Stig Posted January 17, 2022 Share Posted January 17, 2022 Hi. I got this cheap TI-99/4a the other day, but the screen is all messed up. Anyone know how to fix this? Quote Link to comment Share on other sites More sharing options...
+OLD CS1 Posted January 18, 2022 Share Posted January 18, 2022 I suspect VDP RAM problem, though it does not look like one of these masks here https://www.ninerpedia.org/wiki/Troubleshooting Maybe it is a stuck-on bit rather than a stuck-off. 3 Quote Link to comment Share on other sites More sharing options...
Tursi Posted January 18, 2022 Share Posted January 18, 2022 Probably still VRAM, since it made it to the title page. Looks like there's probably an address bit issue combined with a data bit issue Quote Link to comment Share on other sites More sharing options...
+mizapf Posted January 18, 2022 Share Posted January 18, 2022 I can see this: E (>45) → U (>55) A (>41) → Q (>51) N (>4E) → ^ (>5E) I (>49) → Y (>59) In the empty space we have "0 0 0 ..." which means that at every second position in the Screen Image Table there is a >30 ("0") instead of the >20 for space. Also, the character definitions have additional dots. The space character looks like 10 00 10 00 10 00 10 00. Quote Link to comment Share on other sites More sharing options...
Stuart Posted January 18, 2022 Share Posted January 18, 2022 It suggests that the 4th bit is stuck at 1: xxx1xxxx, but it is not consistent - "TEXAS" for example is fine on the top line but corrupt on the bottom line, and some characters that should be affected are displayed OK. After it has warmed up for a bit, do you see the exact same screen as above each and every time you reset it? I'd start by replacing the RAM chip labelled "3" in the photo on the link above and see what you're left with. Quote Link to comment Share on other sites More sharing options...
+mizapf Posted January 18, 2022 Share Posted January 18, 2022 TEXAS is correct on the top line because every second character is changed, but only if the bit is not already set. That is, characters >50 - >5F will not change, while >40 - >4F will turn to >50 - >5F *if* they are on the affected columns. So on the top line we have (T)E(X)A(S) → TEXAS and on the bottom line we have T(E)X(A)S → TUXQS 1 Quote Link to comment Share on other sites More sharing options...
+mizapf Posted January 18, 2022 Share Posted January 18, 2022 Looks like this: I added the following line to the VRAM write function: if ((m_Addr & 1)==0) data |= 0x10; m_Addr is the current video address, data is the value passed to the VDP. Theories? 1 Quote Link to comment Share on other sites More sharing options...
Stuart Posted January 18, 2022 Share Posted January 18, 2022 So is that the lowest order address line pulling that data bit high, but can't pull it low? I think I'd be looking out my box of spare 9918s as well ... Quote Link to comment Share on other sites More sharing options...
+mizapf Posted January 18, 2022 Share Posted January 18, 2022 The problem is with U105 (the chip marked by "3"); maybe setting the LSB address bit (A0 at the VRAM) to 0 forces the data input to 1. However, the address is set in two passes (7 bits, strobed by RAS and CAS), so this is not really convincing yet. Quote Link to comment Share on other sites More sharing options...
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