ti99iuc Posted February 4, 2013 Share Posted February 4, 2013 hi Friends ! i would want to repair a ti99 that have this problem... Someone know how i could solve this problem ? thank you, Ciro. Quote Link to comment Share on other sites More sharing options...
samishal Posted February 4, 2013 Share Posted February 4, 2013 I think that might be a problem with either the RF modulator or the internal VDC. In either case I don't think I'm qualified to suggest how to repair, might be something on Mainbyte tho? Quote Link to comment Share on other sites More sharing options...
ti99iuc Posted February 4, 2013 Author Share Posted February 4, 2013 Thank you, the RF Modulator is working on another TI99... so it's sure that the TI99 computer have some problems... just o do not know the chip i must check.. i have another incomplete TI99 motherboard too... so i could use that for repair... Quote Link to comment Share on other sites More sharing options...
+RetroElectroDad Posted February 4, 2013 Share Posted February 4, 2013 I had two TI-99s that I hadn't used for a long while that had the same problem when I started them up. Corrupt characters and then eventually the computer would hang. Luckily I had two F18As waiting to be installed so I thought I'd at least use one to practice the installation and lo and behold installing the F18A corrected the issue and I had a working TI. Seems it was the VDP chip in my case. Quote Link to comment Share on other sites More sharing options...
Stuart Posted February 4, 2013 Share Posted February 4, 2013 It's more likely I think that one or more of the video RAM chips is faulty. It's displaying a picture OK, but the data is corrupted. If you remove the board you'll see a row of 8 off 4116 (or equivalent) RAM ICs. With everything connected back up again, you *might* find that one or more are running noticeably hotter or colder than the others - that might be a clue as to which needs replacing. Otherwise replace one, test, replace another if still faulty ... Replacements are fairly cheap and common on Ebay. To replace, I find it easiest to cut all the pins on a chip and throw it away, unsolder the remains of the pins and clean up the holes, then fit a socket. 4 Quote Link to comment Share on other sites More sharing options...
ti99iuc Posted February 4, 2013 Author Share Posted February 4, 2013 Perfect, thank you... i will check in next days and will back here Quote Link to comment Share on other sites More sharing options...
kl99 Posted February 4, 2013 Share Posted February 4, 2013 (edited) A guy from our austrian group has seen these video errors a few times (he fixes a lot of 8/16 bit systems), one of the 8 video memory chips could be faulty. Open it and try to see if one of the memory chips is really hot compared to the others, this one needs replacement then: http://www.8bit-home..._reparatur.html http://www.8bit-homecomputermuseum.at/repair/ti_994a_2/ti994a_2_reparatur.html Since the VDP is socketed you can further try removing the VDP chip and putting it back into the socket. Of course you could try a F18A in there which bypasses the 16K VDP Ram all together because it is using it's own 16K as VDP. Edited February 4, 2013 by kl99 1 Quote Link to comment Share on other sites More sharing options...
ti99iuc Posted February 4, 2013 Author Share Posted February 4, 2013 Ok Klaus... thank you ! Quote Link to comment Share on other sites More sharing options...
+mizapf Posted February 4, 2013 Share Posted February 4, 2013 I just tried this in MESS (file tms9928a.c): I replaced m_vram_space->write_byte(m_Addr, data); with m_vram_space->write_byte(m_Addr, data & 0xfb); in order to kill the 0x04 bit. See the result. Not quite, but pretty close, isn't it? Michael 4 Quote Link to comment Share on other sites More sharing options...
ti99iuc Posted February 4, 2013 Author Share Posted February 4, 2013 Ah ! ... ... fault simulator! Quote Link to comment Share on other sites More sharing options...
+mizapf Posted February 4, 2013 Share Posted February 4, 2013 I once thought about adding a feature like "Simulate cartridge port wear-out" which should add random bits to the address and data bus lines. But I also remember the thrill in those days long ago, entering an Extended Basic program, hoping that the cursor that just disappeared on the screen will return, anxiously thinking about another quick save, although this may just trigger yet another crash. I was so glad to have the Geneve and no cartridge problems anymore. Maybe for this reason I never carried on with that thought. But for your current problem: This seems to be a good indication that the RAM circuit for bit 0x04 is faulty. If you look at the schematics you see that the Video RAM is organized as 8 chips with 16K x 1 bit, i.e. each bit of a byte is stored in another chip. Michael Quote Link to comment Share on other sites More sharing options...
ti99iuc Posted February 4, 2013 Author Share Posted February 4, 2013 yes.. good indication... thank you Quote Link to comment Share on other sites More sharing options...
scouter3d Posted February 12, 2013 Share Posted February 12, 2013 Hi, i am the guy Klaus mentioned above (www.8bit-homecomputermuseum.at)... The easy way to find most of the Videoram faults is to "piggypack" a working Ramchip ontop of the Ram's (one after the other) and see if the display changes... Sometimes it is not easy to feel heat differences... Ciao TOM:-) Quote Link to comment Share on other sites More sharing options...
JamesD Posted February 12, 2013 Share Posted February 12, 2013 (edited) I have the same problem on a couple COLECO ADAMs. When I load a game I get black vertical stripes where the VDP can't wrote to or read from memory. That makes it a little more obvious than the garbled characters, which I also get. Definitely RAM. Edited February 12, 2013 by JamesD Quote Link to comment Share on other sites More sharing options...
S1500 Posted February 12, 2013 Share Posted February 12, 2013 Circuit bending! Quote Link to comment Share on other sites More sharing options...
CantStopClicking Posted February 23, 2013 Share Posted February 23, 2013 Hi, i am the guy Klaus mentioned above (www.8bit-homecomputermuseum.at)... The easy way to find most of the Videoram faults is to "piggypack" a working Ramchip ontop of the Ram's (one after the other) and see if the display changes... Sometimes it is not easy to feel heat differences... Ciao TOM:-) Thanks for sharing an easy trouble-shooting tip! -Dano Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.