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About AwkwardPotato

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  1. Make sure the voltages coming off the internal power supply are staying in spec as the machine warms up. If they are, chances are the VDP needs to be replaced (or one of the 4116s is behaving intermittently, but that's unlikely).
  2. The SN76494/94624 (haven't come across a TMS9919-labelled chip yet) share the 447kHz clock with the GROMs on most of the TI boards I've seen. The SN76489/76496 take a 3.58MHz clock input, and I've only seen them installed in 99/4 and some very early 4A boards. Correct me if I'm wrong, but I believe you can tell if you need a 3.58MHz-capable chip if your board has the long coaxial bodge wire between the VDP and PSG.
  3. Probably not. Check that the wires from the power input at the back of the computer are actually attached to the internal power supply, and that there's continuity between the internal power supply and the motherboard. Also make sure that the power switch is good; it is known to fail.
  4. I couldn't think of any other TI hardware with 4116s on it (besides the motherboard, but it doesn't look like what's in the picture). Also the black silkscreen on green soldermask, the only place I can remember seeing that is on the PEB cards. I'll let somebody else go after me since I posted a picture only a couple days ago
  5. Correct, and good call on the soldering quality. That was a quick fix a couple years back when I was touching up the PCB (rubber piece underneath it started rotting), should probably hit a lot of those joints again with some flux.
  6. Try applying new thermal paste on the stock RF shield/heatsink and see if the problem goes away. If the system still acts up afterwards, there's a good chance that the 9918A is failing (chips aren't always completely non-functional when they're damaged). If a VDP can't stay stable even with refreshed thermal paste on the stock heatsink, it's a better idea to replace it than cut up the shield/install heat pipes/custom heatsink, since the problem will likely only continue to get worse.
  7. I would definitely do everything I can to repair and preserve this machine (assuming I could afford it, of course) but would be equally happy to see it go to someone like Ksarul or Fabrice. ☺️
  8. Black screen w/ constant noise is a pretty common TI failure mode. First thing to try is pulling GROMs CD2156 and CD2157 and seeing if that changes anything. If there's no change, the fault most likely lies in one of the ROMs/SRAMs (the four wide chips right next to the CPU). Unfortunately neither the ROMs nor SRAMs are socketed, and there really isn't a good way to narrow down the problem without a logic analyzer or oscilloscope. Replacements for the SRAMs are easy to find on eBay, but ROMs would either need to be programmed yourself or harvested from another board.
  9. Agreed with wierd_w, it's a good idea to make sure it's not the TV before swapping chips. Pretty much all the computers from the time period were slightly-out of spec with their video signals, so the one you have might not display anything with the TI despite working with the C64. It's an especially good idea to rule out the TV first when dealing with the TI though; the "black screen" issue can be caused by a number of components (CPU, 9904, SRAM, ROMs, etc.) One question: with the sound chip installed, do you get any kind of noise when the computer is turned on?
  10. If you're able to see the title screen behind the black bars, but with no color, it's almost certainly a crystal problem. For something like that I see no point in buying another console; the crystal only has 2 pins and would probably take about 10 mins to swap.
  11. Black screen & cacophony usually isn't the VRAM; check the ROMs and 6810 SRAMs.
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