/output. There were numerous hints online, and they all resulted to the fact that we'd need to solder a pin on the Vibra16 IC to get midi to the port. Here's the module removed from the DOS card. The CT2501 IC nearly takes up the entire board.

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I was able to re-wire all of the below as I created my own hydra cable. I wouldn't suggest trying to modify an off-the-shelf cable as they're a sealed unit and have many pins incorrectly wired at the joystick-port-end.

Determining the correct pin

I'd asked online for help from someone who'd already worked out the solution, but hadn't received a quick answer. After rummaging through my junk boxes, I realised I didn't have a card with the correct CT2501 chipset, so I asked a few friends also. No one had the exact model required, so I considered purchasing one on eBay. They were averaging about one-hundred-dwollar shipped, so I decided to try and find high resolution photos online instead.

It turns out that dosdays has a fantastic library of information on the SB16. Included are high-enough-res photos of the rear of the board that allowed me to easily trace pin 12 (MIDI OUT) back to the CT2501 IC. Pin 15 (MIDI IN) wasn't so easy as it disappears under the 74-series IC in the bottom left of the top-side of the board. Fortunately, my main goal was to just get DOOM II throwing MIDI out to my SC-88.

sb16-bottom sb16-top ct2500-close-up

After sitting in Paint.NET for a while, tracing traces, I came to the conclusion that MIDI OUT was Pin 30 of CT2501. In the pictures above, I've traced this in RED on the underside-shot of the board and YELLOW on the zoomed in CPU picture. MIDI IN is traced in BLUE on the underside-shot, but then disappears on the top half under the IC. Images from the web don't provide enough detail to allow me to continue the search! I was happy with just MIDI OUT, so I then started the quick hack-job to solder on a test wire...

ct2501-close-up ct2501-close-up-2 ct2501-close-up-soldered

In no time at all, DOOM II was outputting its soundtrack into my headphones.

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Usually I'd pipe the audio-out from the SC-88 back into the soundcard of the computer that it was connected to, but that's not an option here with the Power Mac 6100 as it only has Microphone In! Oh wait, can I also wire-up Line-In for the DOS Card? I don't think that'd be as straight-forward as I'm sure there'd be a DAC and other components on the path.

External Connections

The port at the back only needed to be one wire, so I initially used an RCA socket.

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I then realised I had no spare RCA plugs on-hand, so I switched the socket to a 3.5mm mono audio jack. This looked nicer anyway.

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From here, a headphone cable was wired through with one of the channels running to pin 12 on the joystick port. This was then plugged into the SC-88 and DOOM II WAS PLAYED AT FULL VOLUME!

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