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jon

black screen

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Hi All,

 

I recently obtained a 99/4a. After purchasing a power supply and video cable (thanks arcade shopper) I attempted to boot the device. The HDMI monitor detects a signal, but the screen is completely black (there are perhaps some evenly spaced but very faint vertical bars?).  fwiw the video cable does work with my C64.  the output voltages on the internal PSU appear to be correct

 

based on what I read here on atariage, I purchased a replacement 9918 on ebay. this did not solve the issue.  based on another post, I then removed the sound chip (SN94624) and tried both 9918s. this did not help. my next attempt was going to be to replace the ROM CD2156NL based solely on a blog post (at nightfallcrew.com).  

 

does this make sense as the next step? are there better ways to identify the source of the issue? thanks!

 

--jon

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I would verify that your TV is able to properly accept low resolution video.  This is a common problem, especially with newer TVs, and affects a great many devices.  My sony PS2 is affected as well-- I have to use a special hardware upscaler with it.

 

If you have an old C64 composite monitor, or some old CRT TV laying around, I would try there first to be sure you aren't chasing ghosts.

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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?

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Unfortunately, I don't have access to any other displays. I am using this av2hdmi upscaler device to display the signal on my hdmi computer monitor.   

 

Sound is another issue.  The upscaler device combines the audio (RCA red/white) with the HDMI signal, but the display doesn't have speakers.  I'm thinking I'll have to hack a pair of headphones and see if I can listen in that way. I'll report back on what if any sounds the 99/4a is making.

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Do you have an old vcr? Those are often inexpensive ntsc/pal frame filters for potentially out of spec devices. Worth a shot to be sure your upscaler is not wigging out on you.

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59 minutes ago, jon said:

Unfortunately, I don't have access to any other displays. I am using this av2hdmi upscaler device to display the signal on my hdmi computer monitor.  

Those things are horrendous.  When they do work, lag's significant and picture quality is awful.  And sometimes they just don't.  So I would by no means treat that as an authoritative indicator of the TI 99's failure.  Even if you haven't got an HDTV, would it be possible to test the TI 99 on a friend's HDTV?  If not, as far as trustworthy upscalers go, the RetroTINK 2x is the most affordable upscaler which can correctly handle Composite video of this sort (and line-double/transcode it to 480p HDMI, which almost anything can handle).  The Framemeister can as well, but is much more expensive (because it is hugely more feature-rich).  The OSSC is also popular and relatively affordable, but does not handle Composite (which is in low demand, due to its offering the worst picture quality short of RF), so it won't be useful for an unmodified TI 99/4A, unfortunately (but is excellent for F18A-modded upscaling, since it's fantastic for 480p transcoding/upscaling). 

 

Results from built in HDTV upscalers can also be a bit all over the place.  Literally anything can happen, including almost perfect (some lag) upscaling (from a fair number of Samsung sets), to things like the wrong colours, or no displayed image at all, or an entirely grey image.  Because outright support-by-design for this type of non-standard video (in which every other scanline is drawn at 59.94kHz and the other half are never drawn) does not exist, on the HDTV market.  Definitely, if I was in your shoes, I'd be looking at either taking the TI 99 to test on a Samsung or Sony or otherwise mainstream HDTV, or otherwise picking up a RetroTINK.  Especially if you have other uses (you mention C64) for good upscaling.  Seems a shame to replace every part in the thing without really knowing for sure that it's broken. 

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ok so I was able to get it hooked up to my TV. I've got the same black screen and a loud incessant noise/tone.  

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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.

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On 9/15/2019 at 5:45 PM, AwkwardPotato said:

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.

 

Ok I pulled the GROMs.  No change :(

 

I haven't been hugely successful at desoldering in the past.  Especially when the board is the thing I'd like to salvage (as opposed to the component itself).  Also, are the SRAMs the 4164s, 4116s  or the MCM6810Ps ?

 

Thanks!

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Alright, I finally received the MCM6810Ps, the sockets and the requisite kid-free time to desolder.  Nothing.  What a bummer.  What's the next step? I'm surely near the giving-up stage now. 

 

Thanks!

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