UsagiElectric Posted Tuesday at 03:18 AM Share Posted Tuesday at 03:18 AM Hi All, I've been working on a TI-99/4 and it's giving me the proper run-around (picture attached of system in question). It's a dead system, black screen and sound chip screaming. In my initial round of testing, I found two bad transistors in the video circuit (weird, but they definitely weren't amplifying) and a bad DRAM (all have now been replaced with 4164s modified). There is effectively no change in the failure mode. There is data on the data bus, addresses on the address bus, none of the data or address lines look to be stuck low or high, all four clocks look good, voltages are not perfect but close (4.9V, -5.2V, 11.4V). I have a spare 99/4 parts donor (it's a rusty nightmare, so no guarantee the parts are good) and a spare 99/4A parts donor (it was also a non-booting machine though), so I've been swapping chips from those machines to test stuff out. So far, I've confirmed the following: - Swapped CPU for two different CPUs, all three behave the same (all three came from non-working systems though...) - DRAM is good, though I think the failure happens before we even get that far (VDP never receives CSW or CSR) - Swapped VDP for three different ones, all four behave the same (generates a video signal, but there's not data in it) - Swapped console ROMs (both) for two different ones, all three pairs behave the same (one pair is from parts donor, the other pair are 28C64s in adapters) - Swapped the GROMs (all) for the spare ones from the parts donor, same behavior - Replaced the 74LS138s on the right edge of the board, no change (this was at the behest of the manual) I'm starting to run low on things it could be. I've tried to follow the TI-99 new technician manual troubleshooting procedure, but it's not getting me there. I'm down to thinking it's either the SRAM chips or the the 16-bit to 8-bit conversion chips (LS245, LS373, LS244). I would kind of expect one line to be stuck high or low if it were the SRAM chips, though I suppose they could fail high impedance? Also thinking that maybe the board is just really picky on voltages? The 4.9V is well within spec, but the 11.4V is right on the bitter edge for the TMS9900. It could still be CPU as well, all three of the CPUs I have tried came out of non-working systems, but I feel like they shouldn't all fail in the exact same way. It could also still be the GROMs, but I'm not sure how to test those in such a way that it rules them out. Of interesting note, if there's a cartridge in the slot, it stays in reset for about a full second longer than if there isn't one. Not sure if that's indicative of anything. If y'all have any other ideas, I'm all ears! 2 Quote Link to comment Share on other sites More sharing options...
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