TI-99/4 doesn't start up!

Spirantho · December 31, 2012

Hi everyone,

I finally got around to setting up my TI-99/4 today to get started on Mystery Fun House (Scott Adams adventure).

Unfortunately, the TI had other ideas.

I was greeted with a blank screen and a constant buzz from the sound chip (which is good - it's alive).

From what I can see, this is a common problem.... which instantly makes me suspect the 4116 ICs as they die so easily. However, I've repaired TI-99/4As before which had dodgy 4116s, and the symptom was different.

Assuming there isn't a 4116 grounding its output and thereby the data line it's on, what's most likely to be at fault? I'm thinking VDP, GROM or ScratchRAM, as obviously the machine isn't able to start up enough to initialise the sound chip.

I'm really hoping that it's not the VDP, but if it is... can I use a TMS9918A in place of the TMS9918? Are the two identical on the outside and just different on the inside? Or do I need a TMS9918 (where on earth am I going to find one of them....?)

If it's the GROM, can I 'borrow' it from a TI-99/4a - or maybe an EPROM? Ditto the scratchpad RAM... that'll be the same as a TI-99/4a.... probably.

Anyone able to give a hint as to where to start? Oh - one last bit of info - when I have a cartridge plugged in (Tunnels of Doom, of course), it 50% of the time creates a video signal (still black screen but the TV syncs to an NTSC display). Usually there's no video at all which is what you expect when the VDP isn't initialised. I *hope* this means it's failing to start up enough to start the VDP, and that the (very scarce) VDP is ok.

Thanks for any help anyone can give!

Edited December 31, 2012 by Spirantho

Spirantho · January 4, 2013

Much as I dislike replying to my own posts, it might be useful to others.

The black screen and constant tone is what happens when nothing is initialised. The usual diagnosis apparently is dead CPU or GROMs (which is really bad news on something as rare as a TI-99/4 as you can't use the /4A GROMs or EPROMs or anything like that).

Luckily, I found the schematic here: http://www.99er.net/download2/index.php?act=view&id=61

This showed me that the CPU gets its clock from a clock divider chip, the 74LS362, a.k.a. TIM9904. Looking with an oscilloscope showed nothing on the clock line! Hence nothing was happening at all. This meant almost certainly the TIM9904 or the crystal had gone. So I got both. Yes, you can still get TIM9904s from LittleDiode. Goodness knows how - mine was probably the first they'd ever sold.

Both arrived this morning.... tried the TIM9904 - no good. Tried the new crystal (48MHz, incidentally)... and it fired up!

One TI-99/4 all running nicely again!

Space Centurion · January 14, 2013

Hi, I just bought a TI-99 that had the same problem when I got it home. After doing some quick web research on the problem, I took it apart and:

-reset the chips (chip-creak)

-removed the cartridge adapter board (apparently they cause system issues when they get dirty)

-cleaned the power supply board with contact cleaner

-cleaner the power switch with contact cleaner

I did a test, and everything worked. :-) I cleaned the edge part of the cartridge board, put it all back together, and everything works! Well...almost, my "P" key doesnt work, but that will be fixed next! I'm not sure if I just got lucky with this, or wether this will work for most units. Either way, just wanted to let you know what I did so hopefully it will help you and others.