cbmeeks Posted November 12 Share Posted November 12 A friend of mine gave me a beige TI-99/4a. When I power it on, I get a black screen and a loud buzzing sound. The PSU is confirmed good and the VDP is confirmed good (so is the cable). The VDP is the only thing socketed. Any idea what the issue could be? I have other units that need repair but are closer to working so I might save this one for last. Thanks! Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/ Share on other sites More sharing options...
Stuart Posted November 13 Share Posted November 13 (edited) If the loud buzzing noise sounds like the sound chip, then it's not executing code (or not executing the correct code) as one of the first things it does at power-on is silence the sound chip. So could be the processor, clock chip, ROMs, scratchpad RAMs, logic chips, etc. Will need further investigation to see what it's actually doing. If you have the means to check that the processor is actually running, then one of the scratchpad RAMs is often the culprit. Edited November 13 by Stuart 2 Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5564967 Share on other sites More sharing options...
Shift838 Posted November 14 Share Posted November 14 3 hours ago, Stuart said: If the loud buzzing noise sounds like the sound chip, then it's not executing code (or not executing the correct code) as one of the first things it does at power-on is silence the sound chip. So could be the processor, clock chip, ROMs, scratchpad RAMs, logic chips, etc. Will need further investigation to see what it's actually doing. If you have the means to check that the processor is actually running, then one of the scratchpad RAMs is often the culprit. On 11/12/2024 at 5:48 PM, cbmeeks said: A friend of mine gave me a beige TI-99/4a. When I power it on, I get a black screen and a loud buzzing sound. The PSU is confirmed good and the VDP is confirmed good (so is the cable). The VDP is the only thing socketed. Any idea what the issue could be? I have other units that need repair but are closer to working so I might save this one for last. Thanks! Sounds like the new console tester I am selling may help to diagnose it. 1 Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5565072 Share on other sites More sharing options...
cbmeeks Posted November 14 Author Share Posted November 14 Oh yeah? Might be useful. I have 9 TI's and only 3 of them work 100%. The others range from non-working keyboard to a very PSU buzzing sound. Some corrupt fonts, etc. Seems like the beige ones are the least reliable. Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5565104 Share on other sites More sharing options...
Shift838 Posted November 14 Share Posted November 14 (edited) 29 minutes ago, cbmeeks said: Oh yeah? Might be useful. I have 9 TI's and only 3 of them work 100%. The others range from non-working keyboard to a very PSU buzzing sound. Some corrupt fonts, etc. Seems like the beige ones are the least reliable. I have the preorder going on right now. I have already ordered the PCBs, they should be here next week for me to start building. https://www.shift838.com/store/ Edited November 14 by Shift838 Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5565115 Share on other sites More sharing options...
Tator Posted November 22 Share Posted November 22 I am trying to repair some non working TI 99/4A. Now I am dealing with a beige console that it's giving me a hard work. It has the classic loud buzzing noise sounds. Thes CPU was faulty, I replaced but still stuck on the buzzing noise. I tested all socketed chips, sound, GOMS and VDP and also removed, tested and socketed the TMS 9901, ROMs, scratchpad RAMs and most of the logic chips, etc. I also compared voltages and signals with a working MOBO, both without the CPU and couldn't find any differences. When I turn it on, and look at the address bus I get differents and weird behavours every time. Some times it's locked sometimes there is activity. ( Even at some point started playing repetitive sounds that seems to be part of a firing game???) I didn't replace any of the 16K RAMS IC's yet. It's my understanding that you need those in order to boot to the initial screen but I assume that if they are faulty thery are not involved in the initial part of the boot process not before the time that does silence the sound chip. But I am not 100% sure that they wouldn't affect this part. Looking at the schematics they are not connected to the main address bus, however I believe thta they may be locking the data bus and therefore may be affecting the initial code and therefore getting weird addresses signals? I know there are many hardware experts here that could answer my doubts or point me to the right path. As always I appreciate the support of the great TI 99 community! Tks! Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5570209 Share on other sites More sharing options...
HOME AUTOMATION Posted November 22 Share Posted November 22 1 hour ago, Tator said: repetitive sounds that seems to be part of a firing game??? Did it sound at all like an angry woodpecker? 1 Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5570265 Share on other sites More sharing options...
Shift838 Posted Thursday at 05:38 AM Share Posted Thursday at 05:38 AM On 11/12/2024 at 5:48 PM, cbmeeks said: A friend of mine gave me a beige TI-99/4a. When I power it on, I get a black screen and a loud buzzing sound. The PSU is confirmed good and the VDP is confirmed good (so is the cable). The VDP is the only thing socketed. Any idea what the issue could be? I have other units that need repair but are closer to working so I might save this one for last. Thanks! Did you ever get this fixed? I am betting if your VDP is good then you have a bad VRAM causing the problem. Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5578886 Share on other sites More sharing options...
cbmeeks Posted Friday at 01:16 AM Author Share Posted Friday at 01:16 AM Unfortunately, I didn't get this fixed. Mainly because I don't have any spare VRAM. I have a single spare CPU and quite a few VDP's but that's it. I was going to wait until I could accumulate some spare parts. Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5579395 Share on other sites More sharing options...
Shift838 Posted Friday at 04:08 AM Share Posted Friday at 04:08 AM 2 hours ago, cbmeeks said: Unfortunately, I didn't get this fixed. Mainly because I don't have any spare VRAM. I have a single spare CPU and quite a few VDP's but that's it. I was going to wait until I could accumulate some spare parts. If I remember right the TI uses 4x TMS4116 DRAMs to make up it's 16kb of VRAM. These can be had really cheap on ebay.. Quote Link to comment https://forums.atariage.com/topic/375287-faulty-ti-994a/#findComment-5579470 Share on other sites More sharing options...
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