Need schematics for Compaq Presario 742 CDS

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Need schematics for Compaq Presario 742 CDS

Postby 8bitbubsy » Tue Mar 19, 2013 7:58 am

This will be a hard one to find, if it's even out there in the world at all!
I need the schematics for this specific model (same mobo used in 720/700 CDS as far as I know). The reason why is because I want to completely disable the internal sound card so it's not even sending any interrupts or getting identified by Windows.

There's no jumper to disable it, and disabling it in the BIOS does *nothing*. Windows 95 still finds it, and it still functions in DOS. I went as far as desoldering the ESS ES688F IC itself, but then the computer wouldn't POST. I soldered it back in and it worked again. I also found some zero-ohm SMD resistor jumpers that lead to certain pins of the sound card IC, desoldered them but it changed absolutely nothing -- the sound card still showed up, it still took IRQ and DMA channels and so on. What a useless pile of s***, but it's the only 486 computer I have and it should do good for some nostalgic action. I wonder if it's possible to cut any specific pins to prevent it from being identified? I'd need to check the ES688F schematics too.

Maybe there's some routines in the BIOS that sets the sound card up, I could possibly dump the ROM and disassemble it. I have an EEPROM reader and a PLCC adapter.

I want to do this because I want to have a Sound Blaster 16 or GUS in the system, for the ultimate retro feeling. Having another card that takes up interrupts and DMA channels is lame, it just feels "fake" when using another sound card.

I hope you understand, this is not important, but it's a quest I want to achieve. :)
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Postby Hardware Junkie » Tue Mar 19, 2013 8:50 am

Yeah that is going to be extremely difficult. Especially since you are trying to do something beyond normal setup for this motherboard. The thing was designed to run as is, and most didn't get manuals printed for them.

They only really documented CPU select and CMOS erase jumpers, everything else was factory
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Postby Hardware Junkie » Tue Mar 19, 2013 9:12 am

You'd have to trace every point from the bus to the sound card, and start disabling connections. Presumably it would have resistors and other items before it hit the sound card, cutting those connections would do it.

I would just figure you would make a mess of the board before it was finished.
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