cbmeeks Posted September 16 Share Posted September 16 If I have a dead SN76494, what would it take to replace it with a SN76489? I have a bunch of those. I know there is something about the clock input on the '489 being 4MHz vs. 500KHz on the '494. Is there some documentation on how to make this mod? Thanks! Quote Link to comment Share on other sites More sharing options...
Gary from OPA Posted September 16 Share Posted September 16 22 minutes ago, cbmeeks said: If I have a dead SN76494, what would it take to replace it with a SN76489? I have a bunch of those. I know there is something about the clock input on the '489 being 4MHz vs. 500KHz on the '494. Is there some documentation on how to make this mod? Thanks! There should be a jumper on the ti99 motherboard to select either the CPUclk (3.58mhz) or the GROMclk (477khz / the default) both clock signals come from the VDP 9918. 2 Quote Link to comment Share on other sites More sharing options...
cbmeeks Posted September 16 Author Share Posted September 16 Ah, that's good news. Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted September 16 Share Posted September 16 2 Quote Link to comment Share on other sites More sharing options...
Gary from OPA Posted September 16 Share Posted September 16 Here is another version taken from sams showing the two different locations. When using CPUclk, it is recommended to use a coaxial cable with shielding to ground or you will even get worse background noise on the sound will there is activity on the data bus. 1 Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted September 16 Share Posted September 16 Just guessing that this is W502... Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted September 16 Share Posted September 16 (edited) This shows the coax... Edited September 16 by HOME AUTOMATION Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted September 21 Share Posted September 21 @Gary from OPA Bit off-topic... Shown in RED, I noticed that leaving W104, W502, connected, and with W500 removed, would provide the 3.58 MHz signal, to the GROMs. Wondering how\if this has any practicality. Also curious what address ranges, U504's, unused outputs, might correspond to. Looks like Pin 14, was partially drawn. Observed that while the other end of W500, is shown as connected to Sheet 1, U100-pin 37. Sheet 1, shows U100-pin 37, connected to Sheet 4, U101-pin 3! Which makes little or no sense. 1 Quote Link to comment Share on other sites More sharing options...
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