Tandy sound modification

Hardware questions and modifications

Tandy sound modification

Postby Vorticon » Thu Feb 18, 2010 8:54 pm

Hi. Can anyone here outline to me how to perform the Tandy sound modification on my PCjr? I understand that it is simply a matter of re-routing the sound chip output. I recently acquired Battle Chess but I have no sound when running it in Tandy mode...

Walid
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Re: Tandy sound modification

Postby jmetal88 » Fri Aug 06, 2010 6:11 pm

I haven't been able to find any docs about this mod, but I did find a PDF of the tech reference (minus appendix B), which does a pretty good job of describing the sound system.

From what I can tell, the problem with some Tandy 1000 games is that they don't set the proper registers to activate the audio signal selector (apparently in the Tandy the audio signals are just all tied together). I've just spent a couple of hours poking around on the PCB with a multimeter, following traces, and I finally figured out the audio selector is the MC15429 chip next to the power board slot. It looks like 3-tone audio from the SN76496 enters the MC15429 chip on pin 5 and PC speaker audio enters the MC14529 on pin 2. Pins 3 and 4 are sidecar audio and cassette audio, so I doubt those would be needed for a Tandy game. According to the datasheet, audio on these pins will exit the chip on pin 9.

The trace from pin 9 of the MC15429 is kind of hard to follow, and I didn't really feel like trying after figuring out the previous info, but it's supposed to go through a low-pass filter and out to the audio amp. I'd probably have better luck if I felt inclined to remove the PCjr PCB from its case, but I don't feel like breaking and re-gluing those plastic tabs again tonight.

Anyway, after looking at all that, it's my guess that the Tandy 1000 audio mod simply consists of soldering jumpers from pins 2 and 5 to pin 9 on the MC14529. Personally, I don't have any reason to test this yet, so try at your own risk. Actually, it might be prudent to use diodes instead of jumpers. Just a thought.

EDIT: Actually, I don't even know if the jumper from pin 2 would be necessary, but I guess that would depend on the game. It would probably be useful if the 3-tone audio and PC speaker were both being used to generate different audio.
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Re: Tandy sound modification

Postby Vorticon » Mon Aug 09, 2010 8:38 pm

It actually turned out that you can activate Tandy sound by setting Port 61 via software. I wrote a small Turbo Pascal utility to do that called Tsound which you can find in the Sound Bits post under Programming.
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Re: Tandy sound modification

Postby jmetal88 » Mon Aug 09, 2010 8:47 pm

Vorticon wrote:It actually turned out that you can activate Tandy sound by setting Port 61 via software. I wrote a small Turbo Pascal utility to do that called Tsound which you can find in the Sound Bits post under Programming.


Cool beans. :)
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Re: Tandy sound modification

Postby jmetal88 » Tue Aug 10, 2010 2:33 pm

jmetal88 wrote:I don't feel like breaking and re-gluing those plastic tabs again tonight.


This probably falls under the category of 'nobody cares', but I decided to just toss out the rear tab. Since the front one is still intact, I can rely on the screws to hold down the back. It's pretty obvious it was glued anyway, so it's not like there was any cosmetic benefit from leaving it in there.

Anyway, I now have the board out and am scrutinizing it to make sure I'm ready to start my other project.
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