Delta Posted April 20, 2023 Share Posted April 20, 2023 Hi all, I have 2 TI-99 4A boards that show a black screen. I have replaced the VDP and it has made no difference. My next step is to replace the two 4K ROM ICs. My question is, where can I get these from? Thanks in advance. Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted April 20, 2023 Share Posted April 20, 2023 3 hours ago, Delta said: I have 2 TI-99 4A boards... Sounds like you already have them! hmmm ...What are the chances that both sets are bad? 3 Quote Link to comment Share on other sites More sharing options...
atrax27407 Posted April 20, 2023 Share Posted April 20, 2023 Console ROMs can be replaced with TI TMS2532 EPROMs. I can provide them. However, check everything else first - it is usually something simple. 2 Quote Link to comment Share on other sites More sharing options...
Delta Posted April 20, 2023 Author Share Posted April 20, 2023 OK... Both motherboards show the same symptoms: white fuzzy horizontal lines moving up the screen. It doesn't seem to "boot" which is why I was considering the ROMs. I don't hear a beep or any other noise. I know the RAM is fine on one board and I am fairly certain the other is OK. I have swapped the VDPs. What should I try next? Quote Link to comment Share on other sites More sharing options...
+arcadeshopper Posted April 20, 2023 Share Posted April 20, 2023 have you checked to see if the cpu is working? Quote Link to comment Share on other sites More sharing options...
Delta Posted April 20, 2023 Author Share Posted April 20, 2023 How would you suggest I do that? I don't have an oscilloscope but I do have a multimeter. Quote Link to comment Share on other sites More sharing options...
+mizapf Posted April 20, 2023 Share Posted April 20, 2023 Stuck READY line? 1 Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted April 20, 2023 Share Posted April 20, 2023 3 hours ago, Delta said: ...white fuzzy horizontal lines moving up the screen. Do the lines look like the ones in the background of the picture shown in this video... Though they do not appear to be highly pronounced, they can make the video cut-out when they move into the vertical sync region at the top of the screen. Perhaps this is hinting at a power supply issue. Quote Link to comment Share on other sites More sharing options...
Delta Posted April 20, 2023 Author Share Posted April 20, 2023 1 hour ago, mizapf said: Stuck READY line? Could you please elaborate? Quote Link to comment Share on other sites More sharing options...
Delta Posted April 20, 2023 Author Share Posted April 20, 2023 (edited) I have attached a picture to show the white horizontal bars moving down the screen. The 4 power points on the motherboard show: - 5V, 5.4V, 0V, 2.9V Edited April 20, 2023 by Delta Quote Link to comment Share on other sites More sharing options...
+arcadeshopper Posted April 20, 2023 Share Posted April 20, 2023 3 hours ago, Delta said: How would you suggest I do that? I don't have an oscilloscope but I do have a multimeter. buy a logic probe or a oscilloscope even the crap oscilloscopes are good enough to tell if you have data lines active Quote Link to comment Share on other sites More sharing options...
+arcadeshopper Posted April 20, 2023 Share Posted April 20, 2023 10 minutes ago, Delta said: I have attached a picture to show the white horizontal bars moving down the screen. The 4 power points on the motherboard show: - 5V, 5.4V, 0V, 2.9V uh where's your 12v? did you test the pwr supply? try a known working one? 1 Quote Link to comment Share on other sites More sharing options...
Delta Posted April 20, 2023 Author Share Posted April 20, 2023 I realized as soon as I posted those voltages that the 12V was missing. The power rails was the first thing I checked a few days ago and the 12V was most certainly there. It looks like the capacitors need replacing on the smaller board. 1 Quote Link to comment Share on other sites More sharing options...
Delta Posted April 20, 2023 Author Share Posted April 20, 2023 (edited) Buy a logic probe or oscilloscope:I will do. Edited April 20, 2023 by Delta Quote Link to comment Share on other sites More sharing options...
+mizapf Posted April 20, 2023 Share Posted April 20, 2023 2 hours ago, Delta said: Could you please elaborate? One of the reasons you get a black screen and nothing else is that the READY input at the processor is held low. This may come from one of the devices on that line. When you turn it on, do you get a beep tone after a second, or is there just blackness? Or random noises from the sound generator? Quote Link to comment Share on other sites More sharing options...
Delta Posted April 21, 2023 Author Share Posted April 21, 2023 (edited) There is no beep tone, only a black background with white horizontal bars moving down the screen. Edited April 21, 2023 by Delta Quote Link to comment Share on other sites More sharing options...
+FarmerPotato Posted April 21, 2023 Share Posted April 21, 2023 5.4 is out of spec. The VDP RAM needs -5V, +5V, +12V. Depending on what else is going on, 5.4V should not damage chips, and it might even go to 5 once the root cause is fixed... The sound chip will make a random, horrible, (but strangely endearing) sound at power-up. This lasts until the console sends it the commands to be silent. So I would even suggest the sound chip 76489 (sometimes SN94624?) is dead, or else it is not getting the clock from video. I think it is socketed and removable in case of damage. A logic probe ($20) is useful to poke at clock pins. The probe tells you if there is a clock, constant high, or constant low. 2 Quote Link to comment Share on other sites More sharing options...
GDMike Posted April 21, 2023 Share Posted April 21, 2023 Oh yeah I always think that if it's not getting sound that it's something with the rom.. and I always forget that the sound Chip itself could be bad..duh.. you're so right. 1 Quote Link to comment Share on other sites More sharing options...
Duewester Posted April 21, 2023 Share Posted April 21, 2023 I'm pretty sure it'll boot without the sound chip installed but not with a bad rom. I spent hours and lotsa dollars just to find out one of my 6810s was bad. HOWEVER, mine made a goshaweful screech when booting that never stopped. Therefore leaving out the sound chip till I figured it out. And, mine didn't even display a white line but did show an occasional scratchy dot or two. 2 1 Quote Link to comment Share on other sites More sharing options...
Delta Posted April 26, 2023 Author Share Posted April 26, 2023 OK...I have replaced all the capacitors on the internal psu board and it has made no difference. Still the same horizontal bars. However, regarding the low voltages, I have noticed that if the PSU board is disconnected from the system board, the output voltages are fine: 5.2V, 11.8V and - 5V. If I connect the PSU board to the system board, the 5.2V and 11.8V drop significantly. The oscilloscope is on the way... Quote Link to comment Share on other sites More sharing options...
Duewester Posted April 26, 2023 Share Posted April 26, 2023 (edited) 2 hours ago, Delta said: OK...I have replaced all the capacitors on the internal psu board and it has made no difference. Still the same horizontal bars. However, regarding the low voltages, I have noticed that if the PSU board is disconnected from the system board, the output voltages are fine: 5.2V, 11.8V and - 5V. If I connect the PSU board to the system board, the 5.2V and 11.8V drop significantly. The oscilloscope is on the way... Good move on an o'scope. I prefer one over a logic probe. Reads like you got a ground on the 12v somewhere. I'd get out the schematics and start tracing down the 12v line. Hmm, ok, so I was just looking at the prints and it looks like if Q500 were shorted, it would put 12v and 5v to grnd. Edited April 26, 2023 by Duewester Quote Link to comment Share on other sites More sharing options...
HOME AUTOMATION Posted April 26, 2023 Share Posted April 26, 2023 On 4/20/2023 at 1:28 PM, Delta said: Is it possible to plug an NTSC modulator, into a PAL /4A? This looks a bit like some kind of video interface issue ...especially if you're getting the same picture, on two different /4As ...I'm guessing that you're using a video modulator. But, I believe an over-load might cause similar symptoms. 3 hours ago, Delta said: If I connect the PSU board to the system board, the 5.2V and 11.8V drop significantly. Anything ...reaching cooking temps?🌡️ 1 Quote Link to comment Share on other sites More sharing options...
Duewester Posted April 26, 2023 Share Posted April 26, 2023 @HOME AUTOMATIONmakes a good point about the video modulator. Just so happens 12v is on pin 1 IIRC, Just coincidence but the Ti I'm restoring had a broken out video connection on the board and would not have had a display had I ever powered it on before fixing it. Things like that are why I open up and inspect a unit before ever powering up. Quote Link to comment Share on other sites More sharing options...
Delta Posted April 26, 2023 Author Share Posted April 26, 2023 (edited) 27 minutes ago, HOME AUTOMATION said: Is it possible to plug an NTSC modulator, into a PAL /4A? This looks a bit like some kind of video interface issue ...especially if you're getting the same picture, on two different /4As ...I'm guessing that you're using a video modulator. But, I believe an over-load might cause similar symptoms. Anything ...reaching cooking temps?🌡️ I don't have an NTSC modulator, just a PAL one which is what I am using. No. All the ICs and RAM stay quite cool. Edited April 26, 2023 by Delta Quote Link to comment Share on other sites More sharing options...
+arcadeshopper Posted April 26, 2023 Share Posted April 26, 2023 33 minutes ago, HOME AUTOMATION said: Is it possible to plug an NTSC modulator, into a PAL /4A? This looks a bit like some kind of video interface issue ...especially if you're getting the same picture, on two different /4As ...I'm guessing that you're using a video modulator. But, I believe an over-load might cause similar symptoms. Anything ...reaching cooking temps?🌡️ din5 vs din7.. so no 2 Quote Link to comment Share on other sites More sharing options...
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